Functional Cardiac Output Using Ventilation Approximates Cardiac Output in a Pig Endotoxemia Model

Introduction: Evaluation of cardiac output (CO) is an important part of critical care, but bedside estimation remains challenging. Alveolar functional fraction (AFF), calculated as the ratio of EtCO2 to PaCO2, reflects pulmonary blood flow and, in the absence of intracardiac shunt, systemic cardiac output (CO) when linked to minute ventilation (MV). We hypothesized that the product of AFF and MV (AFF x MV), termed functional CO (fCO), would strongly correlate with measured CO in a porcine endotoxemic shock model.


Methods: We performed a prospective, observational study in a juvenile pig model of endotoxemic shock with E. coli (0111:B4) lipopolysaccharides. All pigs were invasively ventilated with MV and EtCO2 measurement from Phillips Respironics NM3. PaCO2 was measured using blood gas analysis obtained from a carotid artery catheter. CO was measured using ultrasound dilution with the CO-status (Transonic Systems, Inc, New York). Each measurement was obtained at baseline and at 30-minute intervals for 5 additional measurements following shock, defined as a 25% decrease in blood pressure, 25% decrease in CO, or 25% increase in heart rate. At each measurement, fCO was calculated using AFF and MV. Pearson’s coefficient was used to determine the correlation of fCO with measured CO at each timepoint treating all paired measurements as independent observations.


Results: Thirty-five paired measurements of fCO and CO were available from 6 pigs. All pigs were < 1 year-old, weighed 25.5 to 31.5 kg, and had length 112 to 125 cm. Baseline CO ranged from 2.48 to 6.34 L/min with a decrease to 0.78 to 3.7 L/min with shock. The calculated fCO ranged from 2.88 to 6.23 L/min at baseline and 0.98 to 4.9 L/min after shock. The overall Pearson’s correlation coefficient (r) between CO and fCO was 0.68 (p < 0.001), with r=0.44 (p=0.38) at baseline and r=0.60 (p < 0.001) after endotoxemic shock. fCO was slightly biased toward higher values than CO with mean fCO/CO ratio of 1.45 (95% CI 1.31 to 1.60).


Conclusions: Measurements of fCO using AFF x MV strongly correlated with CO, as measured by ultrasound dilution for both healthy baseline measurements as well as after endotoxemic shock in a pig model. These data provide preliminary evidence that fCO could be useful as a practical surrogate to estimate CO in critically ill patients.