The effect of lung injury and excessive lung fluid, on impedance cardiac output measurements, in the critically ill

Objectives: To investigate the relationship between the attenuation of impedance cardiac output (IC_co) measurements and lung fluid content in critically ill patients.¶Design: Observational study.¶Setting: Intensive Care Unit of a major teaching hospital in Hong Kong.¶Patients: Twenty-four critically ill patients who required a pulmonary artery catheter.¶Measurements and main results: Triplicate thermodilution cardiac output (TD_co) and BoMed NCCOM3 (IC_co) measurements were made simultaneously on a single occasion in each patient. Lung fluid accumulation was assessed by: (a) thoracic impedance (Zo), (b) radiological assessment of chest X-rays using an alveolar consolidation score (0–4) and (c) scoring the degree of hypoxia and use of positive end-expiratory pressure (PEEP). Offsets (TD_co–IC_co)/TD_co, expressed as percentage, were compared with these indices of excess lung fluid. Patients were divided into those with sepsis (n = 13), fluid balance problems (n = 5) and cardiothoracic problems (n = 6). Mean cardiac output values were: 6.7 l/min TD_co (range 3.6–12.9) and 5.2 l/min IC_co (range 2.7–9.0). Overall the TD_co and IC_co values showed great variance, with a bias and limits of agreement of 1.49 ± 4.16 l/min, or ± 69 %. In septic patients, increasing offset was correlated with decreases in Zo (r = 0.73, P = 0.005) and increases in alveolar consolidation score (r = 0.72, P = 0.005).¶Conclusions: The BoMed under-estimates cardiac output in critically ill patients. In septic patients the degree of attenuation of IC_co can be related to the extent of lung injury and fluid accumulation within the thorax.