Importance of pleural pressure for the evaluation of respiratory mechanics

Abstracts

JUSTIFICATIVA E OBJETIVOS: Para a partição das medidas de mecânica do sistema respiratório em seus componentes pulmão e parede torácica, faz-se necessário o conhecimento da pressão pleural. A finalidade desta revisão foi discutir sobre medidas alternativas à obtenção da pressão pleural para o cálculo da mecânica pulmonar, relatar as peculiaridades do método do balão esofágico para obtenção indireta da pressão pleural, as particularidades da obtenção da medida da pressão esofágica em pacientes sedados ou anestesiados, discorrer sobre a medida direta da pressão pleural e sua correlação com a pressão esofágica, assim como relatar sobre o reflexo da PEEP nas pressões pleural e esofágica. CONTEÚDO: A variação da pressão intra-esofágica reflete a variação da pressão intrapleural, podendo ser usada como medida alternativa à pressão pleural direta, no estudo da mecânica dos componentes pulmão e parede do sistema respiratório. A medida da pressão esofágica pode ser realizada por meio de um delicado balão posicionado no interior do esôfago. O método e a técnica foram observados e validados em seres humanos e animais em diferentes condições e posturas corporais. O emprego da PEEP em pacientes sob ventilação controlada mecânica está consolidado; no entanto, existem controvérsias da correlação próxima entre a pressão esofágica e a pressão pleural em pacientes ventilados com PEEP, o que pode resultar em erros de cálculo de mecânica respiratória considerando a pressão esofágica. CONCLUSÕES: O método do balão esofágico é o mais utilizado para a obtenção da medida indireta da pressão pleural. Em pacientes sedados ou anestesiados sem alterações significativas da complacência respiratória, a variação da pressão esofágica corresponde à variação da pressão pleural quando a PEEP é aplicada.

FISIOLOGIA, Pulmonar; FISIOLOGIA, Pulmonar; VENTILAÇÃO

REVIEW ARTICLE

Importance of pleural pressure for the evaluation of respiratory mechanics^*

La importancia de la presión pleural en la evaluación de la mecánica respiratoria

Cláudia Regina Fernandes, TSA

Doutora em Medicina pela Universidade de São Paulo; Chefe do Serviço de Anestesiologia do HUWC/UFC; Responsável pelo CET/SBA HUWC/UFC

^{Correspondence to}

SUMMARY

BACKGROUND AND OBJECTIVES: Pleural pressure has to be known for the partitioning of respiratory system mechanical measurements into their lung and chest wall components. This review aimed at discussing alternative methods to obtain pleural pressure to calculate pulmonary mechanics, at reporting peculiarities of the esophageal balloon method for obtaining indirect pleural pressure, peculiarities of esophageal pressure measurement in sedated or anesthetized patients, at discussing direct pleural pressure and its correlation with esophageal pressure, in addition to reporting on the impact of PEEP on pleural and esophageal pressures.

CONTENTS: Esophageal pressure variation reflects pleural pressure variation and may be used as alternative to direct pleural pressure in the study of lungs and chest wall mechanics. Esophageal pressure may be obtained with a delicate balloon placed inside the esophagus. Method and technique were observed and validated in humans and animals in different conditions and body positions. PEEP is a consolidated method for patients under mechanically controlled ventilation, however there are controversies about the close correlation between esophageal and pleural pressure in patients ventilated with PEEP, which may result in wrong respiratory mechanics calculation based on the esophageal pressure.

CONCLUSIONS: The esophageal balloon is the most common method to obtain indirect pleural pressure. In sedated or anesthetized patients without major respiratory compliance changes, esophageal pressure variation corresponds to pleural pressure variation when PEEP is applied.

Key Words: PHYSIOLOGY, Pulmonar: pleural cavity, respiratory mechanics; VENTILATION: positive end expiratory pressure.

RESUMEN

JUSTIFICATIVA Y OBJETIVOS: Para la partición de las medidas de mecánica del sistema respiratorio en sus componentes pulmón y pared torácica, se hace necesario el conocimiento de la presión pleural. La finalidad de esta revisión fue la de discutir sobre las medidas alternativas para la obtención de la presión pleural para el cálculo de la mecánica pulmonar, relatar las peculiaridades del método del globo esofágico para la obtención indirecta de la presión pleural, las particularidades de la obtención de la medida de la presión esofágica en pacientes sedados o anestesiados, discurrir sobre la medida directa de la presión pleural y su correlación con la presión esofágica, como también relatar sobre el reflejo de la PEEP en las presiones pleural y esofágica.

CONTENIDO: La variación de la presión intra esofágica refleja la variación de la presión intrapleural, pudiendo ser usada como medida alternativa a la presión pleural directa, en el estudio de la mecánica de los componentes pulmón y pared del sistema respiratorio. La medida de la presión esofágica puede ser realizada a través de un delicado globo posicionado en el interior del esófago. El método y la técnica fueron observados y validados en seres humanos y animales en diferentes condiciones y posturas corporales. El empleo de la PEEP en pacientes bajo ventilación controlada mecánica está consolidado, sin embargo existen controversias de la correlación próxima entre la presión esofágica y la presión pleural en pacientes ventilados con PEEP, lo que puede resultar en errores de cálculo de mecánica respiratoria considerando la presión esofágica.

CONCLUSIONES: El método del globo esofágico es el más utilizado para la obtención de la medida indirecta de la presión pleural. En pacientes sedados o anestesiados sin importantes alteraciones de la complacencia respiratoria, la variación de la presión esofágica corresponde a la variación de la presión pleural cuando se aplica a PEEP.

INTRODUCTION

Pleural pressure (Ppl) has to be known for the partitioning of respiratory system mechanics measurements into its lungs and chest wall components. However, the access to pleural cavity to obtain direct measurement has disadvantages for being invasive and for the risk of pneumothorax. So, alternatives were sought, such as esophageal pressure (Pes). It is known for more than one century that intra-esophageal pressure variation reflects pleural pressure variation ¹ and may be used as alternative to direct pleural pressure to study lungs and chest wall mechanics and to evaluate respiratory work during assisted or spontaneous ventilation.

Esophageal pressure measurement through a delicate balloon placed in the esophagus was firstly described more than 50 years ago ². Since then, several human and animal studies were conducted comparing esophageal and pleural pressures to try to validate the method ^3,4. Other studies were performed to validate the esophageal balloon technique ^5,6.

PEEP in patients under mechanically controlled ventilation is consolidated both for anesthesia and intensive care. It is mandatory in the treatment of respiratory distress syndrome ^7,8. Being the study of respiratory mechanics and its sub-components lungs and walls critical to understand some clinical situations, esophageal pressure to indirectly obtain pleural pressure has become a popular method ^9,10. In this context, there are still controversies about the close correlation between esophageal and pleural pressures in PEEP ventilated patients, under sedation or anesthesia, or even with different body positions ¹¹_. This is important because abnormal esophageal pressure values may result in errors in respiratory mechanics calculation and interpretation, which may lead to inadequate management.

This review aimed at discussing alternative methods to obtain pleural pressure to calculate pulmonary mechanics, at reporting peculiarities of the esophageal balloon method to obtain indirect pleural pressure, peculiarities of esophageal pressure measurement in sedated or anesthetized patients, at discussing direct pleural pressure measurement and its correlation with esophageal pressure, in addition to reporting on the impact of PEEP on pleural and esophageal pressures.

ALTERNATIVES TO PLEURAL PRESSURE MEASUREMENT

It is known for more than 120 years that chest pressures variations affect esophageal pressure. Luciani, in 1878 ¹, was the first to publish studies involving esophageal pressure measurements, although the discovery of the method is credited to Ceradini, who has not published his results.

In the search for an alternative method to obtain intra-thoracic pressure, Otis et al. have shown that it is possible to correlate increased venous pressure to increased pulmonary pressure by placing a catheter in the cubital vein; however, the validity of this technique was questionable ¹².

In 1949, Buytendijk has introduced esophageal pressure measurement technique with a balloon in the esophagus ².

Dornhorst et al. have obtained esophageal pressure with a liquid-filled catheter ¹³. Other authors have observed that this system is comparable to esophageal balloon; however it is more sensitive to noises such as heart beats ¹⁴. This method has been primarily used in neonates and infants to determine the mechanics of respiratory system sub-components ^15,16.

Clarysse et al. have measured esophageal pressure with two balloons in a single catheter, separated 10 cm from each other ¹⁷. Rajacich et al. have created a method to monitor esophageal pressure with a balloon irrigated with saline solutuion ¹⁸. Karason et al. have used a double-lumen liquid-filled gastric tube to obtain esophageal pressure. The efficacy of this method was compared to the esophageal balloon ¹⁹.

More recently, the flexible catheter with a microtransducer at the edge was made available to monitor esophageal pressure ²⁰.

Some authors have directly measured pleural pressure with microtransducers coupled to the edge of catheters ^21,22, and others with flat and flexible sensors ²³. Tobin et al. have reported that surface inductive plethysmography is an alternative for indirect pleural pressure measurement through a transducer placed in the sternal fossa ²⁴. However it has some limitations, such as adequate transducer positioning in the fossa, sternal cavity size, adequate fixation of patient's head and neck.

Other investigators have studied transpulmonary gradient through pleural pressure recording, using a minimally invasive technique with rib capsules to promote minor pleural space distortion ^25,26.

ESOPHAGEAL BALLOON METHODS

The most popular esophageal pressure method is the latex air-filled balloon-catheter connected to a pressure transducer system ¹⁴.

The esophagus is a muscle-membranous tube located in the chest and measuring approximately 4 cm diameter and 25 cm length. It is maintained collapsed and is connected to other mediastinal structures by connective tissue and small muscles. There are functional sphincters on its edges. Esophageal pressure variations may be intrinsic or extrinsic. Extrinsic pressures are those originating in the chest, while intrinsic pressures are essentially two: located spasms or changes in tone which may be generalized, and peristaltic waves ³.

Mead et al. have compared short (3 cm length and 1 cm diameter) and long (16 cm length and 0.8 cm diameter) esophageal balloons and have observed that the long balloon shows lower pressure variations with changes in position inside the esophagus ¹⁴. Other authors ²⁷ have also stated that pressure measurement with long balloons is more accurate; however they reported that the best results were obtained when measures were taken in the esophageal medium third, showing that in this region the shape of the pressure-volume curve is not significantly affected by changes in body position.

Petit and Milic-Emili have studied the mechanical properties of human esophageal wall using esophageal balloons of different diameters. They have observed that esophageal balloons with perimeter above 4.8 cm are ideal because they are flaccid and more accurately transmit esophageal wall tension variations.

These authors have noted that specific esophageal elastance is not uniform, has lower value in the lower third and progressively increase in the cephalad direction. The histological difference of muscles in different esophageal regions (striated muscles in the upper third, smooth muscles in the lower edge and mixed muscles in the medium third), with corresponding muscle activity differences, may explain different elastic properties.

The authors infer that, due to differences in specific elastance along the esophageal wall, the difference between intra-thoracic and intra-esophageal pressures will depend on the position of the balloon, and will be lower when it is placed in the lower third where there is more distensibility. This same study has observed more frequent peristaltic waves with the individual in the supine position. However, when a wider and longer cuff was used, the effect of peristaltic waves on esophageal pressure was substantially decreased ²⁸.

The hypothesis that body position could affect the fidelity of esophageal pressure measurement with regard to pleural pressure has led some authors to perform several investigations. Ferris et al. have studied six normal adult patients maintained under spontaneous ventilation and measuring esophageal pressure through a balloon placed in the lower third of the esophagus, in different body positions.

They have observed that esophageal pressure values in the prone and the lateral positions were similar to the value in the upright position, while in the supine position esophageal pressure was much higher, supposing that this increase was related to esophageal pressure exerted by the heart and great vessels by the action of gravity ²⁹. Knowles et al. have evaluated esophageal pressure obtained with a 5.5 cm balloon in four patients in different positions – sitting, supine with 30º cephalic inclination and prone – in different pulmonary volumes and have observed that in low pulmonary volumes, that is, 20% of vital capacity (VC) in the supine position, esophageal pressure was more positive. They have also attributed this fact to esophageal compression by mediastinal content, especially the heart which is moved closer to the balloon by diaphragm displacement ³⁰.

Milic-Emili et al. have shown that esophageal balloon pressure is identical to pleural pressure when pressure difference throughout its structures, that is balloon wall, esophageal wall and different mediastinal structures, is zero ³¹. Balloon pressure increases with volume. Analyzing this aspect, authors have evaluated esophageal pressure in eight patients with increasing pulmonary volumes, and with the balloon filled with different volumes of air. All measures were obtained in the upright position. Results indicated that the effect of balloon volume on esophageal pressure is higher in both vital capacity extremes, especially with high pulmonary volumes 20% above VC. They have concluded that the balloon filled with low volumes, approximately 0.2 mL of air, will more accurately reflect pleural pressure. The ideal volume for the adequate filling of the esophageal balloon should be obtained through the balloon compliance curve. In long balloons, the plateau of this curve is in general between 0.2 to 0.5 mL of air ^32,33.

Gerhardt and Bancalari have observed that airway pressure percentage (Paw) transmitted to the esophagus depends on pulmonary compliance. They have studied 26 premature neonates and have observed low correlation between Paw and Pes variation. Only 5% of Paw was transmitted to the esophagus of children with hyaline membrane disease, and 12% to neonates with patent ductus arteriosus ³⁴.

Always trying to validate the adequate catheter-balloon position in the esophagus, several tests were proposed. Pes validity may be obtained by static Valsalva or Mueller maneuvers against the occluded airway. An approximate matching between esophageal (DPes) and tracheal (DPtr) pressure variation – airway pressure – indicates satisfactory balloon positioning. Since many untrained individuals find difficulties in performing this static maneuver, Baydur et al. have introduced a dynamic alternative to solve this problem. The "occlusion test" consists on airway occlusion at the end of expiration, allowing the patient to do three to five inspiratory efforts with occluded airways.

This technique was tested in 10 volunteers in the sitting, right and left lateral and supine position with the esophageal balloon placed 10 cm above the cardia. Authors have observed that in the sitting and lateral positions the DPes/DPtr ratio was close to the unit, while in the supine position DPes/ DPtr was lower than the unit in seven out of ten volunteers.

These esophageal pressure changes were primarily attributed to cardiac artifacts. Authors have concluded that in some individuals in the supine position, esophageal pressure measured with conventionally placed balloon does not presumably reflect pleural surface pressure. With the repositioning of the balloon in a different level, 5 or 15 cm above the cardia, a position may be found where DPes/DPtr ratio is close to the unit. Pulmonary mechanics measurements estimating pleural pressure are presumably valid, based on esophageal pressure as from this site ³⁵. This test was also validated in anesthetized non-paralyzed patients under spontaneous ventilation ⁹.

Fonseca-Costa and Nardi have also applied the "occlusion test" to validate esophageal pressure in eight volunteers in different positions: sitting with the chest in the vertical position (90º) and in 45º, 0º and -10º angles with the balloon placed 10 cm above the cardia. They have concluded that by carefully repositioning the catheter after each change in body angle, it is possible to obtain acceptable measurements in all studied positions. Major difficulty of the study was observed in the horizontal and head-down positions due to higher interference of heartbeats. Asher et al. have studied this same test to validate the liquid-filled catheter in six healthy neonates in the right lateral, prone and supine positions. They inferred that the catheter placed in the lower esophageal third is more accurate to indirectly determine pleural pressure variation in those patients ³⁶.

Coates et al. have also validated Ppl measure by the "occlusion test" using liquid-filled catheter. The study was performed with eight intubated neonates and measures were obtained in the supine position. Mild pressures were applied to the larynx during the test to prevent air leakage. "Occlusion tests" were performed in the lower esophageal third starting from the cardia until the level where DPes/DPtr was lower than or close to the unit. All them presented a range between the cardia and the carina where DPes/DPtr was very close to the unit ¹⁵. This test has also been used to validate the position of liquid-filled catheter aiming at measuring esophageal pressure of small animals ³⁷.

The "occlusion test" for esophageal balloon validation may also be used during mechanically controlled ventilation under neuromuscular blockers. In this condition it is not possible to perform spontaneous inspiratory efforts so the recording of airway and esophageal pressure is only possible through chest or abdominal compressions. Lantieri et al. have applied airway occlusion associated to mild upper abdomen or rib compressions in paralyzed dogs maintained under mechanically controlled ventilation. They were able to record concomitant oscillations of esophageal and airway pressures, thus validating esophageal balloon position ³⁸. Barnas et al. have performed abdominal compressions associated to airway occlusion in 13 anesthetized paralyzed patients in the postoperative period of cardiac procedures, validating the balloon technique ³⁹.

A study with 1.5 cm catheter-balloon system positioned in the esophagus at 2-cm intervals has analyzed esophageal pressure along intra-thoracic and cervical segments of healthy humans. They have observed that esophageal pressures were progressively more negative from the intra-thoracic to the cervical region, that is, there is a pressure gradient inside the esophagus ³².

Clarysse et al. have used two balloons in a single catheter, placed 10 cm apart from each other to measure esophageal pressure. The lowest balloon was positioned 5 cm above the cardia. They have investigated changes in transpulmonary pressure before and after turning the body 180º. They have observed that transpulmonary pressure is more positive in the upper balloon in both positions, that is, in the medium esophagus in the upright position and in the distal esophagus in the head down position. They have also observed that there was increased transpulmonary pressure gradient with decreased pulmonary volume from 80% to 20% of VC, implying that pressure changes follow global volume changes. They have also found a reverse esophageal pressure gradient in the head down position ¹⁷.

Dechman et al. have investigated the influence of body position, balloon position and lung volume in the "occlusion test" of anesthetized dogs spontaneously breathing and paralyzed under mechanically controlled ventilation. They speculated that Pes more precisely reflects Ppl during inspiration as compared to expiration. They have observed that DPes/DPtr ratio is closest to unit in the paralyzed state, indicating that Pes more precisely reflects Ppl in the presence of muscle paralysis ⁴⁰.

A possible explanation for this finding is the fact that the canine esophagus, as well as man's, is made of striated muscles. Pes measured in the upper esophageal third less accurately reflects Ppl as opposed to what has been observed in the distal third. In the supine position in spontaneous ventilation, the Pes versus Ptr loop has often differed from the unit, while under muscle paralysis the result was consistently very close to the unit. This study has suggested that mediastinal content weight is not responsible for poor "occlusion test" results during spontaneous ventilation in the supine position.

Baydur et al. have analyzed esophageal pressure of volunteers under spontaneous ventilation in three different levels (5, 10 and 15 cm above the cardia) in the presence of different pulmonary volumes in the sitting and supine positions. They observed that during relaxed breathings esophageal pressure has varied 30% more in the lower esophagus as compared to the upper region. During Valsalva's maneuver, DPes/DPtr remained close to the unit in most pulmonary volumes, except for the sitting position in volumes close to 20% VC ⁴¹.

Ingimarsson et al. have investigated possible differences in Pes resulting from changes in body position. They have studied 17 anesthetized paralyzed children (1.5 month to 15.5 years) in the supine and left lateral position. Pressure-volume curve was obtained and it was observed that when Pva = 30 cmH₂O, Pes was very similar in both positions, approximately 11 cm H₂O. However, when Pva = 0 cmH₂O, Pes was very positive in the supine position (6.6 ± 2.2 cmH₂O) and close to zero in the lateral position (1.1 ± 1.9 cmH₂O) ⁴². These results have impaired the validation of the hypothesis that Pes is representative of global Ppl in the supine position.

As to ideal esophageal balloon size for adults, it is currently known that its perimeter should correspond to that of the esophagus, between 4 and 4.8 cm. In practice, thin latex balloons with 0.1 mm thickness, 5 to 10 cm length and perimeter varying from 3.2 to 4.8 cm are adequate. Conventional catheters are made of polyethylene with internal diameter of 1.4 mm and 100 cm length (Figure 1). Volume of air should follow balloon's compliance and in the vast majority, 0.5 mL is adequate for measurements ⁴³. Satisfactory measurements have been obtained in neonates with 30 to 50 mm length, 7.6 mm diameter and 0.045 to 0.075 mm thickness balloons. Air volume should be determined by the pressure-volume curve in vitro, before its introduction in the esophagus ⁴⁴.

ESOPHAGEAL PRESSURE MEASUREMENT AND THE ANESTHETIZED PATIENT

The catheter-balloon system has been used in most different studies on respiratory system and its sub-component mechanics in the anesthetized patient, considering esophageal pressure value as pleural pressure estimate, although some studies question esophageal pressure validation, which would pose problems for ventilatory mechanics calculation.

Higgs et al. have studied 10 patients submitted to general anesthesia with oxygen, nitrous oxide and halothane and maintained under spontaneous ventilation. With the balloon conventionally positioned (10 cm above the cardia) they performed the "occlusion test" and showed that the difference between Pes and Ptr was below 10% in most patients. These authors have evidenced the presence of artifacts caused by heartbeats and posing problems for ventilatory mechanics calculation with the need to reposition the balloon in some cases ⁹.

Some authors have calculated ventilatory mechanics in anesthetized patients through esophageal pressure measurement and reported only that they have fixed the balloon 35 cm apart from the lips of all patients ⁴⁵; or 40 cm above the nose ⁴⁶, regardless of patients' height. No additional test was performed to validate the correct position of the balloon. Others have fixed the balloon at the point where the highest esophageal pressure variation was found, and a constant increase in this pressure was recorded during positive pressure ventilation ^47,48.

Other investigators ^49,50 have performed the "occlusion test" to validate the balloon technique in anesthetized non-paralyzed patients to calculate ventilatory mechanics during sedation or anesthesia. D'Angelo et al. have performed the "occlusion test" in the supine position before anesthetic induction ⁵¹. In some studies on ventilatory mechanics in anesthetized patients, no details were reported on the validation of the esophageal balloon technique before obtaining esophageal pressure ^33,52.

Pelosi et al. investigating ventilatory system mechanics in 10 morbidly obese patients have validated the position of the balloon before muscle paralysis through the "occlusion test", when there were still some ventilatory efforts, and have also used chest X-rays to confirm catheter-balloon position in the medium third of the esophagus ⁵³.

Several methods were developed to obtain esophageal pressure variation to calculate pulmonary mechanics in patients under mechanically controlled ventilation. The single breath method described by Zin et al. ⁵⁴ to measure passive ventilatory system properties ^55,56, the interruption technique ⁵⁷ and the end inspiratory occlusion method (EIOM) are examples. Among them, EIOM with constant flow has been the most widely used method. EIOM is based on the theoretical development proposed by Bates et al. ^58,59, based on pioneer studies by Don and Robson, Rattenborg and Holaday ^60,61. As from those studies, the ventilatory system may be described as being made of multiple compartments connected in parallel, each one with different emptying times defined as time constants.

At a certain zero inspiration moment with flow entry occlusion for a certain period of time (e.g.: 4 seconds), a fast decrease may be observed in the pressure which was previously being generated. This decrease represents the resistive pressure of the system and after this fast decrease there is a gradual pressure curve decrease until a plateau is reached which represents elastic shrinkage pressure. This variation is more evident it the airway trace and less evident in the esophageal pressure curve. Both sub-components represent different physiological processes during gases distribution in lungs. The first is characterized by fast and homogeneous pressure decrease and represents the pressure needed to overcome airway resistance.

The second, slower and not homogeneous ^58,59, would involve pendelluft and stress relaxation phenomena. Pendelluft is the inspired gas redistribution process seen between the units and with unequal time constants during the zero flow phase. It is possible that time constants are unequal due to different lung compartments presenting different fibro-elastic constitutions. The stress relaxation process is the property of the lung of decreasing pressure and distention created with pulmonary inflation when it is inflated and maintained at a constant volume. Pressure decrease would have two reasons. On one hand there would be a realignment of interstitial fibrillar matrix elastic fibers of the pulmonary parenchyma; on the other hand there would be interstitial alveolar film reorganization which would decrease forces generated by superficial stress ⁶². Stress relaxation represents the accommodation of lungs to positive intra-alveolar pressure. Jonson et al. have found that viscoelastic time constant in normal adults is 0.82 ± 0.11 seconds ⁶³. This implies an occlusion time above 2 seconds to obtain the true plateau pressure needed for mechanic calculations.

Auler Jr. et al. have used this method to calculate lung and chest wall elastic and resistive properties of morbidly obese patients and of normal weight patients anesthetized and paralyzed using 5 seconds inspiratory pause ⁶⁴.

DIRECT PLEURAL PRESSURE MEASUREMENT AND COMPARISON WITH ESOPHAGEAL PRESSURE

The access to pleural cavity for pressure measurement has disadvantages because it is invasive and has a high risk of pneumothorax. Ethically, this would be virtually impossible, except when there is already a pleural space drain ⁶⁵. So, esophageal pressure has been used to infer pleural pressure. To evaluate esophageal pressure validity as pleural pressure measurement, Fry et al. have studied the relationship between both pressures in two manners. The first method consisted on the simultaneous measurement of esophageal and pleural pressure in three individuals under spontaneous ventilation. They concluded that esophageal pressure is an accurate method to obtain pleural pressure variations. The second method consisted on the simultaneous recording of both pressures of a young man with minor pneumothorax and they concluded that relative pleural pressure changes were quantitatively reflected by esophageal pressure. They have also observed that pleural pressure was slightly more negative than the esophageal pressure. Authors have inferred that esophageal pressure safely and accurately reflects static and dynamic pleural pressure variations ³.

Cherniack et al. have compared esophageal and pleural pressure of 14 patients, of whom eight were studied under anesthesia and six were conscious during the procedure. They have observed that pleural pressure was consistently more negative during spontaneous ventilation as compared to esophageal pressure. Esophageal pressure variations were not higher than pleural pressure variations under controlled ventilation, both during inspiration and expiration.

Since patients were in the supine position, authors have raised the possibility of mediastinal weight influencing the esophagus and resulting in more positive pressure. They concluded that esophageal pressure is not identical to pleural pressure and does not follow a parallel ratio. In addition, they observed extreme individual variations in both pressures ratio, suggesting that pulmonary mechanics results should be carefully interpreted ⁶⁶. One year later, Attinger et al. have simultaneously measured esophageal and pleural pressure in four patients (two with spontaneous pneumothorax with 10% pulmonary collapse and two with pleural effusion) in spontaneous ventilation and in different positions. They have observed that pleural pressure variation was usually higher as compared to esophageal pressure.

The difference has varied in a patient-by-patient basis and depended on body position. End expiratory pleural pressure was in general more negative as compared to end expiratory esophageal pressure. Both end expiratory pressures were consistently more negative in the sitting position as compared to the supine position, and in most patients it was more negative in the prone position as compared to the sitting position ⁶⁷.

Mead and Gaensler, in 1959, have simultaneously recorded pleural and esophageal pressures in seven patients in the upright position and in five patients in the supine position. They have observed that cardiac oscillations were three times higher in the supine position as compared to the upright position and that this could partially contribute for the higher difference found in this position. They have also observed that the best correspondence between esophageal and pleural pressure was obtained in the upright position ⁶⁸. Absolute pressure values in the supine position have shown consistently more positive esophageal pressure, which may be explained by esophageal compression by mediastinal structures interfering with pulmonary compliance and resistance calculations.

Daly and Bondurant, in 1963, have simultaneously studied pleural pressure in different intercostal spaces (3^rd, 5^th and 8^th) and esophageal pressure in human volunteers in the sitting position. They observed that pleural pressure has varied according to measurement site, and that such values where higher in the lower thoracic region as compared to upper chest. Even with 200 mL pneumothorax, there were no differences in pleural and esophageal pressures ⁶⁹. Most studies have considered absolute pressure values to compare pleural and esophageal pressures. It is currently known that this comparison should be performed by relating pressure variations.

EFFECT OF PEEP ON PLEURAL AND ESOPHAGEAL PRESSURES

PEEP in mechanically controlled ventilation, both during anesthesia or Intensive Care is unquestionable. Respiratory mechanics understanding in any clinical condition is critical for the adequate treatment. Esophageal pressure as indirect pleural pressure measurement is a less invasive alternative to calculate respiratory system elastic and resistive properties. So, it is important to understand whether PEEP interferes with the correspondence of esophageal and pleural pressures.

Chapin et al. have investigated changes in chest (Cc) and pulmonary (Cp) compliance and the required PEEP level to establish adequate pulmonary expansion in 10 anesthetized pigs ventilated with 10 cmH₂O PEEP and Ppl monitoring. They have observed that in normal chest and pulmonary compliance conditions, approximately half (52% ± 9%) of applied airway pressure (Paw) was transmitted to the pleural space, but has increased to 65% ± 9% when Cc was normal and Cp was decreased by pneumatic tourniquet applied to abdomen and chest. Pulmonary hydrochloric acid instillation decreased Cp, median sternotomy increased Cc and in this situation, Paw transmission to pleural space was decreased to 11% ± 6%. Results indicated that both Cc and Cp are significantly important to determine the level of pulmonary expansion and pressure transmission to intrathoracic structures during PEEP ²². This study, however, has not monitored esophageal pressure.

Craven and Wood have studied six anesthetized dogs comparing esophageal and pleural pressure close to the heart in the right lateral position. They have observed that esophageal pressure is very similar to the pressure between the lung and the left pericardium with PEEP levels of 0, 10 and 20 cmH₂O. Although right extra-pericardial pressure is 4.5 to 5 times more positive, authors have related this fact to direct heart weight on pericardial space ⁷⁰. Marini et al. have examined the effect of PEEP on juxtacardiac and esophageal pressures of eight dogs in the prone and supine positions. Juxtacardiac pressure was significantly higher than esophageal pressure in the supine position, while there has been no statistically significant difference between pressures in the prone position. In this same study, authors have recorded esophageal pressure of three humans in the supine, prone and lateral positions, with 0, 10 and 20 cmH₂O PEEP. They have observed heart elevation and significant esophageal pressure increase during PEEP increase. This increase was higher in the prone and lateral positions as compared to the supine position ⁷¹.

O'Quin et al. have studied in dogs the ratio between Pes and Ppl measured in the juxtacardiac space during different pulmonary and chest compliance conditions in different PEEP levels (5, 10, 15 and 20 cmH₂O), all in the supine position. Comparisons were made at the end of expiration. In all compliance conditions, Ppl variation was significantly higher as compared to Pes. This difference depended on Cp and Cc, however it was independent of PEEP. During normal Cp, 61% of applied PEEP was reflected in the pleural space at the end of expiration, while 52% was reflected by esophageal pressure. After Cp decrease, 55% and 46% PEEP were transmitted to pleural and esophageal spaces, respectively ⁷².

Smiseth et al. have studied the ratio between juxtacardiac esophageal and pleural pressures during 10, 20 and 30 cmH₂O PEEP ventilation applied to both lungs, and selectively in a single lung, in eight dogs in the supine position. They have observed that bilateral 10, 20 and 30 cmH₂O PEEP has promoted progressive and similar right and left pleural pressures increase. They concluded that the esophageal balloon does not reflect sharp regional pleural pressure increase when selective PEEP is applied to one lung, and that it consistently underestimates pleural pressures when PEEP is applied to both lungs ⁷³. It is important to stress that these authors considered absolute pressure values rather than pressure variation.

Klingstedt et al. have evaluated in eight humans in the lateral position the effect of PEEP (8 and 16 cmH₂O) solely in the dependent lung, on the compliance and resistance of the same hemithorax. They have observed that selective 8 cmH₂O PEEP increased dependent hemithorax compliance while the non dependent remained unchanged. With 16 cmH₂O PEEP, both hemithoraces compliance were virtually the same. Inspiratory resistance has also progressively decreased with selective PEEP increase, being significant with 16 cmH₂O PEEP. Esophageal pressure was measured with the balloon and authors concluded that esophageal pressure variation does not truly reflect pleural pressure variation when selective PEEP is applied in the lateral position ⁴⁵.

Jardin et al. have observed the effect of airway pressure transmission to the esophagus in 19 acute respiratory failure patients divided in three groups, with mild, moderate and severe total respiratory system compliance decrease. Observations were made at three PEEP levels: ZEEP, 10 and 20 cmH₂O. Measurement variations were checked at the end of expiration and at the end of inspiration. Pes was linearly correlated to tracheal pressure in all patients. In the group with mild Crs decrease, Pes variation was 2.73 ± 0.45%, related to Pt transmission in approximately 37%. In the group with moderate Crs decrease, Pes variation was 3.14 ± 1.04 and transmitted Pt was 32%. In the group with important Crs decrease, variation was 4.13 ± 1.55 and transmited Pt was just 24%. Airway pressure transmission for the esophagus was decreased in the presence of severe respiratory system compliance deterioration ⁴⁶.

Bonnet et al. have determined the magnitude of juxtacardiac pressure changes through esophageal, pericardial and mediastinal pleural pressures, related to 0 to 20 cmH₂O PEEP, increased in 11 patients submitted to cardiac procedures. There has been intra-thoracic pressure increase with increased PEEP levels in all patients. Changes in esophageal pressure were less important, however they presented linear correlation with pleural pressure changes ¹¹.

Rajacich et al. have studied the variation of esophageal pressure per applied PEEP (0, 5, 10, 15 and 20 cmH₂O) of patients in the postoperative period of mycardial revascularization procedures. They have observed that Pes was significantly increased at 10, 15 and 20 cmH₂O PEEP with patients in the supine position ¹⁸.

Smiseth et al. have directly measured extracardiac pleural pressure in eight patients submitted to cardiac procedures. Measurements were obtained in the expiratory period with 5, 10 and 15 cmH₂O PEEP. They have observed that PEEP has progressively increased extracardiac pleural pressure ⁷⁴.

Pelosi et al. have studied six dogs with respiratory failure caused by oleic acid under anesthesia and muscle paralysis in the supine position. They have simultaneously measured esophageal and pleural pressures in the apical and median region and in the more dependent region of the chest, after 5 and 15 cmH₂O PEEP associated to different tidal volumes (low, medium and high). They have observed that pleural surface pressure is underestimated by esophageal pressure when airway pressure is increased. They have shown that absolute Pes value is only useful as a good Ppl estimate for the medium pulmonary region, being consistently different from pulmonary dependent and non dependent region values. However there is satisfactory statistical correlation between Pes and Ppl variation in all pulmonary regions ²³. Auler Jr and Fernandes have observed that in anesthetized paralyzed patients in the postoperative period of cardiac surgeries, there was excellent correlation between esophageal and pleural pressures with intermediate PEEP levels (5 and 10 cmH₂O); however, with zero PEEP these pressures were not correspondent ⁶⁵

Further studies are needed in humans to validate the esophageal balloon in patients with pulmonary compliance changes under PEEP. Some recommendations for the use of esophageal balloon in sedated patients under mechanically controlled ventilation or under anesthesia should be followed ⁷⁵:

• Adequate balloon positioning in the medium third of the esophagus, confirmed by the "occlusion test";

• Catheter-balloon system position should be confirmed by chest X-rays, since most catheters are radiopaque;

• Balloon filling with adequate volume, obtained by balloon compliance test, aiming at recording actual pressure values obtained in the intra-esophageal space;

• It is recommended that esophageal pressure measurements in anesthetized paralyzed patients should be obtained in the supine position with PEEP above 5 cmH

  ₂O, since in this situation, in ZEEP, esophageal and pleural pressures do not correspond. The best correspondence between both pressures is achieved with 10 cmH

  ₂O PEEP.

• Adequate choice of signals monitoring equipment and high sensitivity of transducers are fundamental, in addition to gauging the equipment before each use.

Accurate lung and chest wall elastic and resistive properties calculation is only possible with accurate pleural pressure.

The esophageal balloon is the most popular method to obtain indirect pleural pressure values. The "occlusion test" is critical for adequate balloon positioning. With intermediate 5 to 10 cmH₂O PEEP levels, there is adequate correlation between Pes and Ppl variation in sedated or anesthetized patients without major pulmonary compliance impairment.

REFERENCES

Submitted for publication 7 de outubro de 2005

Accepted for publication 20 de fevereiro de 2006

* Received from Hospital Universitário Walter Cantídio, Universidade Federal do Ceará (UFC), Fortaleza, CE

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Publication Dates

• Publication in this collection
  29 May 2006
• Date of issue
  June 2006

History

• Accepted
  20 Feb 2006
• Received
  07 Oct 2005