assumes that anything that pulses within the tissue bed is arterial blood, hence the name pulse oximeter.

Actually, the pulse oximeter measures the ratio of the pulsatile component of red light absorbed to the

pulsatile component of the infrared light absorbed. This ratio changes with SaO2

. The exact relation

between this ratio and SaO2 has been empirically determined from volunteer studies and is programmed

into the electronics of the oximeter. If any artifacts occur in a pulsatile nature, they may be erroneously

integrated into the equation, causing erroneous SaO2 estimates.

Several things should be remembered when interpreting a pulse oximeter’s output. First, the device

measures SaO2 and not arterial oxygen tension (PaO2

). The PaO2 must drop below 80 mm Hg before

any significant change in SaO2 occurs. As the PaO2 drops below 60 mm Hg, the SaO2

rapidly falls as the

inflection point of the sigmoidal oxyhemoglobin dissociation curve is approached. As a rough rule of

thumb, as SaO2 drops below 90%, the PaO2 can be estimated by subtracting 30 points from the SaO2

.

For example, a SaO2 of 85% corresponds to a PaO2 of 55 mm Hg. Second, the pulse oximeter measures

saturation (milliliters of oxygen per deciliters of blood) and not arterial content or oxygen delivery to

tissues.

Because a traditional pulse oximeter uses only two wavelengths of light, it cannot detect the presence

of carboxyhemoglobin (carbon monoxide poisoning) or methemoglobin. Recently introduced pulse

oximeters incorporate additional wavelengths of light and are capable of detecting carboxyhemoglobin

and methemoglobin ratios as well.58 In addition, continuous measurement of hemoglobin (mg/dL) via

certain pulse oximeters allows the trending of intraoperative hemoglobin during procedures involving

moderate to large blood loss, guiding transfusion therapy. Although the margin of measurement error is

as large as 1 mg/dL when compared to central laboratory complete blood counts, the data do offer a

trend and may reduce unnecessary transfusions in clinical settings without access to point of care blood

gas machines.

Ventilation Monitors

By definition, a patient is appropriately ventilated when arterial carbon dioxide tension (PaCO2

) is 40

mm Hg. Measuring the respiratory rate can document only the presence of ventilation, not its adequacy.

Capnography, or end-tidal CO2 monitoring, is the visual display of the CO2 concentration at the airway.

To understand the utility of capnography, one must understand dead-space (DS) components and how

they affect CO2

removal from the body.59 DS is defined as the portion of the tidal volume (VT) that does

not participate in gas exchange.

VT = DS - VA

The alveolar volume (VA) is the volume of the inspired gas that reaches well-perfused alveoli. The

remainder of the VT, which equals the DS, can be divided into three subcomponents: apparatus dead

space (DSap

), anatomic dead space (DSan

), and alveolar dead space (DSal). At the end of inspiration, the

respiratory apparatus (e.g., endotracheal tube) is filled with inspired gas that should not contain CO2

.

Similarly, all the anatomic airways (trachea, bronchi, and all conducting airways down to the alveoli)

should be filled with inspired gas and should therefore contain no CO2

. In this model, there are two

types of alveoli: those that are well perfused and those that are not perfused. The alveolar gas should

completely equilibrate with the arterial blood and contain CO2 at the same tension as the arterial blood;

ideally, PaCO2 should equal 40 mm Hg. As the patient expires, the CO2 detected at the patient’s mouth

first reflects the DSap gas having no CO2

; followed by the DSan gas, again with no CO2

; and finally the

alveolar gases, containing both DS and well-perfused alveolar gas. When mixed alveolar gas reaches the

airway, it produces a rapid rise in the CO2 concentration to a level somewhere between the

concentration in the alveolar gas (40 mm Hg) and the DSal (0 mm Hg), depending on each component’s

proportion of volume. For example, if half of the alveoli are DSal and PaCO2 equals 40 mm Hg, then the

plateau value of the capnogram should be 20 mm Hg, implying that half of the alveoli are not being

perfused. With inspiration, the CO2 value again drops to 0 until another expiration, and a square wave

appears again as the alveolar gas is detected at the mouth. With each breath, there should be a square

wave, whose height approaches the PaCO2 value as the amount of the DSal gas approaches 0.

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Algorithm 13-2a. Algorithm for managing a patient on chronic buprenorphine therapy. APS, acute pain service; ICU, intensive care

unit; PCA, patient-controlled anesthesia; NSAIDs, nonsteroidal anti-inflammatory drugs.

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Algorithm 13-2b.

In a healthy young adult, there is no significant DSal gas, and the end-tidal CO2 value equals the

PaCO2

. Therefore, the difference between these values indicates the proportion of DSal in the patient.

The presence of a capnogram itself implies that there is metabolism (the production of CO2

), circulation

(blood flow to the lungs), and ventilation (respiratory rate and an intact ventilator circuit).

Providing this information on a breath-to-breath basis, the continuous capnogram is extremely useful

in many critical situations. It can be used as a surveillance monitor of both the respiratory circuit and

the cardiovascular system. Any acute decrease in cardiac output will decrease blood flow to the lungs

and increase the DSal

, causing an acute drop in end-tidal CO2

. For this reason, the device was originally

used during neurosurgical procedures in the sitting position to detect the presence of air emboli. This

principle also allows the detection of pulmonary emboli or any acute drop in cardiac output. In fact, the

only acute catastrophic cardiopulmonary problem that will not be detected by the capnometer is arterial

desaturation. Therefore, the combination of the capnometer and the pulse oximeter creates a dynamic

duo for beat-to-beat and breath-to-breath surveillance of metabolism, circulation, ventilation, and

oxygenation.

Circulation Monitors

14 Hemodynamic stability can be monitored by a variety of methods, the most basic of which is

systemic arterial blood pressure. Intermittent, noninvasive measurement of systemic blood pressure

with an oscillometric blood pressure cuff is the standard in the operating room, and its accuracy equals

that of clinical measurements by auscultation. Blood pressure cuffs can be cycled as quickly as once per

minute, but when used for an extended duration, they should be cycled no more than once every 3 to 5

minutes. When tighter control or observation is required in patients with significant comorbidities or

large swings in hemodynamics due to surgical circumstances, invasive arterial monitoring is used.

Although pressure measurements provided by invasive techniques are different from those of

noninvasive techniques, they usually coincide closely. A continuous invasive arterial tracing can also be

used to assess the adequacy of fluid resuscitation by following the systolic pressure variation (SPV) with

positive-pressure ventilation. As positive-pressure ventilation impedes venous return within a closed

thorax, decreases in systolic pressure associated with a respiratory pattern can be detected. In patients

with sinus rhythm with stable cardiac contractility, the degree of SPV is inversely related to the

intravascular volume status of the patient. The normal range of SPV is 5 to 10 mm Hg. A systolic

pressure decrease of greater than 10 mm Hg during positive-pressure ventilation implies inadequate

preload and the need for more aggressive fluid resuscitation.

In this context, central venous access may be reserved for patients and procedures with the potential

for large, rapid volume resuscitation requirements or expected need for potent vasoconstrictors,

inotropes, or vasodilators not amenable to peripheral administration. Transesophageal

echocardiography (TEE) is now commonly used to assess cardiac function. This technique is easily used

in the anesthetized, intubated patient and can quickly assess systolic and diastolic function as well as

valvular dysfunction. Increasing familiarity in the use of TEE by noncardiac anesthesiologists may result

in the pulmonary artery catheter being reserved for very specific patients demonstrating the need for

pulmonary artery pressure monitoring or continuous cardiac output trending.

Finally, a wave of noninvasive continuous cardiac output monitors has gained the attention of

surgeons and anesthesiologists alike. These monitors, promising the realization of goal-directed therapy,

use indirect means to assess the missing aspect of blood pressure-focused hemodynamic management –

stroke volume. Thoracic electrical bioimpedance, pulse wave transit time, peripheral pulse contour

analysis, volume clamp, and other methods of estimating stroke volume have been in existence for

decades, but have seen a recent rise in interest as variation in intraoperative fluid management may

impact surgical outcomes. However, conflicting studies in real-world critically ill patients have failed to

establish that these monitors are capable of replacing traditional invasive monitoring routes or are

worth establishing as standards of care for all patients undergoing major surgery.60 Despite the absence

of compelling clinical data, many enhanced recovery protocols have begun including some type of

noninvasive cardiac output monitoring to guide fluid therapy.61 It is unclear whether these monitoring

devices offer superior guidance to sound clinical judgment administered by a vigilant anesthesiologist.

“Awareness” and Level of Consciousness Monitors

When delivering a general anesthetic, one of the major imperatives is to achieve and maintain a loss of

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consciousness. Normally, this is achieved through careful observation of vital signs (heart rate, blood

pressure), physical examination signs (movement in a patient without neuromuscular blockade), and

delivery of inhalational or intravenous agents at doses consistent with a loss of consciousness. However,

this “science” is inexact and must account for patient age, comorbidities, chronic medications, surgical

stimulus, and patient-to-patient variability. As a result, patients may rarely experience “awareness under

anesthesia” – a state characterized not only by consciousness but also by recall of intraoperative events.

The laypress and public have increased their scrutiny of this perioperative event.

First, appropriate expectations and effective communication of the anesthetic are required. Recent

literature has demonstrated that patients undergoing regional anesthesia are as likely to report

unpleasant “awareness” as patients undergoing a general anesthetic.62 This is despite the reality that

loss of consciousness is only a goal of general anesthesia. Clearly, anesthesiologists must communicate

the anesthetic plan and expectations more accurately.

Second, several level of consciousness monitors have been developed in hopes of providing the

anesthesiologist with additional objective data to guide their assessment and actions. The current

generation of monitors generally uses electroencephalographic (EEG) analysis to provide the clinician

with an assessment of the relative “depth” of anesthesia achieved. Several commercially available

monitors such as the bispectral index (BIS) from Aspect Medical Systems and Entropy from General

Electric Healthcare are in common clinical use. Data evaluating the value of these EEG-based monitors

in reducing awareness are conflicting, with several large trials producing varying results.63,64 As a

result, the ASA has not adopted awareness monitors as a standard of care and leaves the decision to use

such monitoring technology to each provider, patient, and situation.65 The largest, most recent trials

have failed to demonstrate a measurable reduction in awareness.66,67

COMMON PROBLEMS IN THE POSTOPERATIVE PERIOD

Postanesthesia care units are required in any setting where surgical procedures are conducted. The

increased scope of surgery and the invasive technology used to monitor sicker patients has increased the

service at and training required to operate these facilities. In 2014, the ASA revised the Standards for

Postanesthesia Care (Table 13-15).

The scoring systems used to assess the postoperative patient direct attention to the primary areas of

concern. The postanesthesia recovery score is an attempt to evaluate postanesthesia patient status

(Table 13-16).68 This basic information should be incorporated in a record that provides clear

documentation of postoperative events. Documentation should also include details of postoperative

outpatient care, with a note indicating postoperative telephone contact made to elucidate problems.

Problems should receive appropriate follow-up, and written postoperative discharge instructions should

be provided for the patient.

Investigators have reported that 24% of patients experience a postanesthesia care unit complication.

Nausea, vomiting, and the need for airway support constitute 70% of these complications (Fig. 13-2).69

The need to maintain airway support was by far the most common respiratory complication. The

duration of the procedure as well as ASA classification and type of procedure had a significant bearing

on the incidence of complications in this study. Hypothermia was also a common problem that

prolonged postoperative postanesthesia care unit stay (Fig. 13-3). Hypothermia has the deleterious

effects of altering drug metabolism and delaying recovery. Furthermore, it causes shivering, which

increases the metabolic demand for oxygen.

Among cardiovascular complications in the postoperative period, none is more important or more

difficult to diagnose than myocardial ischemia. The association of perioperative myocardial ischemia

with cardiac morbidity has been clearly documented.26 In a series of high-risk patients undergoing

noncardiac surgery, researchers noted that “early postoperative myocardial ischemia is an important

correlate of adverse cardiac outcomes.”26 Diagnosis is complicated by the fact that only 10% to 30% of

patients suffering documented myocardial infarction have pain and that postoperative ECG T-wave

changes are often nonspecific.70 Instead, one must seek secondary indications of ongoing ischemia or

“angina equivalents,” such as hypotension, arrhythmias, elevated filling pressures, or postoperative

oliguria. Arrhythmias are common and are significant primarily because of the association with

myocardial ischemia or hypoxemia.

Table 13-15 Standards for Postanesthesia Care

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Nausea and vomiting are rarely unifactorial and cause considerable discomfort to patients. Opioids

are responsible for stimulating the emesis center in a significant cohort of patients. These patients may

provide a clear history of opioid sensitivity and anesthetic and analgesic regimens may be tailored to

limit the exposure of these patients to these drugs. In general, however, there is little evidence to favor

one anesthetic or anesthetic technique over another, although propofol appears to have an antiemetic

effect. Nitrous oxide, often considered causative, does not appear to increase the incidence of nausea

according to well-documented studies. It is not unusual for an antiemetic agent to be included

preoperatively or as part of the anesthetic technique, especially in patients with a positive history or

those deemed to be at risk, such as menstruating young women undergoing laparoscopy. Standard usage

includes phenothiazines, butyrophenones, 5HT3 antagonists, and steroids. A multimodal approach that

avoids redosing of a given medication class has been demonstrated to be most beneficial.71 Despite

decades of research, classic medications such as droperidol remain a mainstay of therapy. The FDA

recently added a black-box warning to droperidol due to concerns of QT prolongation, but such concerns

have not been validated when compared to other perioperative medications; as a result, many

institutions continue to use droperidol for postoperative nausea and vomiting prophylaxis.72,73

Table 13-16 Postanesthesia Recovery Score

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Figure 13-2. Major postanesthesia care unit complications by percentage of occurrence and number of patients experiencing each

complication. Nausea and vomiting were the most frequently observed complications. ROMI, rule out myocardial infarction.

(Reproduced with permission from Hines R, Barash PG, Watrous G, et al. Complications occurring in the postanesthesia care unit: a

survey. Anesth Analg 1992;74:505.)

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