Simple method for the continuous noninvasive estimate of cardiac output using the maxima breathing system. A pilot study
The Maxima is a new breathing system which makes it possible to monitor cardiac output (Q̇)noninvasively and to follow trends continuously. When used as a rebreathing system in controlled ventilation, the Maxima selectively eliminates alveolar gas and no fresh gas (V̇F) is eliminated. Without the need for a mixing chamber, eliminated CO2 (FĒCO2) may measured directly. With steady state conditions, and assuming a respiratory quotient of 1, carbon dioxide production (V̇CO2) = V̇F x FĒCO2. The indirect Fick principle applied to CO2 exchange (Q̇ = V̇CO2/venous-to-arterial CO2 content difference (C(a)CO2-C(v)CO2)) may be modified to Q̇ = V̇F x FĒCO2/(C(a)CO2-C(v)CO2). If V̇F in the breathing system is adjusted so that FĒCO2 is equal to C(a)CO2-C(v)CO2, then Q̇ = V̇F. It was found that, in the presence of a normal haemoglobin (13.3 g.dl-1, Q̇ = V̇F when V̇F was adjusted to achieve an FĒCO2 value of approximately 4.1%. Cardiac output estimates were compared with those obtained using a thermodilution technique in five patients undergoing coronary artery bypass grafting and in four patients with septicaemia in the ICU. Cardiac outputs were estimated from 1.VCO2 measurements and then 2. V̇CO2 and haemoglobin (Hb). The results of 28 measurements on nine patients with methods 1. and 2. respectively yielded a correlation with thermodilution measurements: coefficient r = 0.91 and 0.94 with a bias of -10.5% and -0.05% and an accuracy of 14% and 9%.