Correlation of continuous cardiac output measured by a pulmonary artery catheter versus impedance cardiography in ventilated patients

Introduction

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.

Method

Patients requiring mechanical ventilation and a PAC were studied in a combined surgical and medical intensive care unit from October 2002 to October 2003. 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 (Cardiodynamics, San Diego, CA, USA). Measurements were recorded at initial placement and at 12, 24 36, and 48 hours.

Results

Twenty-two patients were studied. The average PaO₂/FiO₂ was 184 ± 85. Sixteen (73%) of the patients required dopamine, norepinephrine and/or dobutamine. Overall the correlation coefficient was r = 0.48 and r² = 0.23. There was a decrease in correlation with time, as shown in Table 1. Six (27%) of the patients had a cardiac output greater than 9 l/min measured by the PAC. The correlation coefficient was r = 0.52 and r² = 0.27. In the 18 patients with a cardiac output less than 9 l/min, the correlation coefficient was r = 0.89 and r² = 0.80.

Table 1

Conclusion

The cardiac output measured by impedance cardiography has good correlation with the PAC in patients with a cardiac output less than 9 l/min. With time, the correlation between the two methods decreases. Further studies are indicated to determine which method is best suited for ventilated patients.