Page 492 - Cardiac Nursing

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468 P A R T III / Assessment of Heart Disease
a derivation of Ohm’s law (pressure flow resistance), if re- VENTRICULAR FUNCTION CURVES
sistance remains constant, there is a direct relationship between
pressure and flow. However, clinically, resistance is seldom con- Knowledge of the relationship between preload, afterload, contrac-
stant. Thus, BP may appear adequate while flow is decreased, or tility, and SV is essential for effective hemodynamic monitoring
conversely BP may be low although perfusion remains ade- and guiding therapeutic actions that modify these hemodynamic
quate. Algorithms and evidence-based guidelines are available variables. Ventricular function curves demonstrate the interaction
V
V
to guide assessment of the physiological and technical factors between preload, afterload, and contractility and the effects of var-
that affect direct and indirect BP measurements and outline the ious disease processes (heart failure [HF], hemorrhage) and thera-
correct performance and interpretation of these BP measure- peutic actions (vasodilator or inotropic drug therapy) on SV and
ments. 22,23,97,98 CO (Fig. 21-8). The family of curves varies for each patient but is
In addition to the physical factors that cause the differences in useful in predicting and evaluating the effects of various thera-
arterial pressure measured in various locations in the body, there peutic interventions. The curves are constructed by plotting the
are also technical factors that affect measurement accuracy (see PA occlusion pressure (PAOP) (or some measure of end-diastolic
Fig. 21-3). 23,99,100 In addition, the oscillometric system directly volume or preload) on the horizontal axis and the CO, CI, or SV
measures the mean pressure and extrapolates the systolic and di- on the vertical axis. A key point is that an increase in SV in re-
astolic pressure based on an algorithm, which may affect the ac- sponse to a fluid bolus (change in preload) cannot be reliably pre-
curacy of the systolic and diastolic pressure measurements. dicted on the basis of the standard preload indices (CVP, PAOP)
For direct and oscillometric BP monitoring the correct refer- or volumetric indices (right ventricular [RV] end-diastolic vol-
ence is the heart. If the transducer or the arm is positioned above ume, global end-diastolic volume), because the response depends
the heart, there will be a decrease in the measured pressure. Con- on ventricular function, as indicated by the slope of the ventricu-
versely, if the transducer/arm is positioned below the heart, there lar function curve. 117 The traditional preload indices remain use-
will be an increase in the measured pressure. 101–103 When the pa- ful in the differential diagnosis and determining a patient’s risk for
tient is in the sitting position, for oscillometric or auscultated BP pulmonary edema.
measurements, the arm should be supported at the level of the
heart (level of the midsternum). If the arm is parallel to the pa-
tient or supported on the armrest the SBP and DBP may be
10 mm Hg higher than if the arm is supported horizontally at heart
level (level of the midsternum) 104–109 and in patients with hyper-
tension the difference in arm position may cause a 20 mm Hg
overestimation of SBP. 108 If the patient is in a lateral recumbent
position, the noninvasive BP measurements taken from the “up
arm” may be 13 to 17 mm Hg lower than those if the patient is
in supine position, and BP measurements from the “down arm” Cardiac Function (CO, SV)
are similar to those taken at supine position or inconsis-
tent. 110–112 If the “up arm” is used, the measured pressure can be
corrected by measuring the distance from the angle-specific phle-
bostatic axis (see Display 21-1) and correcting the pressure (1 cm
0.73 mm Hg or 1 in. 1.8 mm Hg).
Forearm BP measurements may be necessary in cases in which
access to the upper arm is not possible or an appropriate cuff is End-Diastolic Fiber Length
not available (i.e., the existence of morbid obesity or a conical- (PA Occlusion Pressure)
shaped arm). 113 Two factors need to be considered when compar- ■ Figure 21-8 Family of ventricular function curves representing
ing the BP from the upper arm and the forearm. First, if the arm normal, depressed, and severely depressed function. A change in pre-
is in a dependent position, the hydrostatic pressure increases the load is represented by a move up or down a single curve (Frank–Star-
BP in the forearm relative to the upper arm. To correct for this hy- ling principle). Point A to point B and point B to point A reflect an
drostatic effect, the arm should be supported horizontally at heart increase and decrease, respectively, in preload. The response to vol-
ume loading is dependent on the position on the ventricular function
level. The second factor is that the SBP is normally higher in the curve and the shape of the curve. If both ventricles are on the steep
periphery (forearm upper arm), although there is limited re- portion of the curve the SV will increase in response to volume (re-
search comparing noninvasive upper arm and forearm BP in he- sponder). In contrast if the heart is on the flat portion of the curve the
modynamically stable patients. 114–116 SV will not increase (nonresponder). A change in afterload results in
A challenge when performing BP measurements in individ- a shift in the curve that appears similar to that caused by contractil-
uals who are morbidly obese is finding an appropriately sized ity, although the mechanism is different. Point D to E reflects the net
cuff. For every 5 cm increase in arm circumference (starting at effect of a decrease in afterload on a failing heart. This upward and
35 cm) use of a standard cuff leads to an overestimation of SBP lateral shift is the result of two actions. Point D to C reflects an in-
by 3 to 5 mm Hg and DBP by 1 to 3 mm Hg compared with crease in force of contraction and point C to E a decrease in preload
an appropriately sized large cuff. 100 To correctly size the cuff, due to increased systolic ejection. A change in contractility is repre-
sented by an upward or downward shift of the curve, that is, for any
measure the arm circumference half the distance from the el- given preload and afterload, the CO is increased or decreased. In a
bow to the wrist. Cuff size should be similar to the guidelines failing heart, an additional effect of decreased contractility is an in-
97
for upper arm circumference. The cuff should be centered be- crease in preload due to decreased systolic ejection; thus, the net ef-
tween the elbow and wrist and the arm should be supported at fect of a decrease in contractility is to shift the curve down and to the
the level of the heart. left (Point C to G).