Applied Physiology for Surgical Patients
Cardiovascular
Cardiac output is stroke volume times heart rate; stroke volume depends on preload, afterload and contractility. The Frank–Starling relationship means a fluid-loaded ventricle ejects more, until the curve flattens and further filling only raises pressures. Oxygen delivery (DO2) equals cardiac output multiplied by arterial oxygen content; content is dominated by haemoglobin-bound oxygen (1.34 mL per gram of saturated haemoglobin) with a trivial dissolved fraction. Oxygen consumption is normally about 250 mL/min, giving an extraction ratio near 25% and a mixed venous saturation of about 75%. When delivery falls, extraction rises and SvO2 drops; a persistently high SvO2 in shock suggests impaired extraction or a shunt.
Respiratory
The oxyhaemoglobin curve shifts right (reduced affinity, easier unloading) with raised CO2, hydrogen ion, temperature and 2,3-DPG. Functional residual capacity falls with anaesthesia, obesity and supine posture, promoting atelectasis. Pulse oximetry measures saturation, not ventilation - a patient can be profoundly hypercapnic with a normal SpO2 on oxygen.
Renal and gastrointestinal
GFR is autoregulated between mean arterial pressures of roughly 60 and 160 mmHg. Antidiuretic hormone and aldosterone conserve salt and water postoperatively. Gastric acid secretion is driven by gastrin, histamine and acetylcholine; secretin and cholecystokinin govern pancreaticobiliary flow.
Worked example: a 70 kg patient has cardiac output 5 L/min, haemoglobin 15 g/dL and saturation 100%. Arterial content ≈ 1.34 × 15 ≈ 20 mL/dL. DO2 = 50 dL/min × 20 mL/dL ≈ 1000 mL/min. With consumption of 250 mL/min, extraction is 25%, so SvO2 ≈ 75%.