PETCO2 as an estimate of PaC02

Clinical Aspects

Bhavani Shankar Kodali MD

( I ) PETCO2 as an estimate of PaC02

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 It can vary from patient to patient and is dependent on several factors. It increases with age, pulmonary disorders (emphysema), pulmonary embolism, decreasing cardiac output, hypovolemia and anesthesia itself.4,6,-8 It decreases with large tidal volumes and low frequency ventilation.2 In pregnant subjects, as well as in infants and smaller children, the (a-ET)PC022 is lower than in non-pregnant adults and PETCO2 reflects PaC02.3,9,10 Changes in PETCO2 can often be regarded as indicative of changes in PaC02. 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. However, the variations in (a-ET)PC02 during major surgery may be of the same magnitude as the inter-individual variations and caution must be used in the precise prediction of PaC02 from PETCO2 measurements. Several factors such as changes in body position, temperature and pulmonary blood flow, as well as mechanical ventilation and cardiopulmonary bypass, can result in changes in the ventilation perfusion status of the lungs. This in turn alters the alveolar dead space fraction and the slope of phase III, and this affects (a-ET)PC02. Further, there is no consistent correlation between (a-ET)PC02 and the various factors mentioned .11

In steady state, PETCO2 may reflect PaCO2 if alveolar dead space does not change appreciably
 
steadystate

 

PETCO2 may reflect PaCO2 at a new steady state level if (a-ET)CO2 is determined via ABG
 
steadydeadspace

 
If hemodynamics are unsteady, PETCO2 may not reflect PaCO2
unsteadydeadspace


Critically ill neonates:
 
In neonates with mild to moderate lung disease (FIO2 < 0.3 and respiratory frequency < 70/min), the distal sampling of CO2 measurements are preferred to proximal measurements as the former reflect PaCO2 more accurately than the later.12,13 However, in children with severe lung disease even the distal PETCO2 may not be good predictor of PaCO2 because (a-ET)PCO2 gradients varies with changing V/Q relationship of the sick neonate thus making PETCO2 measurements less reliable.14 Under these circumstances, PtcCO2 is more accurate estimate of PaCO2.13 The emphasis here is on more ABG's until V/Q mismatch improves and a more constant (a-ET)PCO2 relationship is established.


Cyanotic heart diseases:

In infants and children with acyanotic heart disease (left to right shunt), PETCO2 is closer to PaCO2 and (a-ET)PCO2 gradient is not significantly different from children with normal circulation.15,16 Further PETCO2 is a reliable estimate of PaCO2.16 However in children with cyanotic heart diseases, PETCO2 underestimates PaCO2 and the (a-ET)PCO2 gradient is increased up to 15 mmHg due a combination of venous admixture and low pulmonary perfusion.17 Under these circumstances, (a-ET)PCO2 is linearly correlated with arterial oxygen saturation (SPO2).17 With a decrease in SPO2 by 10% caused by right to left shunt, the (a-ET)PCO2 gradient can be expected to increase by 3 mm Hg.17


Click here to understand more about the physiology of relationship between (a-ET)PCO2 and alveolar dead space

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. Leigh MD, Jones JC, Motley HL. The expired carbon dioxide as a continuous guide of the pulmonary and circulatory systems during anesthesia and surgery. J Thorac Cardiovasc Surg 1961;41:597-610.

7. Hoffbrand BI. The expiratory capnogram: a measure of ventilation-perfusion inequalities. Thorax 1966;21:518-23.

8. Tulou PP, Walsh PM. Measurement of alveolar carbon dioxide at maximal expiration as an estimate of arterial carbon dioxide tension in patients with airway obstruction. Am Rev Respir Dis 1970;102:921-6.

9. Rich GF, Sconzo JM. Continuous end-tidal CO2 sampling within the proximal endotracheal tube estimates arterial CO2 tension in infants. Can J Anaesth 1991;38:201-3.

10. Shankar KB, Moseley H, Kumar Y, Vemula V, Krishnan A. The arterial to end-tidal carbon dioxide tension difference during anesthesia for tubal ligation. Anaesthesia 1987;42.482-6.

11. Russell GB, Graybeal JM, Strout JC. Stability of arterial to end-tidal carbon dioxide gradients during postoperative cardiorespiratory support. Can J Anaesh 1990;37:560-6.

12 Kirpalani H, Kechagias S, Lerman J. Technical and clinical aspects of capnography in neonates. Journal of Medical Engineering and Technolology 1991;15:154-61.

13 Mcevady BAB, Mcleod ME, Mulera M, Kirpalani H, Lerman J. End-tidal, transcutaneous, and arterial PCO2 measurements in critically ill neonates. A comparitive study. Anesthesiology 1988;69:112-16.

14 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.

15. Fletcher R. Relationship between alveolar deadspace and arterial oxygenation in children with congenital cardiac disease. Br J Anaesth 1989;62:168-76.

16, Fletcher R. Invasive and noninvasive measurement of the respiratory deadspace in anesthetized children with cardiac disease. Anesth Analg 1988;67:442-7.

17. Fletcher R. The relationship between the arterial and end-tidal PCO2 difference and hemoglobin saturation in patients with congenital heart disease. Anesthesiology 1991;75:210-6.
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