(a-ET)PCO₂ gradients or differences- Alveolar dead space

There are three important applications of (a-ET)CO2 differences. 

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.