Cardiac output can be measured continuously by an invasive pulmonary artery catheter (PAC) or by noninvasive impedance cardiography. The purpose of this study is to determine how well the two methods correlate in ventilated patients.
Patients requiring mechanical ventilation and a PAC were studied in a combined surgical and medical intensive care unit from January 2003 to June 2004. Simultaneous measurements of cardiac output were obtained utilizing a Swan-Ganz CCOmbo V catheter (Edwards Lifesciences, Irvine, CA, USA) and impedance cardiography utilizing a BioZ (Cardiodynamic, San Diego, CA, USA). Measurements were recorded at initial placement and at 12, 24, 36, and 48 hours.
Thirty-seven patients were studied. Twenty-six patients were on volume control ventilation and 11 were on pressure control ventilation. The average PaO2/FiO2 was 213±101. Twenty-six (70%) of the patients required dopamine, norepinephrine and/or dobutamine. There were 172 measurements for each method that were evaluated for correlation. Overall the correlation coefficient was r=0.42 and r2=0.17. There was a decrease in the correlation coefficient with time as shown in the table 1. Nine of the patients had cardiac output greater than 9 l/min measured by the PAC. Their correlation coefficient was r=0.52 and r2=0.27. In the 29 patients with a cardiac output less than 9 l/min, the correlation coefficient was r=0.77 and r2=0.59.
The cardiac output measured by impedance cardiography has a better correlation with the PAC in patients with a cardiac output less than 9 l/min. The correlation between the two methods decreases with time.
Further studies are indicated to determine which method is best suited for ventilated patients. Table 1TimeCorrelation coefficient, rInitial placement.4912 hour.4324 hour.4636 hour.3348 hour.36
Daniel Ziegler, one.