CHAPTER 4 • CARDIAC FUNCTION
67
■ FIGURE 4.3 Summary of normal pressures
w ithin the cardiac chambers and great vessels.
The higher of the tw o pressure values (expressed
in mm Hg) in the right ventricle
(RV),
left ventricle
(LV), pulmonary artery
(PA),
and aorta
(Ao)
rep-
resent the normal peak pressures during ejection
(systolic pressure), whereas the lower pressure
values represent normal end of diastole pres-
sure (ventricles) or the lowest pressure (diastolic
pressure) found in the PA and Ao. Pressures in the
right atrium (RA) and left atrium (LA) represent
average values during the cardiac cycle.
heart. Note that the pressures on the right
side of the heart are considerably lower than
those on the left side of the heart, and that
the pulmonary circulation has low pressures
compared to the systemic arterial system. The
pressures shown for the right and left atria
indicate an average atrial pressure during the
cardiac cycle— atrial pressures change by sev-
eral mm Hg as they fill and contract.
Ventricular Pressure-Volume
Relationship
Although
measurements
of pressures
and
volumes over time can provide important
insights into ventricular function, pressure-
volume loops provide another powerful tool
for analyzing the cardiac cycle, particularly
ventricular function.
Pressure-volume loops (Fig. 4.4, bottom
panel) are generated by plotting left ventric-
ular pressure against left ventricular volume
at many time points during a complete car-
diac cycle (Fig. 4.4, top panel). In Figure 4.4,
the letters represent the periods of ventricu-
lar filling (a), isovolumetric contraction (b),
ventricular ejection (c), and isovolumetric
relaxation (d). The EDV is the maximal vol-
ume achieved at the end of filling, and ESV
is the minimal volume (i.e., residual volume)
of the ventricle found at the end of ejection.
The width of the loop, therefore, represents
the difference between EDV and ESV, which is
the SV. The area within the pressure-volume
loop is the ventricular stroke work.
The filling phase moves along the end-dias-
tolic pressure-volume relationship (EDPVR),
or passive filling curve for the ventricle. The
slope of the EDPVR at any point along the
curve is the reciprocal of ventricular compli-
ance, as described later in this chapter.
The maximal pressure that can be devel-
oped by the ventricle at any given left ventric-
ular volume is described by the end-systolic
pressure-volume relationship (ESPVR). The
pressure-volume loop, therefore, cannot cross
over the ESPVR, because the ESPVR defines
the maximal pressure that can be generated
at any given volume under a given inotropic
state, as described later in this chapter.
The changes in pressures and volumes
described in the cardiac cycle diagram and
by the pressure-volume loop are for normal
adult hearts at resting heart rates. Pressure-
volume loops appear very differently in the
presence of valve disease and heart failure as
described in Chapter 9.
CARDIAC OUTPUT
The primary function of the heart is to impart
energy to blood to generate and sustain an
arterial blood pressure sufficient to adequately
perfuse organs. The heart achieves this by con-
tracting its muscular walls around a closed
chamber to generate sufficient pressure to pro-
pel blood from the left ventricle, through the
aortic valve, and into the aorta. Each time the
left ventricle contracts, a volume of blood is
ejected into the aorta. This SV, multiplied by the
number of beats per minute (heart rate, HR),
equals the cardiac output (CO) (Equation 4-1).
Eq. 4-1
CO = SV • HR
Therefore, changes in either SV or heart rate
alter cardiac output.
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