Shock Syndromes I

SVR (also termed total peripheral resistance) is the resistance to flow that must be overcome by the left

ventricle.

SVR is the major determinant of LV afterload.

Systemic vasoconstriction increases SVR, whereas vasodilation decreases SVR.

Skin temperature may be used as an approximation (surrogate) of SVR, in which warm skin

temperature suggests decreased SVR (vasodilation) and cold skin temperature suggests increased

SVR (vasoconstriction).

The right ventricle better tolerates increases in ventricular volume (preload) than increases in afterload.

Contrarily, the left ventricle better tolerates increases in afterload than increases in ventricular volume.

Coronary artery perfusion occurs primarily in diastole. Aortic diastolic pressure must be sufficient to

ensure perfusion of the coronary arteries.

The circulatory system delivers oxygen and vital nutrients to the tissue beds for homeostasis and end-

organ function.

Oxygen bound to hemoglobin is then transported by CO to the tissues. The rate of Do2 is the product

of the CO and the arterial oxygen content (Cao2), as described in Table 1. Metabolic function of tissue

beds requires consistent Do2.

At the tissues, oxygen dissociates from hemoglobin and is taken up by the mitochondria through

systemic capillaries for aerobic metabolism. Oxygen uptake or Vo2 is the rate at which oxygen transfers

from systemic capillaries into the tissues and is the byproduct of CO, Cao2, and venous oxygen content

(Cvo2).

The Fick equation states that CO = Vo2/(Cao2 − Cvo2).

The oxygen extraction ratio (O2ER), or the ratio of Vo2/Do2, is 20%–30% at resting state, meaning that

about 25% of the oxygen delivered to the capillaries is taken up by the tissues. The O2ER is relatively

stable and can accommodate temporary fluctuations in Do2 or Vo2. Sustained Do2/Vo2 mismatches

contribute to tissue hypoxia and deranged metabolic function.

Treatment of shock syndromes should be rapid to minimize permanent tissue and organ damage.

In the early stages of a shock state, blood pressure is preserved through stimulation of the sympathetic

system, release of endogenous vasopressin, and vasoconstriction through the formation of angiotensin

II. The synergy of these actions preserves blood flow and Do2 to vital organs.

Blood flow is prioritized to maximize Do2 to the heart and brain. Consequently, blood flow to extravital

organs (e.g., skin, gut, kidneys) is redirected.

decreases, and overt shock develops.

Hemodynamic variables may be obtained through noninvasive or invasive monitoring devices

(Table 2).

hemodynamic assessment is recommended.