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 impe dance cardiac output (ICco) measurements and lung fluid content in critical ly 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 arte ry catheter. Measurements and main results: Triplicate thermodilution cardiac output (TD co) and BoMed NCCOM3 (ICco) measurements were made simultaneously on a sing le occasion in each patient. Lung fluid accumulation was assessed by: (a) t horacic impedance (Zo), (b) radiological assessment of chest X-rays using a n alveolar consolidation score (0-4) and (c) scoring the degree of hypoxia and use of positive end-expiratory pressure (PEEP). Offsets (TDco-ICco)/TDc o, expressed as percentage, were compared with these indices of excess lung fluid. Patients were divided into those with sepsis (n = 13), fluid balanc e problems (n = 5) and cardiothoracic problems (n = 6). Mean cardiac output values were: 6.7 1/min TDco (range 3.6-12.9) and 5.2 1/min ICco (range 2.7 -9.0). Overall the TDco and ICco values showed great variance, with a bias and limits of agreement of 1.49 +/- 4.16 1/min, or +/- 69 %. In septic pati ents, 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 pat ients. In septic patients the degree of attenuation of ICco can be related to the extent of lung injury and fluid accumulation within the thorax.