Page 508 - Cardiac Nursing

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484 P A R T III / Assessment of Heart Disease
beat-to-beat changes in pulse pressure reflect changes in LV SV. obtained using raw versus processed data from a standard pulse
Unlike the SBP, which is affected by pleural pressure changes, the oximeter or other noninvasive devices (e.g., Finapres) is a sensitive
pulse pressure is affected by only the SV, because the pleural pres- and specific indicator of fluid responsiveness in patients with sep-
sure equally affects the systolic and diastolic pressure. The ab- sis or undergoing cardiac or liver transplantation surgery. 251,267–270
solute PPV% and the threshold to indicate fluid responsiveness is However, performance of POP at the bedside is not feasible.
directly related to the tidal volume (Table 21-8). 246,256 For exam- The PVI, which is a proprietary algorithm embedded in a
ple, in critically ill patients receiving a tidal volume less than 8 standard pulse oximeter system, automatically calculates the
mL/kg the threshold for fluid responsiveness was a PPV% 8%, POP. 266 On a pulse oximeter the perfusion index (PI) is an in-
in contrast to a 8 mL/kg tidal volume where the threshold was dicator of the adequacy of signal quality. The PI, which reflects
a PPV% 12%. 246 After a bolus in a fluid responsive patient the the amplitude of the pulse oximeter waveform, is determined by
PPV will generally decrease, indicating less preload dependence indexing the infrared pulsatile (AC) oximeter signal caused by the
(a shift up a given ventricular function curve), and the greater the pulsating arterial inflow (thought to reflect the beat-to-beat
decrease in the PPV the greater the increase in CI. 233 Changes in changes in SV) against the nonpulsatile (DC) infrared signal,
contractility may also affect the absolute PPV. 256 which reflects the constant amount of light from the pulse oxime-
ter that is absorbed by the skin and nonpulsatile blood flow. The
Detection of Occult Hemorrhage PVI reflects the change in the PI amplitude over a single respira-
The changes in SPV or PPV may also be useful indicators of hem- tory cycle (PVI [(PImax PImin)/PImax] 100). In 25 car-
orrhage or occult blood loss. 257 In experimental hemorrhage in diac surgery patients with a tidal volume of 8 to 10 mL/kg, a PVI
cardiac surgery patients, a change in SPV greater than 4 mm Hg 14% discriminated between responders (↑CI 15%) and non-
was indicative of a significant blood loss. 258 Conversely, in pa- responders with a sensitivity of 81% and 100% specificity, area un-
tients undergoing therapeutic phlebotomy, SPV less than 5 mm der the curve (AUC 0.92), and the PVI was comparable to PPV%
Hg was considered to indicate an absence of hypovolemia. 259 In a (AUC 0.94) and POP (AUC 0.94). 271 In spontaneously
recent animal study, the PPV% increased significantly (12.6% breathing patients, the POP detects changes in intravascular vol-
1.4% to 15.8% 2.0%, p .05) with 18% blood loss, whereas ume 272 and POP 273 and PVI (threshold 19%, sensitivity
the HR, MAP, CVP, PAOP, and SPV did not change significantly 82%, specificity 57%) also predict fluid responsiveness in con-
until there was a 36% blood loss. 260 Caution must be exercised junction with passive leg raising (PLR). 274
when interpreting these values as the VT varied between studies. Factors that may affect the PVI measurement include location of
In addition, hypotension may artificially increase the SPV% and the oximeter probe (finger versus ear or forehead), 275,276 arm posi-
PPV% because of the inclusion of absolute SBP or PP (pulse pres- tion as it affects venous congestion (DC portion of the PI measure-
sure) in the denominator of the equation. 245 ment), and the loss of the signal with severe vasoconstriction. PVI
measurements cannot be obtained in patients with cardiac arrhyth-
Stroke Volume Variation mias, and variations in tidal volume will affect the absolute PVI val-
ues and threshold interpretation. Further research is needed in un-
Stroke volume variation (SVV), which is a derived volumetric in- stable patients, particularly those with changes in vascular tone, in
dicator, can be continuously measured using pulse contour analy- different patient populations and using different proprietary sys-
sis or esophageal Doppler. The SVV is defined as the change in SV tems as the algorithms and thus the PVI values may vary between
over a 30-second period. systems.
V
V
(SV max SV min )
SVV% 100 Limitations of Functional Measures
V
[(SV max
SV min )> 2 ]
V
There are limitations to the use of functional measurements. The
The assumption underlying SVV is that the observed SV SPV, PPV, and SVV cannot be monitored in a spontaneously
changes are respiratory-induced variations. The absolute SVV is breathing patient due to variation in pleural pressure change, 277
also directly related to VT (Table 21-8), and as with other volu- although PLR (described below) and evaluation of the RAP can
metric measurements, the SVV is more closely associated with be performed in these patients. 278 Patients with cardiac arrhyth-
changes in SV than are changes in PAOP and CVP. 249,261 mias have been excluded from all studies; thus, the use of these
Concern has been voiced regarding the method used to measure measures cannot be recommended in this population. There is
the SVV (direct SV measurement versus pulse contour analysis) 262 also limited research in patients with decreased ventricular func-
as reflected in the contradictory results of the ability of SVV to pre- tion. 249
dict fluid responsiveness. 263,264 The contradictory results may re- Changes in tidal volume, 246,279 PEEP, 280 and pulmonary com-
flect differences in the tidal volume, which affects the absolute pliance will alter the magnitude of the ventilator-induced change in
SVV, 265 or the hemodynamic status of the patients studied (stable these indices. The absolute values of these indices are directly related
versus hypovolemic). Finally, to achieve a stable tidal volume, SVV to VT (see Table 21-8). The VT also affects the sensitivity and speci-
analysis can be performed only in patients who are on controlled ficity of the threshold values. In addition, at a low VT (6 mL/kg)
mechanical ventilation and are heavily sedated/paralyzed. the patient may be fluid responsive, yet not reach the threshold be-
cause of insufficient ventilator-induced variation. 281 Conversely, at
Pleth Variability Index a high VT the patient may exceed the threshold because of in-
creased swings in pleural pressure and not fluid responsiveness.
A new noninvasive functional indicator that is under investigation A change in vascular tone may also affect the absolute values
is the pleth variability index (PVI). 266 The respiratory variation in and the thresholds indicating fluid responsiveness. For example, in
pulse oximeter plethysmographic amplitude ( POP), which is an animal model of hemorrhage, the SPV% and PPV% increased