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242 PART 3: Cardiovascular Disorders

Recent reviews on the usefulness of hemodynamic monitoring to
CHAPTER Assessing the Circulation: identify and monitor therapy have been written. Importantly, a recent
1,2
32 Oximetry, Indicator Dilution, consensus statement by 16 experts underscores many of the principles
described below. The essential aspects of circulatory sufficiency, namely
3
and Pulse Contour Analysis the adequacy of D O 2 to sustain metabolic needs, can be assessed by a
combination of routinely available variables. For convenience these will
Michael R. Pinsky be separated into those that measure oxygenation (oximetry) and those
that measure flow (cardiac output).

KEY POINTS
TISSUE OXYGENATION
• No hemodynamic monitoring device will improve patient outcome
unless coupled to a treatment, which itself improves outcome.
• Low venous oxygen saturations need not mean circulatory shock Although mean arterial pressure is a primary determinant of organ perfusion, normo-
tension can coexist with circulatory shock.
but do imply circulatory stress, as they may occur in the setting of
hypoxemia, anemia, exercise, as well as circulatory shock. Since metabolic demand of tissues varies by external (exercise) and inter-
• There is no “normal” cardiac output, only one that is adequate nal (basal metabolism, digestion, fever) stresses, there is no “normal”
or inadequate to meet the metabolic demands of the body. Thus, cardiac output that the bedside caregiver can target and be assured of
targeting a specific cardiac output value without reference to meta- perfusion adequacy. Cardiac output is either adequate or inadequate
bolic need, or oxygen-carrying capacity of the blood, is dangerous. to meet the metabolic demands of the body. Thus, although measures
• Cardiac output is estimated, not measured, by all devices routinely of cardiac output are important, their absolute values are relevant only
used in bedside monitoring (though we shall call it measured in in the extremes and when targeting specific clinical conditions, such
this text). as preoptimization therapy, which will be described below. How then
• Cardiac output estimates using arterial pulse pressure contour does one know that circulatory sufficiency is present or that circulatory
analysis cannot be interchanged among devices and all suffer to a shock exists? Clearly, since arterial pressure is the primary determinant
greater or lesser extent by changes of peripheral vasomotor tone of organ blood flow, systemic hypotension (ie, mean arterial pressure
4
commonly seen in the critically ill. <60 mm Hg) must result in tissue hypotension. Organ perfusion pres-
• Since metabolic demands can vary rapidly, continuous or frequent sure can be approximated as mean arterial pressure (MAP) relative to
tissue or outflow pressure. If intracranial pressure or intra-abdominal
measures of cardiac output are preferred to single or widely spaced pressure increases, then estimating cerebral or splanchnic per-
individual measures. fusion pressure using MAP alone will grossly overestimate organ
• Integrating several physiologic variables in the assessment of the perfusion pressure. However, baroreceptors in the carotid body and
adequacy of the circulation usually gives a clearer picture than just aortic arch increase vasomotor tone to keep cerebral perfusion constant
looking at one variable. if flow decreases, and the associated increased systemic sympathetic
• Integrating cardiac output with other measures, like venous oxygen tone alters local vasomotor tone to redistribute blood flow away from
saturation, can be very helpful in defining the adequacy of blood flow. more efficient O extracting tissues to sustain MAP and global O con-
2
2
˙ . Thus,
sumption (V O 2 ) in the setting of inappropriately decreasing D O 2
although systemic hypotension is a medical emergency and reflects
severe circulatory shock, the absence of systemic hypotension does not
INTRODUCTION ensure that all tissues are being adequately perfused.
The goal of the cardiorespiratory system is to sustain adequate delivery Within this context measures of tissue and blood oxygenation become
of oxygen to the tissues and removal of carbon dioxide to meet their relevant because they define the amount of O within the tissues or
2
metabolic demands. Under normal conditions, this system has signifi- blood and their measures can often be made noninvasively and using
cant flow and O -carrying capacity reserve to handle all but the most indwelling venous or central venous catheters. This oximetric monitor-
2
demanding metabolic stresses or primary organ dysfunction. Indeed, ing offers the potential to assess continuous measures of the adequacy
once overt cardiorespiratory failure is present, the degree of cardiorespi- of blood flow. Furthermore, measures of anaerobic metabolism, such as
ratory impairment is often advanced with existing end-organ ischemic serum lactate levels or arterial blood gas base deficit, often confirm the
5,6
dysfunction. Hemodynamic monitoring plays an important role in the existence of tissue ischemia. Noninvasive pulse oximetry allows for the
management of these critically ill patients with cardiovascular dysfunc- continuous measure of arterial oxygen saturation and is universally used
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tion. It is profoundly useful in the titration of therapies in patients in acute care settings, despite no evidence that it improves outcomes.
with known cardiovascular disease processes, like hemorrhagic shock, Pulmonary artery catheterization allows for the continuous measure of
acute mitral regurgitation, cor pulmonale, left-sided heart failure, and mixed venous oxygen saturation (Sv O 2 ) through reflectance oximetry.
vasoplegic shock wherein knowing the underlying pathophysiologic Central venous catheterization, commonly used to secure a stable intra-
process, physiologic state, and titrating-specific therapies represents vascular infusion site, allows for the sampling of superior vena caval
the centerpiece of cardiovascular support. However, monitoring is also central venous oxygen saturation (Scv O 2 ) and by reflectance oximetry
useful in identifying problems before they deteriorate to shock and/or continuous Scv O 2 )measures as well. The principles behind these mea-
in the management of high-risk patients with proven therapies. In this sures, their usefulness, and limitations are described below.
chapter, we will discuss monitoring the elements of oxygen delivery Hemoglobin varies in its light absorption spectrum as it binds to
), namely cardiac output and blood oxygen saturation. Hemoglobin oxygen, carbon monoxide, nothing (deoxygenation), or is in the ferric
(D O 2 state (methemoglobin) (Fig. 32-1). All oximeters estimate blood or tissue
, but usually does not change rapidly
is the other variable defining D O 2 O saturation by measuring the differential absorbance bands of various
and can be estimated from venous oximetry. Although circulatory shock paired or triple spectral signals across the 600 to 1000 nm bandwidth.
2
to meet metabolic demands, targeting-specific
occult or obvious tissue hypoperfusion and ischemia is not only unwar- ■ ARTERIAL PULSE OXIMETRY
is due to inadequate D O 2
values across all patients in an attempt to prevent
cardiac output or D O 2
ranted but potentially harmful, as the needs of different patients and the Arterial blood O saturation (Sa O 2 ) can be estimated quite accurately at
2
same patient over time can vary widely. the bedside using pulse oximetry. The routine use of pulse oximetry
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