the right atrium, initiates the second phase of the
cardiac cycle. The AV node picks up, amplifies, and
focuses the electrical impulse that has passed
through the atria, sending it along the BUNDLE OF
HISand the bundle branches in wavelike fashion.
The impulse causes the ventricles to contract
simultaneously. The right ventricle pumps blood
to the lungs for OXYGENATION, and the left ventricle
pumps oxygenated blood into theAORTAfor the
arterial network to carry through the body.
The PULSErepresents a completed cardiac cycle.
The heart of an adult at rest completes about 80
cardiac cycles each minute. ARRHYTHMIA, VALVULAR
HEART DISEASE, CORONARY ARTERY DISEASE (CAD),
ISCHEMIC HEART DISEASE(IHD), congenital defects of
the heart, and damage to the heart such as occurs
with HEART FAILUREor HEART ATTACKare among the
conditions that can disrupt the cardiac cycle.
See also BLOOD PRESSURE; BUNDLE BRANCH; CON-
GENITAL HEART DISEASE; SICK SINUS SYNDROME.
cardiac enzymes Proteins the HEART releases
into the bloodstream when HEART ATTACKor other
circumstances cause damage to the heart MUSCLE
(MYOCARDIUM). BLOODtests that measure the levels
of these enzymes help doctors determine whether,
and how long ago, a heart attack has taken place.
All muscle tissue releases certain enzymes when
injured, so the combination of enzymes present in
the blood provides the most useful clues as to the
source of the injury. The cardiac-specific enzymes
that indicate heart attack are cardiac troponin-T
and cardiac troponin-I. The levels of these
enzymes in the blood rise 3 to 6 hours after dam-
age to the heart and remain elevated for 7 to 10
days.
Nonspecific enzymes that may suggest heart
attack include creatine kinase (CK), aspartate
aminotransferase (AST) and lactate dehydroge-
nase (LDH). Elevations of these enzymes occur
whenever there is significant damage to muscle
tissue of any kind. To cardiologists determining
whether a person has had a heart attack, it is the
rise and fall of the enzyme levels that are more
useful than the levels themselves at any one point
in time. Creatine kinase MB (CK-MB), one of the
protein components of creatine kinase, rises more
rapidly and dramatically when the damage is to
heart muscle, providing a strongly suggestive
marker. CK-MB rises within a few hours of heart
damage though returns to normal in about 24
hours. Cardiologists evaluate cardiac enzyme lev-
els in combination with other clinical evidence
such as ELECTROCARDIOGRAM(ECG) and ECHOCARDIO-
GRAM to confirm the diagnosis of MYOCARDIAL
INFARCTION(death of heart muscle cells due to lack
of oxygen).
See also CARDIAC CATHETERIZATION.cardiac intensive care unit (CICU) A specialized
unit within a hospital that provides comprehen-
sive medical care for people recovering from HEART
ATTACKor receiving treatment for other life-threat-
ening cardiovascular diseases. Large hospitals have
separate units for medical patients (such as those
who have had HEARTattacks but not surgery) and
surgical patients (such as those who have had
CORONARY ARTERY BYPASS GRAFT(CABG), heart valve
replacement, or other operations on the heart); in
smaller hospitals a single specialized unit provides
care for both kinds of patients. The nurses and
ancillary health-care staff who work in CICUs
have specialized training in using the monitoring
equipment and caring for patients who have seri-
ous cardiovascular conditions. Most CICUs restrict
visitors and visiting times to protect patients and
allow them to receive adequate rest as well as the
intensive nursing care their conditions require.
See also CARDIAC REHABILITATION; HEART TRANS-
PLANTATION; MECHANICAL VENTILATION.cardiac output The volume of BLOODtheHEART
pumps out to the body each minute. Cardiac out-
put is an important measure of the heart’s effi-
ciency. Many cardiovascular diseases, such as
ARRHYTHMIAand HEART FAILURE, can limit cardiac
output. Cardiologists measure cardiac output as
the combination of HEART RATEand stroke volume
(the amount of blood the left ventricle ejects into
the AORTAwith each contraction). There are sev-
eral methods for determining stroke volume,
including dye injection and thermal differential.
In a person whose cardiovascular system is
healthy, cardiac output increases with increased
physical activity such as exercise, in which both
heart rate and the force of the heart’s contractions
increase. A heart with damage due to disease such
as CARDIOMYOPATHYor HEART FAILURE, or as a conse-cardiac output 29