Jon-Emile S. Kenny MD [@heart_lung] Hemodynamic assessment, by any means, demands a shrewd familiarity with mechanical heart-lung interaction. The two ventricles communicate in series and in parallel; each ventricle’s pressure-volume characteristics and loading conditions pulsate between systole and diastole. And around the heart and pericardium lies the respiratory pump – the lungs within the thorax – itself varying by pressure, volume and frequency. We shouldn’t be surprised, therefore, at the fallibility of distilling these exceedingly thorny relations into a simple output measurement [e.g. pulse pressure variation, vena cavae dimension change]; especially, if we desire an index which is universally applicable within the varied pathologies of the ICU. Accordingly, linking the physiologies of the cardiac and thoracic pumps will facilitate acquisition and interpretation of hemodynamic data as well as provide rationale for optimizing respiratory mechanics.