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Revista Brasileira de Anestesiologia

Print version ISSN 0034-7094

Rev. Bras. Anestesiol. vol.51 no.4 Campinas  2001

http://dx.doi.org/10.1590/S0034-70942001000400010 

MISCELLANEOUS

 

Controlled pressure mechanical ventilation with anesthesia closed system for low weight patients: experimental study*

 

Emprego da ventilação mecânica com pressão controlada em circuito circular de anestesia para pacientes de baixo peso: estudo experimental

 

Empleo de la ventilación mecánica con presión controlada en circuito circular de anestesia para pacientes de bajo peso: estudio experimental

 

 

Denise Tabacchi Fantoni, M.D.I; Sandra Mastrocinque, M.D.II; Silvia Renata Gaido Cortopassi, M.D.III; Elton R. Migliatti, M.D.II; José Otávio Costa Auler Junior, TSA, M.D.IV

IMédica Veterinária, Professora Livre Docente, FMVZ/USP
IIMédico Veterinário, Aluno de Mestrado em Cirurgia, FMVZ/USP
IIIMédica Veterinária, Professora Doutora, FMVZ/USP
IVProfessor Titular da Disciplina de Anestesiologia, FM/USP

Correspondence

 

 


SUMMARY

BACKGROUND AND OBJECTIVES: Pediatric low flow anesthesia requires adequate equipment which, when available, is extremely expensive, thus seldom used. This study aimed at evaluating low flow anesthesia in rabbits using a closed rebreathing circuit in a new pediatric pressure controlled ventilator for anesthesia.
METHODS: Ten rabbits were randomly assigned to two groups. Group I individuals were ventilated with the airway pressure limit set to 15 cmH2O, while in group II the setting was 20 cm H2O. Anesthesia was induced with muscular xylazine (10 mg.kg-1) and ketamine (25 mg.kg-1), followed by maintenance with isoflurane after tracheal intubation. After 20 minutes, 0.1 mg.kg-1 intravenous pancuronium was administered and controlled ventilation was established. Ventilator parameters were: RR - 30 mpm, I:E ratio 1:2.5 and inspiratory time 0.6 sec, in addition to plateau pressures. Fresh gas flow was 300 ml.min-1 (total). Parameters were collected every 20 minutes for one hour and data were submitted to analysis of variance for repeated measures (p < 0.05).
RESULTS: Re-inhaled CO2 decreased significantly in group II from an initial value of 15 mmHg during spontaneous ventilation to a mean value of 2.4 mmHg during controlled ventilation. In group I, the drop was from 19.2 mmHg (initial) to 3.6 mmHg. Comparing both groups, significant differences were observed in venous pH, PaCO2, PvO2 and a slight difference between MBP and DBP. The 15 cmH2O group showed important respiratory acidosis, while the 20 cmH2O had normal pH and PaCO2 values. Since expired volume values were similar in both groups, such differences in pH and blood gases observed could be related to low pH levels seen in group I. Each animal consumed a mean value of 2 ml isoflurane during the 120 minutes of the study.
CONCLUSIONS: With proper equipment, it is possible to use low flow anesthesia with pressure controlled ventilation and closed system in very low weight patients.

Key words: ANIMAL: rabbits; EQUIPMENTS: respiratory system, closed circuit; VENTILATION: mechanical


RESUMO

JUSTIFICATIVA E OBJETIVOS: A anestesia com baixo fluxo, em pacientes pediátricos, requer equipamentos adequados, no entanto, os disponíveis no mercado são de alto custo, o que limita o seu uso. Este estudo avaliou a anestesia com baixo fluxo em coelhos, empregando circuito fechado, modo de pressão controlada em novo ventilador pediátrico para anestesia (VPL-5000A-Vent-Logos).
MÉTODO: Dez coelhos foram distribuídos aleatoriamente, sendo que o grupo I foi submetido à pressão de 15 cmH2O e o grupo II à de 20 cmH2O. A anestesia foi realizada com xilazina (10 mg.kg-1) e cetamina (25 mg.kg-1) associados, por via muscular, seguida de manutenção com isoflurano, após intubação orotraqueal. Após 20 minutos, administrou-se pancurônio (0,1 mg.kg-1) por via venosa e a ventilação controlada foi iniciada. Os parâmetros ajustados no ventilador foram: FR 30 mpm, freqüência I:E 1:2,5 e tempo de inspiração 0,6 segundo, além das pressões de plateau. O fluxo de gases frescos empregado foi 300 ml (total). Os parâmetros foram coletados a cada 20 minutos durante uma hora. Os dados obtidos foram submetidos à análise estatística de variância para medidas repetidas (p < 0,05).
RESULTADOS: O CO2 reinalado diminuiu significativamente no grupo II, de 15 mmHg, durante a ventilação espontânea, para um valor médio de 2,4 mmHg, durante a ventilação controlada. No grupo I, diminuiu de 19,2 mmHg (inicial) para 3,6 mmHg. Comparando-se os dois grupos, diferenças significativas foram encontradas em relação ao pH venoso, PaCO2, PvO2 e discreta diferença entre a PAM e PAD. O grupo de 15 cmH2O apresentou importante acidose respiratória, enquanto o de 20 cmH2O obteve valores normais de pH e PaCO2. Uma vez que os valores de volume expirado entre os grupos foram semelhantes, tais diferenças entre pH e gases sangüíneos apresentados pelos grupos podem estar relacionadas aos baixos níveis de pH observados no grupo I. Verificou-se consumo médio de 2 ml de isoflurano por animal durante os 120 minutos de estudo.
CONCLUSÕES: Com equipamento adequado é possível empregar anestesia de baixo fluxo, ventilação com pressão controlada e circuito fechado em pacientes com peso muito baixo.

Unitermos: ANIMAL: coelho; EQUIPAMENTOS: sistema respiratório, circuito fechado; VENTILAÇÃO: mecânica


RESUMEN

JUSTIFICATIVA Y OBJETIVOS: La anestesia con bajo flujo, en pacientes pediátricos, requiere equipamientos adecuados siendo caros los disponibles, y limitando su uso. Este estudio evaluó la anestesia con bajo flujo en conejos, empleando circuito cerrado, modo de presión controlada en nuevo ventilador pediátrico para anestesia (VPL-5000A-Vent-Logos).
MÉTODO: Diez conejos fueron distribuidos aleatoriamente, siendo que el grupo I fue sometido a presión de 15 cmH2O y el grupo II a la de 20 cmH2O. La anestesia fue realizada con xilazina (10 mg.kg-1) y cetamina (25 mg.kg-1) asociados, por vía muscular, seguida de manutención con isoflurano, después intubación orotraqueal. Después de 20 minutos, se administró pancuronio (0,1 mg.kg-1) por vía venosa y la ventilación controlada fue iniciada. Los parámetros ajustados y el ventilador fueron: FR 30 mpm, frecuencia I:E 1:2,5 y tiempo de inspiración 0,6 segundo, además de las presiones de plateau. El flujo de gases frescos empleado fue de 300 ml (total). Los parámetros fueron colectados a cada 20 minutos durante una hora. Los datos obtenidos fueron sometidos a análisis estadística de variancia para medidas repetidas (p < 0,05).
RESULTADOS: El CO2 reinhalado diminuyó significativamente en el grupo II, de 15 mmHg, durante la ventilación espontanea, para un valor medio de 2,4 mmHg durante a ventilación controlada. En el grupo I, de 19,2 mmHg (inicial) para 3,6 mmHg. Comparándose los dos grupos, diferencias significativas fueron encontradas en relación al pH venoso, PaCO2, PvO2 y discreta diferencia entre la PAM y PAD. El grupo de 15 cmH2O presentó importante acidosis respiratoria, en cuanto el de 20 cmH2O obtuvo valores normales de pH y PaCO2. Una vez que los valores de volumen expirado entre los grupos fueron semejantes, tales diferencias entre pH y gases sangüíneos presentados por los grupos pueden estar relacionadas a los bajos niveles de pH observados en el grupo I. Se Verificó consumo medio de 2 ml de isoflurano por animal durante los 120 minutos de estudio.
CONCLUSIONES: Con equipamiento adecuado es posible emplear anestesia de bajo flujo, ventilación con presión controlada y circuito cerrado en pacientes con muy bajo peso.


 

 

INTRODUCTION

There has been an increasing interest in the use of closed circuits in children in the last years. Such circuits associated to high precision ventilators allow for the use of lower fresh gas flows, better moisture and heat preservation and clearly less anesthetic consumption and operating room pollution1. An additional benefit is the possibility of using different ventilatory modalities and resources, such as positive end expiratory pressure (PEEP) available in such equipment, which are certainly needed for high risk patients. However, in our country, pediatric anesthesia has been a challenge for the anesthesiologist due to the lack of adequate equipment. In many centers, non-rebreathing circuits such as Bain’s or Mapleson D systems are still widely use in pediatrics with manually controlled ventilation. These circuits use high fresh gas flows leading to high anesthetic consumption and environmental pollution. Another great disadvantage of such practice is the lack of accurate parameters monitoring, such as expired volume and airway pressure, among others.

A major drawback for the use of adult ventilators and anesthesia closed circuits, which could also be used for low weight patients, is their high cost. The possibility of low flow anesthesia in children would be another great advantage of this new equipment available in the market.

This study aimed at evaluating a closed circuit anesthesia machine using a pressure controlled time-cycled ventilatory mode with low fresh gas flow. Small animals (rabbits) were used to check the efficacy of this ventilator during anesthesia.

 

METHODS

Ten male and female rabbits weighing 3.5 to 5 kg were used. Anesthesia was induced with muscular xylazine (10 mg.kg-1) and ketamine (25 mg.kg-1), followed by isoflurane under mask. Tracheal intubation was performed with Magill’s cuffed tubes (2.5 to 3.0 of internal diameter). Anesthesia was maintained with isoflurane in 100% oxygen. Animals remained under spontaneous ventilation for 20 minutes until stabilization of the anesthesia machine parameters. Marginal artery and ear vein were catheterized for blood pressure monitoring as well as fluid (2 ml.kg.h-1 lactated Ringer’s) and drugs administration, respectively. Pancuronium (0.1 mg.kg-1) was then administered and time-cycled pressure controlled ventilation was installed for 60 minutes. Ventilator parameters were: respiratory rate of 30 movements per minute, I:E ratio of 1:2.5 and inspiratory time of 0.6 seconds.

Fresh gas flow was 300 ml.min-1 delivered by a non pressure gauged flowmeter. Animals were then randomly distributed in two groups of five animals each. The ventilatory peak pressure was limited to 15 cmH2O in Group I (G15) and 20 cmH2O in Group II (G20). Ventilator gas flow was adjusted to generate a square pressure curve, compatible with the desired modality.

To evaluate adequate ventilation, expired air carbon dioxide concentration, FiO2 and hemoglobin peripheral oxygen saturation were continuously registered. Serial pH, blood gases and plasma bicarbonate concentration were also measured.

Using a ventilation monitor, inspired and expired volume, minute volume, peak, mean and plateau pressures, as well as pulmonary compliance were measured too. This monitor has assured respiratory modality through the visualization of pressure curves. To monitor anesthesia, inspired and expired isoflurane concentration, electrocardiogram and blood pressure were continuously evaluated.

A pneumatic time-cycled system with inspiratory and expiratory phase control was used for this study with the following adjustable controls: flow, inspiratory time, expiratory time, maximum pressure and PEEP, in addition to an alarm detecting disconnection or lack of cycle.

The system is made up of four sub-sets (Figure 1):

1. One time cycling subset with pressure limit;

2. One “Bag in a Bottle” system;

3. One subset made up of a combination of valves which allow the device to operate in controlled (manual or mechanical) or spontaneous ventilation, and;

4. One CO2 absorber reservoir.

During the expiratory phase, a sub-atmospheric pressure acts between the internal reservoir bag (7) and the dome (8), allowing it to be filled with the gaseous mixture after going through the unidirectional valve (1). Another unidirectional valve (2) prevents the expired volume to return to the bag (7).

During the inspiratory phase, a gas volume (generated by the flow control during inspiratory time) compresses the internal reservoir bag (7). The speed of this compression is a function of the flow set, which should mimic a manual compression.

The unidirectional valve (1) closes and the valve (2) opens, allowing the bag’s content (7) to reach the patient.

An expiratory valve (3) holds the tidal volume until the previously set airway pressure (maximum inspiratory pressure) is reached.

From this point on, the volume in excess escapes by opening and overcoming the unidirectional valve (4), and may go:

a) to the external reservoir bag (6);

b) to the atmosphere or anti-pollution system through the pop-off valve (5), if the external reservoir bag (6) is full;

c) directly to the atmosphere if the system is open.

An additional gas flow (AGF) placed between valves (1) and (2) allows the second valve to be kept open, regardless of AGF and provided the internal reservoir bag (7) is full. This allows the small patient to ventilate without resistance. During the expiratory phase, the internal reservoir bag (7) is filled with the gaseous mixture coming from the external reservoir bag (6), as well as from the volume expired by the patient (after going through the CO2 absorber). If the system in use has no CO2 absorber, atmospheric air is then aspirated.

The valve (9) connects the external wall of the internal reservoir bag (7) to the atmosphere, and this happens with the equipment off (spontaneous and manually controlled ventilation). This valve also acts as a safety valve for the “Bag in a Bottle” system by preventing the pressure at this point to go beyond 80 cm H2O.

The expiratory branch was chosen to accurately measure mouth pressure, what was done before the valve (3). This measurement doesn’t reflect the resistive pressure of flow going through the inspiratory way.

The configuration may be converted to a non CO2 absorbing system when one or both corrugated tubes (10 and 11), which connect the ventilator to the absorbing canister, are disconnected. Such procedure may be performed when a faster emergence is desired.

The equipment allows intermittent mandatory ventilation, provided that expiratory time be increased.

Results obtained were analyzed by a computer system (INSTAT), being data submitted to ANOVA analysis of variance and Student’s t test to compare means of both groups. Significance level of 5% was established.

 

RESULTS

Results are shown in table I, table II and table III. There were no statistically significant differences between both experimental groups as to control and spontaneous ventilation moments.

During spontaneous ventilation, high inspired and expired CO2 values were observed in both groups. With controlled ventilation, there has been a ventilation increment, confirmed by PETCO2 normalization. Inspired CO2 decreased and remained in values compatible with an adequate ventilation.

Airway pressures, inspired and expired volumes suffered significant increases during controlled ventilation as compared to spontaneous ventilation.

When comparing both experimental groups, the only significant differences were seen with PaCO2, PvO2, PvCO2 and venous pH. The group ventilated with 20 cmH2O had lower PaCO2 (figure 2) and higher PvO2 and pH values. As to heart rate and blood pressure, there was no significant difference between groups or between evaluations performed before and after controlled ventilation.

During controlled ventilation, isoflurane consumption was 2 ml per animal.

After 90 minutes of anesthesia, isoflurane was withdrawn and animals were extubated uneventfully.

 

DISCUSSION

The lack of adequate and reliable anesthesia ventilation equipment has historically limited the use of pediatric low flow techniques in our country. This concept is also applied to veterinary anesthesia in small animals as well to “latu sensu” pediatric anesthesia. There are several definitions to low flow anesthesia. One of them refers to the use of 0.5 to 1 L.min-1 of fresh gas flow, while minimum flow implies the use of values below 0.5 L.min-1 1. There is no distinction regarding body weight. Recently, Tobin et al.2 have evaluated the efficacy of a closed system with adult bellows to provide minute volume to pediatric test lung. Authors have shown that when an adult closed system is used in children, ventilation depends basically on respiratory rate, inspiratory peak pressure and lung compliance rather than the ventilation mode, suggesting that it is feasible to use an adult closed system for pediatric patients. Igarashi et al.3 have also shown that the use of a closed low flow anesthesia system (600 ml.min-1) is very feasible in children.

In our study, the pressure controlled closed system allowed adequate ventilation and oxygenation of low weight individuals without the inconvenience of high fresh gas flows and with low consumption of inhalational anesthetics.

The ventilation modality provided high and adequate expired volumes (EV), even with narrow tracheal tubes and constant respiratory rate of 30 movements per minute. Tobin et al.2, in an experimental study, have observed that in a normal compliance lung, tube diameter, respiratory rate and PEEP values are limiting factors to obtain adequate expired volumes. In our study, however, despite PEEP values variations with the use of a 3.5 tube and a respiratory rate of 30 movements per minute, adequate EV volumes were reached, confirmed by ventilation parameters seen throughout the study. In ventilation, especially pediatric one, resistive forces may impair alveolar ventilation, especially if associated to sudden compliance variations. Animals ventilated with 20 cm H2O have clearly shown such fact.

By concept, lung ventilation in closed system anesthesia depends on the driving force generated by the “ventilator”, and the hole of gas flow is to supply the anesthetic agent and oxygen to tissue needs.

According to current concepts of low fresh gas flow anesthesia, our study was performed with minimum flow anesthesia 1,4,5, since we used 300 ml.min-1 of oxygen flow. To date, in spite of initial calculations in which the choice of gas flow used in low flow anesthesia is a function of oxygen consumption per kilogram, such definition does not take into account patient’s weight. Igarashi et al.3 have used 600 ml.min-1 of fresh gases in pediatric patients with mean weight of 16.4 kg to study low flow anesthesia with sevoflurane. Perkins et al.6 have used 800 ml.min-1 in children with mean weight of 15 kg.

There are several advantages in the use of low fresh gas flows: decreased operating room pollution, better moisture and temperature maintenance of inspired gases and lower anesthetic consumption1,4,5.

Baxter 5 has reported a 25% anesthesia costs decrease with fresh gas flow reduction to at 1 L.min-1. Perkins et al. 6 have observed an approximately 58% decrease in isoflurane consumption while Igarashi et al. 3 have noticed that sevoflurane consumption was 1/7 of the total used during anesthesias with conventional oxygen flows (6 L.min-1). In our study, isoflurane consumption was 2 ml for one-hour anesthesia.

According to Eger 7, the increasing use of the new generation inhalational anesthetics, sevoflurane and desflurane, shall increase even more the use of low flow techniques. For having a low solubility, such anesthetics are absorbed in smaller amounts. Also, due to their lower potency, high partial pressures in the anesthesia circuit are needed. With high fresh gas flows, most exhaled gas is wasted, and a great amount of anesthetics has to be vaporized and delivered into the anesthesia circuit to re-establish high partial pressures needed to maintain an adequate anesthesia depth 1,8. For all these reasons, the only way to optimize the use of the new agents is to use a lower fresh gas flow.

In most studies dealing with low flow anesthesia in children, anesthesia machines are highly sophisticated, what must be related to greater hospital investments. Our study has evaluated a new equipment which has shown to be very safe and easy to use. Spending less with equipment and using less anesthetic drugs, it is possible to invest in the purchase of ventilation and anesthetic gases monitoring devices, mandatory for low fresh gas flow anesthesia.

In conclusion, in our study, the use of the ventilator (VLL-5000) coupled to conventional flowmeters and vaporizers was efficient and safe in gaseous exchange, maintaining an adequate anesthesia depth in the species under investigation and allowing the use of low gas flows.

 

ACKNOWLEDGEMENTS

The authors thank Dr. Humberto do Val for lending the equipment for the experiment, as well as for supplying technical information on it.

 

REFERENCES

01. Baum JA - Low-flow anaesthesia. Eur J Anaesthesiol, 1996;13:432-435.         [ Links ]

02. Tobin MJ, Stevenson GW, Horn BJ e al - A comparison of three modes of ventilation with the use of an adult circle system in an infant lung model. Anesth Analg, 1998;87:766-771.         [ Links ]

03. Igarashi M, Watanabe H, Iwasaki H et al - Clinical evaluation of low-flow sevoflurane anaesthesia for paediatric patients. Acta Anaesthesiol Scand, 1999;43:19-23.         [ Links ]

04. Baker AB - Low flow and closed circuits. Anaesth Intensive Care, 1994;22:341-342.         [ Links ]

05. Baxter AD - Low and minimal flow inhalational anaesthesia. Can J Anaesth, 1997;44:643-653.         [ Links ]

06. Perkins R, Meakin G - Economics of low-flow anaesthesia in children. Anaesthesia, 1996;51:1089-1092.         [ Links ]

07. Eger EI - Economic analysis and pharmaceutical policy: a consideration of the economics of the use of desfluorane. Anaesthesia, 1995;50:45-48.         [ Links ]

08. Auler Jr JOC, Carmona MJ, Barbas CV et al - The effects of positive end-expiratory pressure on respiratory system mechanics and hemodynamics in postoperative cardiac surgery patients. Braz J Med Biol Res, 2000;33:31-42.         [ Links ]

 

 

Mail to:
Dr. José Otávio Costa Auler Junior
Address: Rua Guarará, 538 Apto 151 Jardim Paulista
ZIP: 01425-000 City: São Paulo, Brazil

Submitted for publication September 29, 2000
Accepted for publication March 2, 2001

 

 

* Received from Laboratório de Anestesiologia da Faculdade de Medicina Veterinária e Zootecnia da Universidade de São Paulo