Explain cardiovascular physiology in O&G, including pregnancy adaptation, uteroplacental circulation and shock physiology.

Cardiovascular Physiology in O&G

Clinical Overview

Cardiovascular physiology is the physiology that keeps obstetric patients alive. It explains why a healthy pregnant woman can lose a dangerous volume of blood before her blood pressure falls, why stenotic valve disease deteriorates in late pregnancy, why pulmonary oedema can appear after delivery, why aortocaval compression matters during resuscitation, and why pre-eclampsia is an endothelial-circulatory disease rather than "hypertension only".

The cardiovascular system has one central job: deliver enough oxygenated blood to tissues at an adequate pressure while preserving venous return and organ perfusion. Pregnancy changes every part of that task. The mother expands plasma volume, lowers systemic vascular resistance, increases cardiac output, remodels the heart, feeds a low-resistance uteroplacental circulation and then reverses much of the load abruptly after birth.

The safest Primary mental model is:

vascular relaxation first -> volume expansion follows -> cardiac output rises -> uteroplacental flow becomes a major circuit -> labour and delivery add acute stress -> postpartum autotransfusion unmasks disease

The same physiology feeds directly into shock management, fluid prescriptions, cardiac disease in pregnancy, hypertensive disease, sepsis, anaesthesia, operative delivery and postpartum care.

Core Knowledge

The Circulation as a Circuit

Cardiovascular physiology becomes easier when the circulation is treated as a circuit rather than a collection of numbers. Pressure drives flow. Resistance opposes flow. Cardiac output is the blood volume pumped per minute. Venous return is the blood volume returning to the heart per minute. In steady state, cardiac output and venous return must match.

The simplified relationship is:

mean arterial pressure = cardiac output x systemic vascular resistance

This equation prevents two common mistakes. First, normal blood pressure can hide falling cardiac output if vasoconstriction is compensating. Second, hypotension can occur despite a high cardiac output if systemic vascular resistance collapses, as in distributive septic physiology.

In non-pregnant resting adults, cardiac output is often roughly 4 to 6 L/min, with lower body-size-adjusted values in smaller women. Because a small woman and a large man cannot share one "normal" number, output is sometimes indexed to body surface area as the cardiac index (cardiac output divided by surface area in m², roughly 3 L/min/m²). Pregnancy then raises output by about 30-50%, so the same absolute output may be normal before pregnancy and inadequate later in gestation or labour.

One Heartbeat: the Cardiac Cycle and Starling's Law

Before scaling up to the whole circulation, anchor on a single beat, because everything else is built from it. In each cycle the ventricle first fills during diastole, then contracts to eject blood during systole. Filling has two phases: early passive inflow as blood falls from atrium to ventricle, then a final top-up from atrial contraction. That atrial "kick" supplies only a small fraction of filling in a normal heart, so losing it (for example in atrial fibrillation) is barely noticed by a woman with a healthy ventricle but can be decisive in a stiff, poorly filling heart such as mitral stenosis.

The single most useful first principle is the Frank-Starling relationship: within limits, the more the ventricle is stretched during filling, the more forcefully it contracts. Stretch is set by the end-diastolic volume (the preload), which in turn depends on circulating volume, venous tone, body position and the pumping of leg muscles. This is why preload matters so much in obstetrics:

more venous return -> more ventricular stretch -> more forceful contraction -> larger stroke volume

The relationship has a ceiling. Past the optimum stretch, further filling no longer increases output and an overloaded, failing ventricle moves down the curve instead. A second, separate lever is contractility — the intrinsic vigour of the pump at any given stretch. Catecholamines and inotropes raise it; hypoxia, acidosis, hypercapnia, ischaemia and cardiomyopathy lower it. Heart rate is the third lever, and the fourth is afterload, the pressure the ventricle must overcome to open the aortic valve and eject. Holding these four in mind — preload, contractility, rate and afterload — lets you read almost any obstetric haemodynamic problem.

The Normal Pregnancy Haemodynamic Arc

Pregnancy cardiovascular adaptation begins early, before the uterus is large. Relaxin, progesterone, oestrogen, nitric oxide, prostacyclin and vascular remodelling reduce arterial tone. The maternal circulation then expands volume to refill the relaxed vascular tree.

Typical changes are approximate, not fixed: