Page 395 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 34: Judging the Adequacy of Fluid Resuscitation 265

not produce variations in stroke volume unrelated to fluid status. Also, may be real. Pulmonary or cerebral edema, abdominal compartment
patients must be passively ventilated, without inspiratory or expiratory syndrome, acute right heart strain, and oliguria are all conditions that
respiratory muscle activation. Unless the patient is therapeutically para- raise the potential risk. Especially when these conditions are present,
lyzed, this requires carefully assessing the ventilator pressure and flow the clinician should attempt to identify patients unlikely to benefit from
waveforms, as well as examining the patient. Finally, acute RV dysfunc- fluids, in order to spare them potential harm. Which dynamic predictor
tion should be excluded by echocardiography in patients at risk of this. to use depends on available monitors; local expertise; and whether the
The first dynamic predictor to be used widely was PPV. In one of patient is passively ventilated or actively breathing.
the most influential studies, 40 subjects with sepsis were mechanically If careful assessment of patient and ventilator waveforms shows that
ventilated (tidal volumes of 8-12 mL/kg), therapeutically paralyzed, and the patient is passive, in sinus rhythm, without acute cor pulmonale,
instrumented with PACs. Four parameters (right atrial pressure [Pra], then the tidal volume should be adjusted temporarily to approximately
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PAOP, systolic pressure variation [SPV], and PPV) were judged for their 10 mL/kg predicted body weight. Then the variation in pulse pressure,
ability to predict the response to a fluid challenge. The areas under the IVC diameter, LVOT-VTI, stroke volume, or brachial artery flow
receiver operating characteristic curves for PPV and SPV (0.98 and velocity should be measured. If the degree of variation predicts fluid
0.91, respectively) were outstanding and far superior to those for Pra responsiveness, a discrete fluid bolus should be given and the circula-
and PAOP (0.51 and 0.40, respectively). Furthermore, a threshold value tion reassessed. This process of prediction and fluid bolus should be
for PPV of 13% (calculated as maximum pulse pressure minus mini- repeated until the indication for fluids has resolved; the circulation is
mum pulse pressure divided by the average and converted to percent) demonstrated not to be fluid responsive; or the predictor is judged to
discriminated responders and nonresponders with excellent sensitivity be a false positive. Not surprisingly, this approach will lead to treatment
and specificity. PPV has proved to be reliable in sepsis, hemorrhage, that differs from a CVP-based approach, usually by restricting fluids in
following cardiac surgery, and in other settings. 44,46,61-64 fluid-unresponsive patients with low CVP and more often prescribing
The same cardiopulmonary interactions that produce PPV also cause inotropes. There is little reason to prefer any one of the measures of
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variations in arterial blood flow velocity. Transesophageal echocardiog- variation, although measuring the LVOT-VTI demands more expertise
raphy has been used to judge aortic flow variability prior to fluid chal- than the arterial catheter-based measures or the other ultrasonographic
lenge, showing excellent performance in discriminating responders and techniques. If the patient is passive but the preconditions for validity of
nonresponders. 31,47,65 Similar utility has been shown by measuring blood ventilation-induced predictors are not met, PLR is recommended with
flow velocity in the brachial artery, a less invasive approach. 66,67 measurement of the effect at 1 minute.
During passive mechanical ventilation, inferior vena caval diameter If the patient is breathing actively, PLR is the best validated predictor.
tends to increase during lung inflation (as Pra rises) and tends to Alternatives include the degree of inspiratory collapse of the IVC, where
decrease during expiration (to the extent that the heart is on the steep greater than 50% appears to predict a fluid response based on anecdotal
portion of the Starling curve). In two separate studies of ventilated, experience. The inspiratory fall in the right atrial pressure can also be
septic subjects, IVC diameter variation was highly accurate in predict- used but, like the inspiratory collapse of the IVC, validity is uncertain.
ing fluid responsiveness (eg, positive and negative predictive values
93% and 92%, respectively). 34,37 Because the superior vena cava (SVC)
is surrounded by pleural, rather than abdominal, pressure, it may be CONCLUSION
preferable for predicting fluid responsiveness. In one study of patients
with sepsis and acute lung injury, an SVC collapsibility index above After the initial fluid resuscitation, many patients who have traditional
36% predicted a significant, fluid-induced rise in cardiac output with a indications for a fluid challenge will not actually respond. Such fluid
sensitivity of 90% and specificity of 100%. 68 challenges may not be only ineffective, but harmful. While further stud-
Finally, the inspiratory decrement in right atrial pressure has been ies should attempt to confirm and quantify this harm, we think that
proposed to identify fluid responders among spontaneously breathing current knowledge is sufficient to guide practice safely. We advocate
patients, and could serve as an alternative to PLR. Right atrial pressure that fluid boluses be considered critically rather than simply being
tends to fall in those with a fluid-responsive circulation, but not in given reflexively. When a patient develops indications for a fluid bolus,
nonresponders: In 33 mixed medical and surgical ICU patients, some the potential for harm should be considered and, if there is reasonable
of whom were mechanically ventilated but actively inspiring (assured by potential for harm, a dynamic predictor should be used to limit fluid
noting at least a 2-mm Hg inspiratory fall in PAOP), an inspiratory drop infusion only to patients who are likely to benefit. We believe there is
in Pra (measured at the base of the a wave) less than 1 mm Hg served room for much further study to identify whether this approach confers
to predict responsiveness to an adequate fluid bolus. Cardiac output improved outcomes in critically ill patients.
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increased by at least 250 mL/min in 16 of 19 patients with a positive
inspiratory response and only 1 of 14 patients with a negative response.
The importance of an adequate inspiratory fall in pleural pressure, nec- KEY REFERENCES
essary to shift the cardiac function curve sufficiently, was emphasized
by a study of 21 mechanically ventilated subjects, also actively inspiring. • Cavallaro F, Sandroni C, Marano C, et al. Diagnostic accuracy of
The inspiratory change in Pra did not distinguish fluid responders from passive leg raising for prediction of fluid responsiveness in adults:
nonresponders, perhaps because the ventilatory assistance prevented systematic review and meta-analysis of clinical studies. Intensive
much fall in pleural pressure. This study calls into question the valid- Care Med. 2010;36:1475-1483.
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ity of the inspiratory fall in CVP. Further, this predictor relies on the • De Backer D, Heenen S, Piagnerelli M, et al. Pulse pressure varia-
absence of expiratory muscle activation and a regular cardiac rhythm. tions to predict fluid responsiveness: influence of tidal volume.
Intensive Care Med. 2005;31:517-523.
A PRACTICAL APPROACH • Feissel M, Michard F, Faller JP, et al. The respiratory variation in
inferior vena cava diameter as a guide to fluid therapy. Intensive
When persistent or recrudescent hypotension, tachycardia, or oliguria Care Med. 2004;30:1834-1837.
raises the question as to whether fluids would be helpful, the intensivist • Kumar A, Anel R, Bunnell E, et al. Pulmonary artery occlusion
should estimate the probability of harm from a fluid bolus. For many pressure and central venous pressure fail to predict ventricular
patients, the risks of fluid expansion are trivial and, in such a case, an filling volume, cardiac performance, or the response to volume
adequate fluid bolus should be infused rapidly, while measuring clini- infusion in normal subjects. Crit Care Med. 2004;32:691-699.
cally relevant outcomes. For others, however, the risks of fluid infusion

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