Central and peripheral pulmonary vascular resistance: Implications for who should undergo pulmonary thromboendarterectomy

Pulmonary thromboendarterectomy remains a technically challenging procedure with variable outcomes with regard to improvement in pulmonary function. Reducing the resistance to flow between the pulmonary valve and the pulmonary capillary bed is the key aim of surgery. The resistance to flow is due to the combination of resistance due to the central clot and distal capillary resistance. We hypothesise that the use of fluid mechanics in combination with modern radiology and electronic circuit theory can potentially predict who should or should not undergo a thromboendarterectomy. Electronic circuit theory of two resistors in series was utilised to demonstrate the concept of a model of a central clot and the peripheral pulmonary capillary bed. A simplified 2D model of the lungs utilising finite element analysis and Poiseuille's law was constructed for proof of principle. Modelling predicts that cardiac output and anatomical obstruction interplay and can have profound effects on the outcomes after thromboendarterectomy. Identical pulmonary artery pressures, due to differing cardiac outputs and identical anatomical obstructions due to thrombus can have very different physiological outcomes with regard to changes in pulmonary artery pressure. Modelling the pulmonary vasculature to determine central and peripheral pulmonary vascular resistance may help in predicting who should undergo pulmonary thromboendarterectomy. Mathematical modelling can potentially predict which patients have haemodynamically significant clots in their pulmonary arteries that thromboendarterectomy may potentially help in the setting of pulmonary capillary disease.

Pressure and oxygen debt on bypass - potential quality markers of perfusion?

No markers of quality of perfusion pressure and oxygen delivery during cardiopulmonary bypass (CPB), to complement rewarming rate, maximum temperature on rewarming, lowest haematocrit, and blood glucose, exist. Using the electronic acquisition of blood pressure on bypass (JOCAP system), the percentage of time perfusion pressure was below 30, 40, 50, 60 and 70 mmHg, average deviance, confidence interval, median, mode, standard deviation, variance, and average, maximum and cumulative oxygen debt were calculated. Numerous different readouts of achievement of maintenance of constant pressure on bypass and oxygen debt are now easily achievable with perfusion electronic data management systems. Mean, median, and mode offer poor discrimination of pressure control during CPB. Percentage of time perfusion pressure was below 30, 40, 50, 60 and 70 mmHg, average deviance, confidence interval, and standard deviation all have discriminatory power, but need clinical correlation for their significance. A composite score involving non-pressure readouts (e.g. oxygen delivery, arterial and venous saturations, and flow rates) may need to be integrated into any perfusion quality marker. Assessment of adequacy of constant perfusion pressure and oxygen delivery may allow the scientific evaluation of pressure and oxygen delivery on bypass for patients to be compared accurately. Currently, in studies involving CPB, blood pressure targets are stated with no quantitative assessment of adequacy of achievement of these targets. Electronic data monitoring during cardiopulmonary bypass, when correlated with clinical outcome, may help to provide a marker of quality of perfusion pressure during CPB and may, indeed, allow patient-specific perfusion pressure strategies to be developed.