Cardiac physiology, invasive measurements and imaging
Cardiovascular physiology becomes useful when a measurement is connected to a defined clinical question. During isovolumetric ventricular contraction, the atrioventricular valves have closed but the semilunar valves have not opened. Ventricular pressure rises without ejection. A pressure-volume loop separates loading conditions from contractility: an isolated increase in afterload can increase end-systolic volume and reduce stroke volume, whereas increased contractility changes the end-systolic relationship. Actual patients can change preload, afterload, rate and contractility simultaneously, so identify the isolated assumption in an examination problem.
Left ventricular coronary perfusion occurs predominantly during diastole. Tachycardia shortens diastolic time and increases oxygen demand, helping explain vulnerability of the subendocardium. Myocardial oxygen delivery also depends on arterial oxygen content and coronary flow, not saturation alone. A normal saturation can coexist with substantially reduced oxygen content in anemia. When using the Fick relationship, divide measured oxygen consumption by the arterial-minus-mixed-venous oxygen content difference. Match units: content expressed per deciliter requires conversion when reporting liters per minute. For an explicitly supplied example, oxygen consumption of 240 mL/min and a content difference of 60 mL/L yield a cardiac output of 4 L/min. An assumed rather than measured oxygen consumption introduces additional uncertainty.
Resistance calculations require a pressure difference divided by flow. With mean pulmonary artery pressure 46 mmHg, wedge pressure 14 mmHg and cardiac output 4 L/min, pulmonary vascular resistance is 8 Wood units. This value is not a complete etiological diagnosis. A precapillary hemodynamic pattern can occur in several pulmonary hypertension groups, so exclusion of thromboembolic, lung and other causes still matters. Confirm measurement conditions, waveform quality and relevant respiratory effects before interpreting catheter values.
Imaging modalities answer different questions. Echocardiography evaluates anatomy, flow and ventricular response; Doppler gradients are sensitive to velocity and flow. Cardiac magnetic resonance can characterize scar, edema and other tissue features, while coronary CT depicts coronary anatomy. Ischemic late enhancement commonly involves the subendocardium in a coronary distribution, unlike many nonischemic patterns. OCT uses light and offers high spatial resolution; IVUS uses ultrasound and can assess deeper vessel structures. Neither an anatomical stenosis percentage nor a single image automatically establishes that a lesion explains symptoms or mandates an intervention. Combine image quality, physiology, clinical presentation and the actual management question.