Physiology of capnography
A terminal upswing at the end of phase III
Occurs in obese patients and in pregnant subjects where thoracic compliance is reduced
Fast alveoli empty fast initially and then slow down. Most of the expirate leaves in the first second at almost constant PCO2. Towards the end of expiration, there is only a small amount of expired gas flow thereby increasing PCO2 steeply.
The combined result of these two emptying patterns is as follows
Based on the lung model studies, it has been suggested that, in obese subjects with healthy lungs, there may be two lung compartments with different mechanical and V/Q properties (fast and slow compartments).1,5 Lung model studies were also used to study the alveolar CO2 concentrations from fast and slow compartments during passive expiration where the expiratory flow patterns resembles a "die away" exponential function.1,5 The figure above shows alveolar PCO2 curves obtained from fast and slow compartments.
From the fast alveolar compartments, there is a rapid initial emptying of gases. The high initial expiratory flow rate implies that most of the gases leaving the alveoli has a rather constant FCO2 and is responsible for the near-horizontal initial part of phase III in CO2 trace. However, as the expiratory flow decreases towards the end of expiration, the CO2 content of the expired air increases markedly, producing a terminal steep rise or a ‘knee’ in the FCO2 tracing. This is because, in the later part of expiration, the delayed alveolar emptying results in higher FCO2 due to the continuous release of CO2 into the alveoli. Normally, the alveolar gases with high CO2 may remain within the airways (anatomical dead space) and are not analyzed by the CO2 sensor at the mouth. However, the use of large tidal volumes and low-frequency ventilation enables these gases to reach the mouth where the CO2 sensor registers high FCO2.
The slow compartments empty at a more even pace producing a positive slope from the beginning of phase III, as more time is allowed for CO2 to accumulate in the alveoli. As the expiratory flow rate decreases towards the end of expiration, the slope of phase III increases steadily.
The combined effect of the emptying characteristics of different alveoli determines the shape of the alveolar plateau in an capnogram during IPPV.
The low total thoracic compliance of obese patients is associated with rapid initial emptying of the lungs and phase III of the resulting capnogram predominantly resembles the one produced by the fast compartment: a near-horizontal phase III, or phase III with minimal slope and a terminal upswing (phase IV) similar to phase IV of the SBTN2 curve.
Pregnant subjects resemble obese subjects in some features, namely, reduced functional residual capacity (FRC) and low total thoracic compliance. Additionally, CO2 production is increased in pregnancy. Hence, phase IV may also occur in pregnant subjects during general anesthesia/IPPV with large tidal volumes.4,6-8 The presence of phase IV also contributes to the negative (a-ET)PCO2 values, where end-tidal PCO2 may exceed paCO2, which may be observed in obese and pregnant subjects.1,7-9
1. Fletcher R. The single breath test for carbon dioxide. Thesis. Lund. Sweden 1980.
2. Moon RE. Camporesi EM. Respiratory monitoring. In Miller RD, ed.Anesthesia, ed 5. New York: Churchill Livingstone, 2000:1255-95.
3. Nunn JF. Applied Physiology, ed 3. London: Butterworths, 1989.
4. Bhavani-Shankar K, Kumar AY, Moseley HSL, Hallsworth RA. Terminology and the current limitations of time capnography: A brief review. J Clin Monit 1995;11:175-82.
5. Fletcher R, Jonson B, Cumming G, Brew J. The concept of dead space with special reference to the single breath test for CO2. Br J Anaesth 1981;53:77-88.
6. Fletcher R. Arterial to end-tidal CO2 tension differences. Anaesthesia 1987;42:210-11.
7 Shankar KB, Moseley H, Kumar Y, et al. The arterial to end-tidal carbon dioxide tension difference during anaesthesia for tubal ligations. Anaesthesia 1987;42:482-6.
8. Bhavani-Shankar K, Moseley H, Kumar AY. Delph Y. Capnometry and anaesthesia. Can J Anaesth 1992;39:6:617-32.
9. Shankar KB, Moseley H, Kumar Y, et al. The arterial to end-tidal carbon dioxide tension difference during cesarean section anaesthesia. Anaesthesia 1986;41:698-702.