Determinants of Cardiac Function

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🌱 來自: Huppert’s Notes

Determinants of Cardiac Function🚧 施工中

Determinants of Cardiac Function

•   Preload: Myocardial stretch before contraction (i.e., the end-diastolic length of the cardiac fibers), which affects the force of contraction

-   Proxy: End-diastolic volume (EDV), end-diastolic pressure (EDP)

-   Frank-Starling curve: ↑Venous return → ↑SV + ↑CO

•   Increases preload: ↑Blood volume, exercise, sympathetic (reduced venous compliance), ↑HR (longer filling time)

•   Afterload: Force to push against. Two types: Wall stress and vascular resistance

-   Proxy = systemic or pulmonary vascular resistance, ventricular wall stress

-   Wall stress = (pressure × radius) / (2 × wall thickness); ↑ Wall stress = ↑ O2 demand

-   Systemic vascular resistance (SVR) (component of LV afterload) = (MAP – RAP) / CO

•   Increases LV afterload: Hypertension, aortic stenosis, systolic heart failure (↑ Wall stress)

-   Pulmonary vascular resistance (PVR) (component of RV afterload) = (mPAP – PAWP) / CO

•   Increases RV afterload: PE, pulmonary arterial hypertension

•   Contractility: Inherent ability of the myocardium to contract independent of preload or afterload

-   Proxy: None (SV, CO, EF provide an impression; however, all are affected by preload/afterload)

•   Increases contractility: Sympathetic (catecholamine via β1 receptor), inotropic drugs (e.g., dobutamine via B1 receptor, digoxin via .intracellular Ca2+), tachycardia (via Ca2+ buildup)

•   Decreases contractility: Parasympathetic (ACTH via M2 receptor), beta blocker/calcium channel blocker, CHF/MI, acidosis, hypoxia/hypercapnia

There are three ways to increase stroke volume (i.e., the area of a pressure-volume curve; Figure 1.4):

FIGURE 1.4: Pressure volume loop of the cardiac cycle and impact of preload, afterload, and inotropy. A) Cardiac pressure-volume loop and key physiologic events. B) Interactions between preload and afterload at constant inotropy. C) Interdependent effects of changes in afterload. D) Interdependent effects of changes in inotropy.

-   Increased preload → ↑SV (Frank-Starling)

-   Decreased afterload → ↑SV (increases linearly along the end-systolic pressure-volume relationship [ESPVR])

-   Increased contractility → ↑xSV (increased slope of ESPVR line)