A system has been designed to determine cardiac output noninvasively. The system's main component is a closed breathing circuit and it measures oxygen uptake (VO2), carbon dioxide elimination (VCO2), and end-tidal CO2 partial pressure (PET). As an integral part of the system, periods of CO2 rebreathing can be automatically implemented. The CO2 partial pressure of oxygenated mixed venous blood (Pv) is obtained from the measured exponential rise of the PET value during such a CO2 rebreathing maneuver. A new method is described for estimating the pulmonary blood flow, alveolar ventilation, cardiac output (CO), and mixed venous oxygen saturation (SVO2) from PV, PET, VO2, VCO2, tidal volume, and arterial oxygen saturation. The method was evaluated in 6 anesthetized and mechanically ventilated pigs. A wide range of cardiac output, shunt fractions, and dead space to tidal volume ratios were induced by combinations of bronchoalveolar lavage, hypervolemia, hypovolemia, and variable levels of positive end-expiratory pressure (PEEP). The bias between the CO obtained with the noninvasive technique (CO L/min) and the thermodilution CO (Qt L/min) was 0.13 L/min (SD = 0.78 L/min) and the correlation was N = 64; R = 0.92; CO = 0.95*Qt + 0.38. The bias obtained for double determinations with the noninvasive CO technique was 0.3 L/min (SD = 0.5 L/min). The bias between the noninvasive estimates of Svo2 and the directly measured values was 1.1% (SD = 9.3%). For double determination with the noninvasive technique the bias was -0.9% (SD = 4.7%). It is concluded that in mechanically ventilated pigs the proposed method produces good estimates of CO and SVO2 also in the presence of significant ventilation/perfusion mismatch.
With a single breath perturbation, the differential Fick method can yield cardiopulmonary information using 2-3 breaths only and with a minimum of interference with the patient. Complete data analysis results in multiple determinations of the Qp and ELV values which improve the attainable precision. Our investigation points to the possibility to determine Qp, CO(Fick) and ELV also during spontaneous breathing, by using the natural tidal variations of V and P.
New mathematical algorithms have been applied to a computer controlled closed breathing circuit system for non-invasive measurement of cardiac output (COniv). This system has been described in an animal study. Forty patients were studied 5 and 18 hours after cardiac surgery using the thermodilution technique as the reference (COtd). The variables entered into the algorithms for COniv were oxygen uptake, carbon dioxide elimination, end-tidal carbon dioxide partial pressure, tidal volume and arterial oxygen saturation. Mixed venous carbon dioxide partial pressure was obtained from an automatically implemented short rebreathing manoeuvre. Pulmonary perfusion was calculated by a modified Fick equation for carbon dioxide and the shunt flow added to obtain COniv. During mechanical ventilation, there was a good agreement between COtd and COniv (r = 0.8). The bias was -0.14 l/min and the precision was 0.77 l/min. The reproducibility of COniv was 0.03 l/min and for COtd -0.03 l/min with a standard deviation of the difference being 0.35 l/min for COniv and 0.31 l/min for COtd. In awake, but sedated extubated patients, the method proved unsatisfactory on account for uneven tidal volumes and difficulties with leakage around the mouth piece. We conclude that this new technique provides reliable and reproducible measures of cardiac output in sedated, ventilated patients.
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