The relationship between the arterial to end-tidal P(CO2) difference and hemoglobin saturation in patients with congenital heart disease

Abstract

In right-to-left (RL) intracardiac shunting, the venous blood that is added to the oxygenated blood in the left heart is both poor in oxygen and rich in carbon dioxide. Thus, any given degree of arterial desaturation is associated with an obligatory arterial to end-tidal carbon dioxide tension difference (Pa(CO2) - PET(CO2)). This paper presents a theoretical analysis of the relationship between Pa(CO2) - PET(CO2) and arterial hemoglobin saturation (SA(O2)) in cyanotic heart disease. Using the shunt equation as a starting point, a curvilinear, negative correlation between Pa(CO2) - PET(CO2) and Sa(O2) can be demonstrated. The slope of the regression of Pa(CO2) - PET(CO2) against Sa(O2) is shown to be positively correlated to Hb concentration, Pa(CO2), and the respiratory quotient R. The slope of the regression is also slightly increased at relatively high SA(O2)s and at high inspired oxygen fractions, although these latter factors are of lesser significance. However, in addition to the above primary effects of RL shunting, secondary effects may occur if pulmonary perfusion is reduced sufficiently to cause ''alveolar hypoperfusion,'' which also creates an alveolar dead space. Primary and secondary effects are additive. This theoretical analysis is illustrated with a study of 27 children with congenital heart disease. Their lungs were ventilated with a Servoventilator 900 C, and carbon dioxide single-breath tests were obtained on-line with the use of a computerized system based on the Siemens-Elema carbon dioxide analyzer 930. Blood was sampled for Pa(CO2) measurement and arterial Hb saturation was measured by pulse oximetry (Sp(O2)). The relationship between Pa(CO2) - PET(CO2) and Sp(O2) was found to agree with that predicted by theory, confirming that in cyanotic heart disease, Pa(CO2) - PET(CO2) increases by 0.2-0.4 kPa (2-3 mmHg) for every 10% reduction in Sp(O2). Awareness of this relationship is necessary when attempting to estimate Pa(CO2) from PET(CO2) during anesthesia in cyanotic children.