An investigation to show the effect of lung fluid on impedance cardiac output in the anaesthetized dog

Background. Accumulation of lung fluid in the critically ill patient is believed to attenuate impedance cardiac output (COIC) measurements. However, this phenomenon has never been shown experimentally. Methods. In eight anaesthetized and ventilated dogs (weight 15-22 kg) a high-precision flow probe was placed on the ascending aorta via a left thoracotomy incision and the direct cardiac output (COFP) was measured. Simultaneous COIC measurements were made using a Rheo-CardioMonitor (ACMA, Singapore). Lung oedema was induced by intravenous oleic acid 0.1 mg kg(-1). Lung fluid was assessed by the decrease in basal thoracic impedance (Z(b)). Percentage errors between the two methods (COIC-COFP) were calculated and compared as Z(b) decreased at 1 Omega intervals. Results. During the experiment mean Z(b) decreased from 35.9 (SD 5.2) to 27.8 (6.5) Omega (P = 0.0037). This occurred over a period of 225 (range 112-338)min and Z(b) decreased by 1 Omega every 51 (22-68)min. The presence of excessive lung fluid was confirmed at post-mortem. Before lung oedema was induced, COIC was 1.5 (0.6) litre min(-1) and the corresponding value of COFP was 1.5 (0.7) litre min(-1) (data from eight dogs). As Z(b) decreased, and lung fluid accumulated, the error between COIC and COFP widened (P < 0.0001, ANOVA for repeated measures). Eventually, COIC decreased to 0.7 (0.3) litre min(-1) and the corresponding value of COFP was 1.2 (0.3) litre min-1 (Delta Zb = 5 Omega, data from six dogs). Mean arterial pressure, central venous pressure and systemic vascular resistance were kept constant. Conclusion. The presence of lung fluid attenuates COIC measurements with respect to COFP.