Page 378 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

248 PART 3: Cardiovascular Disorders

dilution, as described above. These two systems analyze the arterial measures to define volume responsiveness. However, over the past 15
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pulse differently. The PiCCO system uses a pulse contour analysis simi- years numerous studies have validated the utility of positive-pressure
lar to that originally described by Hamilton and Remington in 1947, ventilation-induced dynamic changes in either arterial pulse pressure
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and the LiDCO system uses a pulse power analysis. The LiDCOrapid™, or stroke volume, referred to as pulse pressure variation (PPV) 10,11 and
Vigileo™ (Edwards Life Sciences), and MostCare™ (Vytech, Padova, Italy, stroke volume variation (SVV), 53,54 respectively, to predict which criti-
using the Pressure Recording Analytical Method [PRAM]) systems are cally ill patients will be volume responsive (see chap. 34). A threshold
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noncalibrated and estimate cardiac output from the arterial pressure value of either PPV or SVV >10% to 15% defines volume responsive-
pulse using assumptions about arterial compliance using proprietary ness when patients are passive while ventilated with a tidal volume of
algorithms. The LiDCOrapid uses the pulse power approach of its pre- 8 mL/kg or more. These parameters are not accurate during arrhyth-
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decessor LiDCOplus, whereas the Vigileo estimates arterial compliance mias and spontaneous breathing because of varying R-R intervals and
from the wave form pressure distribution and the MostCare relies only ventricular interdependence-induced changes in LV diastolic compli-
on the raw arterial pressure waveform. Recent head-to-head compari- ance, respectively. In those cases, one can perform a passive leg raising
sons of most of the invasive and minimally invasive devices in critically maneuver and note the transient increase in cardiac output. Postural
ill patients demonstrated significant intradevice variability, suggesting changes such as passive leg raising have been used for many years as
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that if one were to use these devices, it is best to use one or two devices a means to transiently increase venous return. The legs are raised to
only and become familiar with their strengths and limitations, rather 30° above the chest and held for 1 minute and the maximal increase in
than use several over time in the same patient. The level of accuracy and cardiac output recorded. This maneuver approximates a 300-mL blood
precision of each device needs to be understood as the data cannot be bolus in a 70-kg patient that persists for approximately 2 to 3 minutes. 56
superimposed from one system to another. The main advantage of these The excitement about the arterial pulse contour analysis devices
arterial pressure–based cardiac output monitoring systems over PAC- comes from their ability to rapidly measure PPV, SVV, and also the
derived measurements is their less invasive nature. dynamic changes in cardiac output in response to PLR. These param-
Since all these devices presume a fixed relation between pressure eters are profoundly robust across multiple clinical trials and allow the
propagation along the vascular tree and LV stroke volume, if vascular bedside caregiver immediate insight into the volume responsiveness
elastance (reciprocal of compliance) changes, then these assumptions of their patient. 51,57 Furthermore, when coupled with preoptimization
may become invalid. Thus, a major weakness of any pulse contour device therapy in high-risk surgical patients a PPV minimization strategy
is the potential for artificial drift in reported values if major changes markedly improved outcome. Even when used as an adjuvant to
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in arterial compliance occur. These points were illustrated well for the document volume responsiveness using traditional fluid boluses, with
PiCCO device in an animal model (Fig. 32-2) but probably apply to the goal of achieving supranormal D O 2 levels, these monitoring devices
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a greater or lesser extent to all devices. In an attempt to understand the facilitate the easy implementation of treatment protocols that reduce
potential magnitude of this pathophysiologic effect, Hatib et al measured acute postoperative complications 59-63 and improve long-term patient-
both aortic and radial arterial resistance, compliance and impedance in centered outcomes. And at the end of the day, is not that why we care
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a pig model before and after the induction of hyperdynamic hypoten- for our patients?
sive endotoxic shock (Fig. 32-5). Normally, the arterial pulse pressure
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increases as blood flow moved peripherally owing to the normal elastic Acknowledgment: This study was supported in part by NIH grants
properties of the arterial tree. However, in the hyperdynamic hypotensive HL067181, and HL073198.
state of fully developed sepsis the pressure trend is reversed with higher Conflicts of interest: The author is a paid consultant to Edwards
central than peripheral pulse pressures owing to a marked increase in LifeScience and LiDCO Ltd and has stock options with LiDCO Ltd. The
peripheral vascular compliance presumably due to endotoxic vasople- author is the inventor of a University of Pittsburgh-owned US patent on
gia, whereas aortic compliance is increased, presumably due to intimal using pulse pressure and stroke volume variation to diagnose and treat
edema. Clearly, algorithms that were developed assuming one specific hemodynamic insufficiency.
physiological state will degrade in the other.

FUNCTIONAL HEMODYNAMIC MONITORING KEY REFERENCES

A primary question asked of all hemodynamically unstable patients is • Cannesson M, Besnard C, Durand PG, Bohe J, Jacques D. Relation
their response to volume resuscitation (ie, will they increase their car- between respiratory variations in pulse oximetry plethysmo-
diac output in response to fluid loading?). Knowing fluid responsive- graphic waveform amplitude and arterial pulse pressure in ven-
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ness would be a very valuable piece of information in planning acute tilated patients. Crit Care. 2005;9:R562-R568. [first clinical study
resuscitation strategies. Since efforts to rapidly increase blood flow are showing that the pulse oximeter plethysmographic density profiles
important to minimize tissue ischemia and organ dysfunction, defining could be used as a surrogate for arterial pulse pressure to assess
those treatments that will quickly increase cardiac output are essential. volume responsiveness]
Traditionally, bedside caregivers gave all hemodynamically unstable
and/or hypotensive patients a rapid fluid bolus to assess their volume • Gomez H, Torres A, Polanco P, et al. Use of non-invasive NIRS
responsiveness. If heart rate decreased, blood pressure increased, or during a vascular occlusion test to assess dynamic tissue O
2
other measure of tissue perfusion suggested improvement (eg, increased saturation response. Intensive Care Med. 2008;34:1600-1607. [first
urine output, increase sensorium, decreased serum lactate), the patients clinical study to examine the determinants of St O 2 change during the
were deemed volume responsive and given massive fluid resuscitation vascular occlusion test]
until no longer responsive. Hemodynamic monitoring serves to help • Hadian M, Kim H, Severyn DA, Pinsky MR. Cross-comparison
define those patients likely to be volume responsive and to monitor their of cardiac output trending accuracy of LiDCO, PiCCO, FloTrac
course during fluid resuscitation because volume overload itself is also and pulmonary artery catheters. Crit Care. 2010;14:R212. [clini-
dangerous. Static measures of cardiac filling pressure or even volumes cal study comparing most commercially available cardiac output
do not predict who is going to be volume responsive and who will not. monitoring devices to each other]
Using the traditional threshold of at least a 15% increase in cardiac out- • Hamilton WF, Remington JW. The measurement of stroke volume
put in response to a 300-mL colloid bolus to define volume responsive- from the pulse pressure. Am J Physiol. 1947;148:14-24. [the original
ness, Michard and Teboul in their systematic review were unable to study on use of pressure pulse to assess flow in a canine model]
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identify any consistent clinical data to support the use of these traditional

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