Print version ISSN 0034-7094
Rev. Bras. Anestesiol. vol.58 no.5 Campinas Sept./Oct. 2008
Influence of tracheal gas insufflation during capnography in anesthetized patients*
Influencia de la insuflación de gas traqueal sobre la capnografía de pacientes anestesiados
Ana Carolina OrtizI; Masashi Muneshika, TSAII; Fernando Antônio Nogueira da Cruz Martins, TSAIII
Disciplina de Anestesiologia, Dor e Terapia Intensiva da EPM da UNIFESP
IIProfessor Adjunto; Doutor da Disciplina de Anestesiologia, Dor e Terapia Intensiva da EPM da UNIFESP
IIIMestre e Doutor em Ciências; Responsável pelo CET-SBA do Hospital da Beneficência Portuguesa de São Paulo/SEMESP Anestesiologia
OBJECTIVES: Tracheal gas insufflation (TGI) consists in injecting gas in
the trachea (usually oxygen). It is used in patients with acute respiratory
distress syndrome (ARDS) to reduce capnometry. In Anesthesiology, the reduction
in capnometry can be useful, but there are no studies on the reduction in capnometry
using TGI. The objective of this study was to evaluate the changes caused by
TGI on capnometry in anesthetized patients.
METHODS: Eleven patients, ages 18 to 60 years, ASA I or II, without lung diseases were evaluated prospectively. After tracheal intubation, a TGI catheter was inserted 2 to 3 cm from the carina. Patients underwent volume-controlled ventilation. The volumetric capnography curve was recorded during 20 minutes and blood was drawn to determine the PaCO2. Twenty minutes after TGI was instituted, the capnograph curve was recorded and blood was drawn once more to measure PaCO2. The end-tidal partial pressure of CO2 (PETCO2) and PaCO2 were evaluated before and after TGI. The capnography curve was observed before and during TGI.
RESULTS: PaCO2 and PETCO2 without TGI were: 33.48 ± 6.81 and 36.91 ± 6.54 mmHg (mean ± standard deviation), respectively, and after TGI, 33.85 ± 8.31 and 36.55 ± 7.93 mmHg, respectively. Parameters were not statistically different before and after TGI, both for PaCO2 and PETCO2 (p = 0.65 and 0.82). The capnography curve showed changes in alveolar air during expiration.
CONCLUSIONS: The use of TGI did not result in a reduction in PaCO2 or PETCO2, but it altered the morphology of the capnography curve.
Key Words: ANESTHESIA, General; MONITORING, CO2: capnography; VENTILATION: tracheal gas insufflation.
Y OBJETIVOS: La insuflación de gas traqueal (TGI - Tracheal Gas Insufflation)
es una técnica que consiste en inyectar gas en la tráquea (generalmente
oxígeno). Se usa en pacientes portadores del síndrome de la angustia
respiratoria del adulto para reducir la capnometría. En Anestesiología,
la reducción de la capnometría puede ser útil pero no hay
estudios sobre la reducción de la capnometría con el uso de la
TGI. El presente estudio evaluó las alteraciones proporcionadas por la
TGI sobre la capnografía en pacientes anestesiados.
MÉTODO: Se evaluaron prospectivamente 11 pacientes, entre 18 a 60 años, ASA I o II, no neumopatas. Después de la intubación traqueal fue insertado catéter para TGI a 2 o 3 cm de la carina. Los pacientes fueron sometidos a la ventilación controlada a volumen. Se registró la curva de capnografía volumétrica durante 20 minutos y se recogió una muestra de sangre para medir el PaCO2. Después de 20 minutos de TGI se registró la curva de capnografía y se recogió una nueva muestra de sangre para medir el PaCO2. Se evaluó la presión parcial de CO2 al final de la expiración (PETCO2) y PaCO2, antes y después de la TGI. Se observó una curva de capnografía, antes y durante la TGI.
RESULTADOS: La PaCO2 y PETCO2 sin TGI fueron respectivamente (media ± desvío-estándar): 33,48 ± 6,81 y 36,91 ± 6,54 mmHg y después de la TGI, 33,85 ± 8,31 y 36,55 ± 7,93 mmHg, no habiendo sido registrada diferencia estadística entre los valores antes y después de la TGI, tanto para la PaCO2 como para la PETCO2 (p = 0,65 y 0,82). La curva de capnografía presentó alteraciones en la fase de expiración del aire alveolar.
CONCLUSIONES: La aplicación de la TGI no propició la reducción de la PaCO2 ni la PETCO2, pero sí que alteró la morfología de la curva de capnografía.
Tracheal gas insufflation (TGI) is a technique associated with mechanical ventilation, which consists of injecting gas in the trachea (usually oxygen) continuously or only during a specific phase of the respiratory cycle 1-3. Usually flows of up to 5 L.min-1 are used, but the best phase of the respiratory cycle for its application is still unknown 4. The use of TGI in the expiratory phase decreases the possibility of pulmonary hyperinsufflation and here relies its applicability in situations of low pulmonary complacency 5.
Tracheal gas insufflation is used as an adjunctive method of the pulmonary ventilatory protection strategy in patients with acute respiratory distress syndrome (ARDS) since it allows the use of lower tidal volumes and respiratory rates 2,3,6,7. Besides, it can reduce the respiratory work in intubated patients with neuromuscular weakness 8. In ARDS, TGI reduced PaO2 by 13 to 17% and when associated with hypercapnia it showed a 30% reduction. Tracheal gas insufflation did not affect oxygenation 9.
The effects on PaCO2 depend on the ratio between the dead space and tidal volume (VD/VT). Minimal changes in residual volume cause huge changes in PaCO2. This implies in the use of TGI when the VD/VT ratio is elevated (permissive hypercapnia) 6,10.
During the use of TGI, one should ideally monitor PEEP, peak inspiratory pressure, airway resistance, pulmonary complacency, and tidal volume to prevent possible complications 11. Monitoring the removal of CO2 by TGI can be done by analyzing arterial blood gases or capnography. Capnometry is the measurement of exhaled CO2 at the extremity of the endotracheal tube against time. Volumetric capnography consists in demonstrating on a chart the volume of CO2 eliminated as a function of time 12.
The capnogram is divided in two segments (expiration and inspiration) and 4 phases (0, I, II, and III).
Phase 0 represents inspiration, and expiration is divided in 3 phases (I, II, and III): phase I represents the anatomical dead space, phase II the mixture of gas in the alveolar and anatomical dead spaces, and phase III the plateau of alveolar gas 13.
By using volumetric capnography, it is possible to calculate the mean expired CO2 (volume of expired CO2 divided by the expired volume). The mean concentration of expired CO2 (PETCO2) is used to calculate the dead space using Bohr-Enghoff's equation: (PACO2 PETCO2) / PACO2, where PACO2 represents the partial pressure of alveolar CO2 12,14-16.
The curve is important to identify the portion of the tidal volume that corresponds to the exhaled CO2 in the proximal airways 16. It is possible that TGI modifies PETCO2. However, so far there are no studies demonstrating which changes occur in the capnography curve, especially in anesthetized patients without restrictive or obstructive pulmonary disease undergoing mechanical ventilation.
The objective of the present study was to evaluate qualitative and quantitative changes of TGI on capnography in anesthetized patients under mechanical ventilation.
After approval by the Ethics on Research Committee of the Hospital São Paulo of the Universidade Federal de São Paulo (UNIFESP) / Escola Paulista de Medicina, 11 patients, 6 females and 5 males, with ages varying from 18 and 60 years, were evaluated prospectively.
All patients were classified as ASA I or II according to the American Society of Anesthesiologists ASA, and none had restrictive and/or obstructive pulmonary disease.
Patients underwent surgeries of the airways or extremities (limbs).
Exclusion criteria were: pregnancy, overweight (characterized as BMI > 35), pulmonary diseases, laparoscopic, thoracic or upper abdominal surgery, patients who developed intraoperative hemodynamic instability, and surgical positioning other than the horizontal dorsal decubitus.
Preoperatively: Patients included in the study underwent prior pre-anesthetic evaluation and signed an informed consent approved by the Ethics Committee. Whenever necessary patients received pre-anesthetic medication composed of 15 mg of oral midazolam thirty minutes before the surgery.
Intraoperatively: after monitoring was instituted (electrocardioscopy, non-invasive blood pressure, and pulse oximeter), a peripheral vein was catheterized with a 20G Teflon® extracath.
After oxygenation with 4 L.min-1 of 100% O2 for 3 minutes, intravenous anesthetic induction with sufentanil (0.5 to 1.0 µg.kg-1), propofol (2 mg.kg-1), and rocuronium (0.6 mg.kg-1) was initiated. Patients were intubated with a 7.0-mm (female) or 8.0-mm (male) tracheal tube. Afterwards, a special catheter for TGI was inserted 2 to 3 cm from the carina through a connector tube (Figure 1).
Patients underwent volume-controlled ventilation with a Monterey® anesthesia machine (Nikey - K. Takaoka - Brazil) with a TGI outlet.
Patients were ventilated with a VT of 6 mL.kg-1, respiratory rate (RR) of 16 bpm, PEEP of 6 cmH2O, inspiratory/expiratory ratio of 1:2, and inspired fraction of oxygen of 0.4 (air and O2). A tidal volume lower than recommended was used for greater approximation with the dead space, which improves the efficacy of the TGI. To maintain minute volume it was necessary to increase the respiratory rate.
Total intravenous anesthesia with propofol (100 to 200 µg.kg-1.min-1) and sufentanil (0.01 to 0.05 µg.kg-1.min-1), using two infusion pumps, was used for maintenance.
Initially, the data analysis program of the ventilatory mechanics and volumetric capnography Cosmo plus (Novametrix®, USA) monitor was calibrated for 10 minutes and the date and time of the connection with a notebook was recorded.
The 30-cmH2O recruitment maneuver was performed for 30 seconds. After assessing and recording the volumetric capnography curve for 20 minutes, arterial blood was drawn for the determination of the PaCO2.
Tracheal gas insufflation was initiated through a catheter with 5 L.min-1, as determined by the anesthesia device used. This flow is pre-determined by the manufacturer and it cannot be changed. After 20 minutes, the capnography curve was examined and recorded and a new blood sample was drawn to assess the levels of PaCO2.
At the end of the procedure, mechanical ventilation with a tidal volume of 7 mL.kg-1, respiratory rate of 10 bpm, and PEEP of 5 cmH2O, was reinstituted.
Parameters evaluated: end-expiratory partial pressure of CO2 (PETCO2) and PaCO2 with and without TGI were evaluated. Capnography curves were assessed before and during TGI.
Statistical analysis: paired t test was used to compare the mean levels of PETCO2 and PaCO2 before and after TGI. A p < 0.05 was considered statistically significant.
Table I shows PaCO2 and PETCO2 before and after TGI.
PaCO2 and PETCO2 did not show statistically significant differences before and after TGI.
Figure 2 shows the changes in capnography caused by TGI.
A reduction in phase III with the introduction of the tracheal gas (O2) was observed; however, in the inspiration phase (phase 0), the capnography tracing showed an elevation. The value detected for the end-expiratory partial pressure of CO2 (PETCO2) by the capnograph (mainstream type) is identified (arrow).
Signs of CO2 rebreathing were detected by capnography, and auto-PEEP by the flow versus time curve, where the flow did not pass through zero before starting the inspiratory phase during the period of mechanical ventilation with VT of 6 L.min-1 and RR of 16 bpm. Those changes affected all patients.
In the present study, no differences in PaCO2 and PETCO2 before and after TGI were observed. This probably occurred because TGI was not triggered in every cycle. It is known that TGI in the anesthesia machine used is programmed to be triggered when the flow is equal to zero (flow x time curve). However, the presence of auto-PEEP secondary to the increased respiratory rate (16 bpm) did not allow TGI to be triggered more often, hindering the interpretation of its efficacy. This could also have been responsible for rebreathing of CO2.
It is possible that, if a lower respiratory rate were used, auto-PEEP would not have occurred and, therefore, conditions would have allowed more frequent entry of tracheal gas. The choice of a higher than normal respiratory rate was due to the need to maintain ventilatory minute-volume, since the tidal volume (6 mL.kg-1) could have compromised alveolar ventilation. This conduct prevented the risk of hypoventilation.
The increase in the capnography tracing during the beginning of inspiration could be explained as follows: during inspiration, it was observed that when TGI was triggered, the capnograph recorded the entry of gas, which resulted in a decrease in phase III. However, it is possible that in the following inspiration part of the expired CO2 had accumulated in the inspiratory arm of the Y tube of the inspiratory system, causing an elevation in capnography. This could also explain the lack of change in PaCO2 and PETCO2.
On the other hand, TGI has been effectively used in patients with hypercapnia, especially in ARDS6. In the present study, patients had a normal V/Q ratio and PaCO2, characterizing a study sample with normocapnia and without pulmonary diseases. Thus, it is possible that in normal individuals TGI does not have the same efficacy as in patients with pulmonary disease, especially ARDS.
As for capnography, the changes observed on the morphology of the curve at each cycle of gas insufflation resulted in a bizarre curve and can lead to wrong conclusions regarding what is happening to CO2 elimination in anesthetized patients on mechanical ventilation and TGI. The typical notch in the curve in phase 0, caused by gas insufflation in TGI, could be mistakenly interpreted as the presence of spontaneous ventilation by the patient, similar to what happens when the effects of the neuromuscular blocker has worn off and the patient has resumed spontaneous ventilation associated with the mechanical ventilation in the presence of general anesthesia.
Under the conditions of the present study, one can conclude the TGI did not result in a decrease in PaCO2 or PETCO2, but it was capable of changing the capnography of the patients evaluated.
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Correspondence to: Submitted em 26
de dezembro de 2007 *
Received from CET-SBA da Disciplina de Anestesiologia, Dor e Terapia Intensiva
da Escola Paulista de Medicina (EPM) da Universidade Federal de São Paulo
(UNIFESP), São Paulo, SP
Dr. Fernando A. Martins
Rua Dr. Diogo de Faria, 513/121 - Vila Clementino
04037-001 São Paulo, SP
Accepted para publicação em 23 de junho de 2008
Submitted em 26
de dezembro de 2007
* Received from CET-SBA da Disciplina de Anestesiologia, Dor e Terapia Intensiva da Escola Paulista de Medicina (EPM) da Universidade Federal de São Paulo (UNIFESP), São Paulo, SP