Clinical Aspects

Monitoring PaCO₂

Measurements of PETCO₂ constitute a useful non-invasive tool to monitor PaC02 and hence the ventilatory status of patients during anesthesia or in the intensive care unit. In normal individuals, the (a-ET)PC02 may vary from 2-5 mmHg.1-5 The PETCO₂ is even more useful if its relationship to PaC02 can be established initially by blood gas analysis. Thereafter, changes in PaC02 may be assumed to occur in parallel with those in PETCO₂ thus avoiding repeated arterial puncture provided there are no major hemodynamic changes or respiratory abnormalities that may alter alveolar dead space and hence, (a-ET)PC02. (For details -Physiology section)

Monitoring alveolar dead space

The (a-ET)PCO₂ is a measure of alveolar dead space, and changes in alveolar dead space correlate well with changes in (a-ET)PCO₂.1 An increase in (a-ET)PCO₂ suggests an increase in dead space ventilation. Hence (a-ET)PCO₂ is an indirect estimate of V/Q mismatching of the lung.

Monitoring clinical progress of a critical patient

In patients with severe lung disease or hemodynamic instability, the PETCO₂ may not be good predictor of PaCO₂ because (a-ET)PCO₂ gradients vary with the changing V/Q relationship of the lungs, thus making PETCO₂ measurements less reliable.6 The emphasis here is on more ABG’s until the V/Q mismatch improves and a more constant (a-ET)PCO₂ relationship is established. Establishment of constant (a-ET)CO₂ implies a good improvement in the V/Q status of the patient.

References:

1. Nunn JF, Hill DW. Respiratory dead space and arterial to end-tidal CO₂ tension difference in anesthetized man. J Appl Physiol 1960;15:383-9.

2. Fletcher R, Jonson B. Deadspace and the single breath test for carbon dioxide during anaesthesia and artificial ventilation. Br J Anaeasth 1984;56:109-19.

3. Shankar KB, Moseley H, Kumar Y, Vemula V. Arterial to end-tidal carbon dioxide tension difference during cesarean section anaesthesia. Anaesthesia 1986;41:698-702.

4. Askrog V. Changes in (a-A)CO₂ difference and pulmonary artery pressure in anesthetized man. J Appl Physiol 1966;21:1299-1305.

5. Bhavani Shankar K, Moseley H, Kumar AY, Delph Y. Capnometry and Anaesthesia. Canadian J Anaesth 1992;39:6:617-32.

6. Phan CQ, Tremper KK, Lee SE, Barker SJ. Noninvasive monitoring of carbon dioxide: A comparison of the partial pressure of transcutaneous and end-tidal carbon dioxide with the partial pressure of arterial carbon dioxide. J Clin Monit 1987;3:149-54.