Estimation of Mixed Venous Pco2 for Determination of Cardiac Output in Children (Introduction)

Estimation of Mixed Venous Pco2 for Determination of Cardiac Output in Children (Introduction)Study objectives: Cardiac output (Q) can be estimated noninvasively during exercise by employing C02-rebreathing techniques (equilibrium and exponential) to estimate the oxygenated mixed venous Pco2 (Pvco2). It has been found in adults and children that the equilibrium method underestimates Q as a result of overestimation of PvC02, unless PvC02 is “downstream corrected.” In adults, it has been found that the exponential method does not require downstream correction and yields values similar to those obtained by the equilibrium method with downstream correction. The objectives of this study were as follows: to test whether the exponential method gives similar results to the equilibrium method with downstream correction in children; to verify that downstream correction is required in children; and to test whether a single equation could be used in adults and children to predict Q from oxygen consumption (Vo2). asthma mist asthma relief

Design: Descriptive.

Setting: Exercise laboratory of a university hospital.

Participants: 23 children (16 boys, 7 girls) with a mean age of 11.0±1.9 years (7.1 to 13.9 years), and 12 adults (7 men, 5 women) with a mean age of 33.6±7.2 years (24 to 48 years).

Interventions: While performing steady-state exercise on an ergometer, Pvco2 was determined in 14 children using both the equilibrium and exponential methods, and in all other subjects using the equilibrium method alone.

Measurements and results: For the 14 children who underwent testing by both the equilibrium and exponential methods, the uncorrected equilibrium PvC02 was significantly different from both the corrected PvC02 and the exponential PvC02. We found a strong relationship between Q (L/min), calculated using the downstream corrected values of PvCO£, and Vo2 (L/min) (r2=0.95), and this relationship was similar to that obtained by dye dilution in other studies. When weight was included, it was determined that one equation could be used for children and adults: Q (L/min)= 1.42+5.80 Vo2 (L/min)+0.06 wt (kg), r2=0.97, SEY=0.67.

Conclusions: C02-rebreathing techniques can be used to determine Q in children; the exponential method gives values that are similar to the equilibrium method with the downstream correction; and one prediction can be used for Q in adults and children.

Cardiac output (Q) can be determined noninva-sively during exercise using the indirect C02-rebreathing method. By this technique, oxygenated mixed venous Pco2 (PvC02) is estimated using one of two methods: the equilibrium method of Collier or the exponential method of Defares.2 In the equilibrium method, a relatively high concentration of C02 is rebreathed, which leads to an equilibration of C02 in the lungs and blood and a plateau in the end-tidal Pco2 (PetC02, at equilibrium known as PeqC02). In the exponential method, a low concentration of C02 is rebreathed, which results in an exponential rise in the PetC02.

In exercising adults, it has been found that the equilibrium method overestimates PvC02, as determined by PeqC02, resulting in an underestimation of the Q in comparison to dye-dilution measures. This led to the development of a correction factor for PvC02 that is known as the “downstream correction factor” (PvC02=PeqC02—[0.24 • PeqC02-11]). However, this correction does not appear to be necessary using the exponential method. The reason for this downstream correction with the equilibrium method remains unclear, but it is likely related to the absence of outward movement of C02 from the blood to the rebreathing bag during this method.

In children, there has been some controversy over whether downstream correction is necessary when using the equilibrium method to determine Q. Godfrey and coworkers found that the overestimation of PvC02 that had been seen in adults’’ was also present in children, but Godfrey suggests that the use of the downstream correction in children gives an overly high estimate of Q, so that the uncorrected plateau value for PvC02 should be used. Paterson and coworkers, however, found that the use of the downstream correction in children gave values that were close to those obtained by dye dilution on similar populations, and that the equilibrium method with downstream correction allowed reliable and consistent measurement of Q.

It has been demonstrated recently that the exponential method gives Q estimates similar to those of the equilibrium method, after the downstream correction, during steady-state exercise in cystic fibrosis patients and healthy control subjects over the age of 14 years. The exponential method is well tolerated, as it requires the use of a lower concentration of C02 than does the equilibrium method, and it can be used during progressive exercise testing. The aims of the present study were as follows: to compare the exponential method for determination of PvC02 using new computer analysis techniques with the equilibrium method in children; to confirm that downstream correction of PvC02 was required in healthy children from 7 to 13 years of age when the equilibrium method was used; and to test whether a single equation could be used in children and adults for prediction of Q from oxygen consumption (Vo2).

We hypothesized the following in exercising children: PvC02 obtained by the exponential method would be similar to that obtained by the equilibrium method using the downstream correction; when plotted against Vo2, corrected values for Q would be similar to those obtained by others using dye dilution; and when plotted against Vo2, the values for Q in children and adults would not differ when downstream correction was employed.

Category: Cardiac Output

Tags: equilibrium rebreathing method, exercise, exponential rebreathing method