What happens to PETCO2 when there is pneumothorax?
I have been asked this question by several colleagues. The answer, however, is not simple. It depends on the status of a pneumothorax (evolution), cause of pneumothorax, and the content of the pneumothorax (air/oxygen/nitrous oxide versus carbon dioxide). Pneumothorax can affect the PETCO2 as well as the shape of the capnograms.
If the pneumothorax is progressing towards a tension pneumothorax, then obviously the PETCO2 will decrease due to decreases in cardiac output.
If a pnemothorax is small, one may not find any difference, and therefore it is non-diagnostic. However, as the pneumothrorax increases in size, the changes in capnography depend on the tidal volume delivered which is a function of the mode of ventilation, i.e., 'pressure controlled' versus 'volume controlled'. If pressure controlled, then tidal volume will decrease, resulting in a gradual increase in the PaCO2 and PETCO2 as well (hypoventilation). However, the PETCO2 may decrease due to inadequate sampling of alveolar CO2 due to the tidal volume becoming progressively smaller. Moreover, as tension pneumothorax increases in size, thereby decreasing cardiac output, PETCO2 decreases. A volume controlled ventilation is more likely to increase intra-thoracic pressure with increasing pnuemothorax which imposes a mechanical impedance to the circulation and results in a decrease in PETCO2.1
If the cause of pneumothorax is due to an obstruction to exhalation, an obstructive capnogram (prolonged phase II and increased slope of Phase III) with increasing PETCO2 can be observed. In a case report by Smith et al,1 a normal capnogram changed to an obstructive picture in 45 minutes, following the induction of general anesthesia and IPPV. Ventilation was increased to offset increasing PETCO2. Ten minutes later, circulatory collapse occurred with decreasing end-tidal PCO2 to 15 mm Hg. Examination of the chest revealed bilateral tension pneumothorax. Although capnography was being used continuously, the altered CO2 waveform that occured went unrecognized. The authors retrospectively felt that important diagnostic clue could have led the authors to search for and correct the cause of expiratory obstruction very early in the evolution of this event. The cause of the pneumothorax was a defective bacterial filter of the breathing circuit.
An obstructive pattern on a capnogram has been reported when a patient developed pneumothorax during laparoscopic Nissen fundoplication.2 There was also an increase in peak inspiratory pressures and wheezing was also noted on ascultation. Several puffs of albuterol nebulizations were administered which resulted in a cessation of the wheezing. However, the obstructive pattern on the capnogram persisted. There was a gradual decrease in the oxygen saturation towards the conclusion of the surgery. A postoperative X-ray revealed a 100% left-sided pneumothorax.
The obstructive pattern on the capnogram is probably due to compression of the airways by the pneumothorax.
Reports of characteristic changes of the descending limb can be found in the literature. A staircase effect on the descending limb of the capnogram is seen in the presence of a pneumothorax in neonates.3,4 When chest tubes are correctly positioned, a staircase effect may indicate chest tube occlusion.3
Carbon dioxide pneumothorax:Capnopneumothorax
When pneumothorax occurs as a complication of CO2 induced pneumoperitoneum, it results in an increase in the PETCO2.5-9 This could be an early warning sign of CO2 pnuemothorax when associated with increases in the peak inspiratory pressure. In one study, PETCO2 and PaCO2 increased in all patients who developed CO2 pneumothorax, and ventilation was increased to offset increases in PETCO2.5 However, no changes in CO2 waveform were observed in this study. In a retrospective of 968 laparoscopic surgical cases, PETCO2 greater than 50 mm Hg and operative times greater than 200 minutes were predictors of the development of pneumothorax and/or pneumomediastinum.9
If pneumothorax is not diagnosed and it progresses into a tension pneumothorax, then it may result in a decrease in PETCO2 secondary to circulatory collapse.
1. Smith EC, Otworth JR, Kaluszyk, P. Bilateral tension pneumothorax due to a defective anesthesia breathing circuit filter. J Clin Anesth 1991;3:229-234.
2. Manger D, Kirchhoff GT, Leal JJ, Laborde R, Fu E. Pneumothorax during laparoscopic Nissen fundoplication.
3. Smalhout B, Kalenda Z. An Atlas of Capnography. Kerckebosche Zeist.The Neterhlands. 2n ed. 1981:163.
4. Curley MAQ, Thompson JE. End-tidal CO2 monitoring in critically ill infants and children. Pediatric Nursing 1990;16;397-403.
5. Joris JL, Chiche JD,Lamy ML. Pneumothorax during laproscopic fundoplication: diagnosis and treatment with positive end-expiratory pressure. Anesth Analg 1995;81:993-1000.
6. Perke G, Fernandez A. Subcutaneous emphysema and pneumothorax during laparoscopy for ectopic pregnancy removal. Acta Annesthesiol Scan 1997;41(6):792-4.
7. Peden CJ, Prys-Roberts C. Capnothorax: implications for the anaesthetist. Anaesthesia 1993;48:664-6.
8. Chui PT, Gin T, Chung SC. Subcutaneous emphysema, pneumomediastinum and pneumothorax complicating laparoscopic vagotomy. Anaesthesia 1993;48(11):978-81.
9. Murdock CM,Wolff AJ, Van GeemT. Risk factors for hypercarbia, subcutaneous emphysema, pneumothorax, and pneumomediastinum during laparoscopy. Obstet Gynecol 2000;95:704-9.