INFLUENCE OF INTRACARDIAC LEFT-TO-RIGHT S HUNTS ON THERMODILUTION MEASUREMENTS OF CARDIAC-OUTPUT

Thermodilution measurements of cardiac output (CO) by means of Swan-Ga nz catheters, in a strict sense, represent pulmonary arterial blood fl ow (PBF). In principle, this is also true in the presence of intracard iac left-to-right shunts due to atrial or ventricular septal defects. However, early recirculation of indicator may give rise to serious met hodological problems in these cases. We sought to determine the influe nce of intracardiac left-to-right shunts on different devices for ther modilution measurements of CO using an extracorporeal flow model. Meth ods. Blood flow was regulated by means of a centrifugal pump that at t he same time enabled complete mixing of the indicator after injection (Fig.1). Pulmonary and systemic parts of the circulation were simulate d using two membrane oxygenators and a systemic-venous reservoir to de lay systemic recirculation of indicator. Control measurements of PBF ( Q(p)) and systemic (Q(s)) blood flow were performed by calibrated elec tromagnetic flow-meters (EMF). Blood temperature was kept constant usi ng a heat exchanger without altering the indicator mass balance in the pulmonary circulation. Left-to-right shunt was varied at different sy stemic flow levels applying a Q(p):Q(s) ratio ranging from 1:1 to 2.5: 1. Thermodilution measurements of PBF were performed using two differe nt thermodilution catheters that were connected to commercially availa ble CO computers. Additionally, thermodilution curves were recorded on a microcomputer and analysed with custom-made software that enabled i terative regression analyses of the initial decay to determine that pa rt of the downslope that best fits a monoexponentially declining funct ion. Extrapolation of the thermodilution curve was then based on the r espective curve segment in order to eliminate indicator recirculation due to shunt flow. Results. At moderate left-to-right shunts (Q(p):Q(s ) < 2:1) all thermodilution measurements showed close agreement with c ontrol measurements. At higher shunt flows (Q(p):Q(s) greater than or equal to 2.1), however, conventional extrapolation procedures of CO co mputers considerably underestimated PBF (Fig. 2). This was particularl y true when a slow-response thermistor catheter was used (Fig. 3). The reason for this underestimation of Q(p) was an overestimation of the area under curve because of inadequate mathematical elimination of ind icator recirculation by standard truncation methods (Fig.4). However, curve-alert messages of the commercially implemented software did not occur. A high level of agreement could be consistently obtained using a fast-response thermistor together with individual definition of extr apolation limits according to logarithmic regression analyses. Discuss ion and conclusion. Under varying levels of left-to-right shunt, both the reponse time of thermodilution catheters and the algorithms for ca lculation of flow considerably influenced the validity of thermodiluti on measurements of PBF in an extracorporeal flow model. The use of com puter-based regression analyses to define the optimal segment for mono exponential extrapolation could effectively eliminate indicator recirc ulation from the initial portion of the declining thermodilution curve and showed the closest agreement with EMF measurements of Q(p). The q uality of thermodilution curves with respect to recirculation peaks in the flow model was slightly better than in clinical routine. Neverthe less, the clinical applicability of the modified extrapolation algorit hm could be illustrated during pulmonary thermodilution measurements i n an exemplary patient with a ventricular septal defect (Fig. 5). PBF at extremely high shunt ratios, however, cannot be assessed by monoexp onential extrapolation in principle (Fig. 6). Insufficient elimination of indicator recirculation resulted in flow values that closely resem bled systemic rather than PBF. This finding is in accordance with a ma thematical analysis of the underlying Steward-Hamilton equation if an infinite number of recirculations would be included in the area under curve.