A Simple Method for the Continuous Noninvasive Estimate of Cardiac Output Using the Maxima Breathing System. A Pilot Study

Abstract

The Maxima is a new breathing system which makes it possible to monitor cardiac output (Q) 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 (&Vdot;F) is eliminated. Without the need for a mixing chamber, eliminated CO2 (FĒCO2) may be measured directly. With steady state conditions, and assuming a respiratory quotient of 1, carbon dioxide production (&Vdot;CO2) = &Vdot;F x FĒCO2. The indirect Fick principle applied to. CO2 exchange (&Qdot; = &Vdot;CO2/venous-to-arterial CO2 content difference (CaCO2-CvCO2)) may be modified to &Qdot; = &Vdot;F x FĒCO2/(CaCO2-CvCO2). If &Vdot;F in the breathing system is adjusted so that FĒCO2 is equal to CaCO2-CvCO2, then &Qdot; = &Vdot;F. It was found that, in the presence of a normal haemoglobin (13.3 g.dl–1), & Qdot; = &Vdot;F when &Vdot;F 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. &Vdot;CO2 measurements and then 2. &Vdot;CO2 and haemoglobin (Hb). The result 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%.