Page 127 - Critical Care Nursing Demystified

112 CRITICAL CARE NURSING DeMYSTIFIED

an LVAD. The second category of patients requiring an LVAD are those who
need circulatory support until a heart transplant can be performed. This cat-
egory is termed “bridge to transplant.” The LVAD is designed to support a
failing heart via flow assistance. The pump diverts quantities of systemic
blood flow around a failing ventricle. Ultimately, cardiac workload will be
reduced and circulation maintained. Flow rates between 1 and 6 L per minute
are used to maintain adequate cardiac output while decreasing ventricular
workload.

Cardiovascular Medications Used in Critical Care

5 Medications that affect the CV system may best be explained by how they
affect the patient’s stroke volume (SV). Recall that the cardiac output is the
amount of blood the heart pumps throughout the system in 1 minute and is
calculated by taking the heart rate times of the SV. The SV is the amount of
blood that is pushed out of the left ventricle with each heartbeat. SV is made
up of three components: preload, contractility, and afterload. The compo-
nents of SV are manipulated by cardiac medications in order to improve Downloaded by [ Faculty of Nursing, Chiangmai University 5.62.158.117] at [07/18/16]. Copyright © McGraw-Hill Global Education Holdings, LLC. Not to be redistributed or modified in any way without permission.
cardiac performance. First, we will look at medications that affect the preload
of the heart.

Preload

Preload is defined as the ability of the heart’s muscle fibers to stretch at the
end of diastole. The components that affect this ability to stretch are deter-
mined by the amount of blood volume or pressure of that blood returning to
the heart. Any medications that increase the preload will increase the stroke
volume and therefore the workload of the heart, thus increasing the heart’s
oxygen need. The opposite occurs as well. Any medication that decreases the
preload will decrease the stroke volume and ultimately decrease the workload
of the heart. In diseased hearts, the critical care nurse will use more of the
latter type of medications. Table 3–6 shows medications that enhance or
reduce preload.
Medications like nitroglycerin and sodium nitroprusside (Nipride) are
called vasodilators because they work to decrease the amount of wall tension
in the arteries and veins, thus decreasing or reducing the preload. A decrease
in pressure in the vascular circuit results in a decrease in peripheral vascular
resistance (PVR). The decrease in PVR results in a decrease in blood pressure.