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In their recent paper on atrial septostomy as a treatment for severe pulmonary arterial hypertension, Reichenberger et al1 measured cardiac output before and after this intervention using both the thermal dilution and Fick methods. We were interested by their choice of dilution technique. In these patients atrial septostomy was intended to produce a right to left interatrial shunt, as evidenced by the fall in arterial oxygen saturation following the procedure. Cardiac output measurement by the thermal dilution method described in their paper relies upon calculation of the area under the temperature curve, measured by a thermistor placed in the pulmonary artery, following an injection of cold saline into the right atrium or superior vena cava. In simple terms, the greater the area under the curve (purists would perhaps say “over the curve” since the injectate produces a transient fall in blood temperature in the pulmonary artery), the lower the derived cardiac output. It is unclear why the authors would choose such a method to estimate cardiac output following atrial septostomy when it would be expected that a proportion of the injectate would pass directly into the left atrium through the interatrial septal defect, producing an erroneous overestimate of cardiac output. A reliable method of measuring blood flow within the pulmonary artery after the procedure might be expected, at least initially, to show exactly the opposite result—namely, a fall in pulmonary arterial flow caused by the right to left shunt. We postulate that the explanation for their observed good correlation between the thermodilution and Fick cardiac outputs is that, before the procedure, both were reliable methods and that, after the procedure, the true cardiac output increased and was correctly measured by the indirect Fick method but was artefactually increased, despite a fall in pulmonary arterial blood flow, when measured by thermodilution.
We thank Dr McCann and colleagues for their comments on our paper in which cardiac output measurements were performed using thermodilution methods following creation of an atrial shunt after atrial septostomy in patients with severe pulmonary hypertension.1
We agree with the authors that cardiac output measured with the thermodilution method can give overestimated readings in the presence of an existing intercardiac shunt and that the Fick method is the method of choice. We have therefore calculated the cardiac output using both methods and, interestingly, found a very good correlation in our patient population between both methods before and after shunt creation (r = 0.83 and r = 0.78, respectively), allowing us to present data measured with the thermodilution method. However, cardiac output measured by thermodilution was significantly lower than the calculation based on the Fick method. This has been described in tricuspid regurgitation.2,3 In our patient population the mean cardiac index increased by 31% after atrial septostomy measured with the thermodilution method and by 29% when measured with the Fick method.
The most important message of our paper is that creation of the small interatrial shunt improves cardiac output independently of the method used for its calculation. This is accompanied by a significant reduction in oxygen saturation (from 93.2% to 87.4%), but systemic oxygen transport is increased. This improves the patients’ symptoms and has the potential to influence prognosis in this selected population of patients with severe pulmonary arterial hypertension.