Page 327 - Clinical Application of Mechanical Ventilation

Hemodynamic Monitoring 293

Stroke Volume and Stroke Volume Index

Stroke volume (S.V.) is calculated by dividing the cardiac output (C.O.) by the heart
rate (HR). The stroke volume index is calculated by dividing the stroke volume by
the body surface area (BSA).

C.O.
S.V. =
HR
S.V.
S.V.I. =
BSA
The stroke volume is determined by three factors: contractility, preload, and
contractility: Pumping strength
of the heart. Contractility may be afterload. Contractility is the pumping strength of the heart. Some conditions that
increased by improving the blood may lower the contractility of the heart include extremes of myocardial compliance
volume or by positive inotropic
medications. (too high or too low), and excessive end-diastolic volume. Preload is the end-diastolic
stretch of cardiac muscle fiber, expressed in pressure units (mm Hg or cm H O).
2
Hypovolemia and shock are two conditions that usually cause a decreased preload.
The stroke volume is Afterload is the tension or pressure that develops in the ventricle during systole (con-
determined by three factors:
contractility, preload, and traction), expressed in pressure units (mm Hg or cm H O). Afterload is usually
2
afterload. increased in conditions of downstream flow obstruction or excessive volume.

Oxygen Consumption and Oxygen
Consumption Index

The oxygen consumption reflects the amount of oxygen consumed in one min. The
oxygen consumption index reflects the amount of oxygen used relative to the body
size. They are calculated as follows:
# #
VO = Q * C(a@v)O 2
2
T
#
# VO
VO index = 2
2
BSA
Pulmonary Vascular Resistance

The pulmonary vascular resistance (PVR) measures the blood vessel resistance to
pulmonary vascular
resistance (PVR): Resistance of blood flow in the pulmonary circulation. For example, PVR is elevated in pulmo-
the arterial system into which the nary hypertension or left heart obstruction (e.g., mitral valve stenosis).
right heart is pumping. Normal
range is 50–150 dynes.sec/cm . 5 (PAP - PCWP) * 80
PVR =
C.O.

Systemic Vascular Resistance

The systemic vascular resistance (SVR) measures the blood vessel resistance to blood
flow in the systemic circulation. For example, SVR is elevated in systemic hypertension.
(MAP - RAP) * 80
SVR =
C.O.

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