Comparison between cardiac output measured by thermodilution technique and calculated by O2 and modified CO2 Fick methods using a new metabolic monitor

Objective: To calculate cardiac output from dual oximetry with carbon dioxide production (VCO₂) and oxygen consumption (VO₂) measured by a new metabolic monitor, and to compare these values with measurements made simultaneously using the thermodilution method during the steady state condition. Design: Prospective, comparative clinical study. Setting: The adult postsurgical intensive care unit (ICU) of a University Hospital. Patients: Twenty mechanically ventilated postsurgical patients (70.7 ± 7.8 years of age; range 50–84). Measurements and results: A new metabolic monitor (Puritan-Bennett 7250, Carlsbard, USA) connected to a ventilator (Puritan-Bennett 7200) was used to measure VCO₂ and VO₂. Measurements of arterial (SaO₂) and mixed venous (SvO₂) oxygen saturations were made using pulse and venous fiberoptic oximeters. Cardiac output starting from VCO₂ (CO_VCO2) was obtained according to Mahutte's formula: CO_VCO2 = VCO₂/[k (SaO₂− SvO₂)], where k represents a constant. The value for each patient was determined from the initial measurements of thermodilution cardiac output (COtd), VCO₂, SaO₂ and SvO₂. CO_VCO2 calculated from the previous equation was compared to the COtd. Cardiac output calculated from the traditional O₂ Fick equation (CO_VO2) was compared to the COtd. All patients were studied over a period of 120 min at 15-min intervals in reasonably stable conditions. CO_VCO2 was closely related to COtd (r = 0.94; SEE = 0.79; p = 0.0001; n = 180) with a bias of − 0.10 and a precision of 0.45 l/min. The mean percent difference between the two methods was − 2.2 ± 8.3 %. CO_VO2 was related to COtd (r = 0.77; SEE = 0.79; p = 0.0001; n = 180) with a bias of − 0.57 and precision of 0.86 l/min. The mean percent difference between the two methods was − 10.8 ± 16.0 %. Conclusions: In stable patients, cardiac output measurements obtained from dual oximetry with VO₂ and VCO₂ measured by this new metabolic monitor, show good correlation with measurements made using the thermodilution method. The values of cardiac output calculated from VCO₂ are more accurate and precise than values from VO₂. The validity of these measurements in hemodynamically unstable patients and during various modes of mechanical ventilation seems warranted. Received: 5 February 1997 Accepted: 16 June 1997