Hemodynamic Principles & Response to Stress

Hemodynamic Principles

Hemodynamics is the study of how pressure, flow, and resistance interact to move blood through the cardiovascular system, and three relationships recur throughout echocardiographic quantification.

Frank-Starling relationship

The Frank-Starling law states that, within physiologic limits, increasing ventricular preload (end-diastolic stretch) increases stroke volume — greater sarcomere stretch increases troponin C's calcium sensitivity and improves actin-myosin cross-bridge cycling. This relationship is the physiologic basis for why venous return, volume status, and atrial contraction all directly influence stroke volume independent of any change in contractility. Plotted as ventricular function curves (stroke volume or stroke work against preload), the Frank-Starling relationship also explains why the failing or dilated ventricle sits on a flatter curve: a given rise in preload produces a smaller stroke-volume gain, and inotropic agents or afterload reduction — rather than further volume loading — shift the curve upward instead of simply moving further along it.

Ohm's law analog

Just as electrical current relates to voltage and resistance, blood flow relates to pressure and resistance:

ΔP = Q × R

where ΔP is the pressure gradient driving flow, Q is flow (cardiac output, or flow across a valve), and R is resistance. This relationship underlies every gradient and resistance calculation in echocardiography. Rearranged as R = ΔP ÷ Q, it produces vascular resistance — for example, pulmonary vascular resistance (PVR) is calculated as PVR = (mean PA pressure − mean pulmonary capillary wedge pressure) ÷ cardiac output, expressed in Wood units, and multiplied by 80 to convert to dyn·s·cm⁻⁵ (1 Wood unit = 80 dyn·s·cm⁻⁵). Rearranged instead as ΔP = Q × R, the same relationship explains why a fixed stenotic orifice produces a higher gradient at higher flow states — the physiologic basis of a "low-flow, low-gradient" pattern in aortic stenosis.

Poiseuille's law

Poiseuille's law describes resistance through a rigid tube:

R = 8ηL ÷ (πr⁴)

where η is viscosity, L is tube length, and r is radius. The critical testable relationship is that resistance is inversely proportional to the fourth power of the radius — halving a vessel or orifice radius increases resistance sixteen-fold. This is why small measurement errors in valve or vessel diameter produce large errors in calculated area and flow, and why even modest anatomic narrowing produces a disproportionately large hemodynamic effect.

Determinants and Normal Values of Cardiac Output

Cardiac output (CO) is the ultimate measure of forward pump performance:

CO = heart rate × stroke volume

Indexing to body surface area (BSA) allows comparison across patients of different sizes and is standard practice whenever CO or SV is reported clinically.

Cardiac reserve

The difference between resting cardiac output and the maximum output the heart can generate under demand is called cardiac reserve. A healthy heart can typically increase cardiac output roughly four- to five-fold above resting levels during maximal exercise — a resting CO near 5 L/min can rise to approximately 20–25 L/min at peak exertion in a healthy adult. Cardiac reserve depends on the same three systolic determinants covered in Section 3.2 (preload recruitment, augmented contractility, and controlled afterload) plus heart-rate reserve and adequate diastolic filling time; disease that limits any one of these mechanisms — a stiff, poorly relaxing ventricle, fixed valvular obstruction, significant coronary stenosis, or chronotropic incompetence from conduction disease or medication — blunts the achievable output rise and is exactly the physiologic gap that stress testing is designed to expose.

Hemodynamic Response to Exercise and Pharmacologic Stress

A normal cardiovascular system increases cardiac output several-fold during exercise through a coordinated response:

• Heart rate rises first and most steeply, driven by sympathetic activation and vagal withdrawal — the dominant early mechanism for increasing CO.
• Contractility increases under sympathetic (beta-adrenergic) stimulation, shifting the end-systolic pressure-volume relationship and increasing stroke volume for a given afterload.
• Venous return and preload increase initially, aided by the skeletal muscle pump and sympathetic venoconstriction, supporting stroke volume via the Frank-Starling mechanism — though at very high heart rates, shortened diastolic filling time can limit further preload recruitment.
• Systemic vascular resistance falls in exercising skeletal muscle beds due to local metabolic vasodilation, even as overall systolic blood pressure rises, allowing forward flow to increase despite the higher resistance-independent workload.
• Ejection fraction should rise or remain normal with exercise or pharmacologic (dobutamine) stress in a normal heart; a segment that fails to hypercontract, or a global EF that falls or fails to augment, represents an abnormal response.

This normal augmentation pattern is precisely what stress echocardiography (covered in Chapter 9) exploits diagnostically: comparing resting and stress images for new or worsening wall-motion abnormalities uncovers inducible ischemia that is invisible on a resting study, because a coronary stenosis that permits adequate resting flow may not accommodate the several-fold flow increase that normal exercise or pharmacologic stress demands.