b. Increases in MetHb produce an underestimation when Spo₂ is >70% and an overestimation when Spo₂ is <70%.

c. COHb produces artificially high results.

C. Proper Use and Interpretation. Factors that may be present during pulse oximetry that affect its accuracy and reliability include dyshemoglobins, dyes (methylene blue, indocyanine green, and indigo carmine), nail polish, ambient light, light-emitting diode variability, motion artifact, and background noise. Electrocautery can interfere with pulse oximetry if the radiofrequency emissions are sensed by the photodetector.

D. Indications. Pulse oximetry has been used in all patient age groups to detect and prevent hypoxemia. Quantitative assessment of arterial oxygen saturation is mandated by the ASA monitoring standards.

E. Contraindications. There are no clinical contraindications to monitoring arterial oxygen saturation with pulse oximetry.

F. Common Problems and Limitations

1. Arterial oxygen monitors do not ensure adequacy of oxygen delivery to or utilization by peripheral tissues and should not be considered a replacement for arterial blood gas measurements or mixed central venous oxygen saturation.

2. Unless the patient in breathing room air, pulse oximetry is a poor indicator of adequate ventilation. (Patients who have been breathing 100% FiO₂ may be apneic for several minutes before desaturation is detected by the pulse oximeter.) After the P_AO₂ has fallen sufficiently to cause a detectable decrease in SpO₂, further desaturation may occur precipitously when the steep part of the oxyhemoglobin dissociation curve is reached.

III. MONITORING OF EXPIRED GASES

A. Principles of Operation. The patient’s expired gas is likely to be composed of a mixture of oxygen (O₂), nitrogen (N₂), carbon dioxide (CO₂), and anesthetic gases (N₂O and halogenated agents). Infrared absorption spectrophotometry (IRAS) gas analysis devices by transmitting light through a pure sample of a gas over the range of infrared frequencies creates a unique infra-red spectrum (like a fingerprint) for that gas.

B. Proper Use and Interpretation. Expired gas analysis allows detection of critical events (Table 25-1).

C. Interpretation of Inspired and Expired Carbon Dioxide Concentrations

1. Capnometry is the measurement and numeric representation of the CO₂ concentration during inspiration and expiration.

2. A capnogram is a continuous concentration–time display of the CO₂ concentration sampled at a patient’s airway during ventilation (divided into four distinct phases) (Fig. 25-2).

a. The first phase (A–B) represents the initial stage of expiration. Gas sampled during this phase occupies the anatomic dead space and is normally devoid of CO₂.

b. At point B, CO₂-containing gas presents itself at the sampling site, and a sharp upstroke (B–C) is seen in the capnogram.

c. Phase C to D represents the alveolar or expiratory plateau. At this phase of the capnogram, alveolar gas is being sampled. Normally, this part of the waveform is almost horizontal. However, when ventilation and perfusion are mismatched, Phases C to D may take an upward slope.

d. Point D is the highest CO₂ value and is called the end-tidal CO₂ (ETCO₂). ETCO₂ is the best reflection of the alveolar CO₂ (P_ACO₂).

e. Unless Rebreathing of CO₂ occurs, the baseline approaches zero.

f. The ETCO₂–PACO₂ gradient typically is around 5 mm Hg during routine general anesthesia in otherwise healthy, supine patients.

3. Capnography is an essential element in determining the appropriate placement of endotracheal tubes. The presence of a stable ETCO₂ for three successive breaths indicates that the tube is not in the esophagus.

a. A continuous, stable CO₂ waveform ensures the presence of alveolar ventilation but does not necessarily indicate that the endotracheal tube is properly positioned in the trachea. (Endobronchial intubation cannot be ruled out until breath sounds are auscultated bilaterally.)

b. A sudden drop in ETCO₂ to near zero followed by the absence of a CO₂ waveform heralds a potentially life-threatening problem that could indicate malposition of an endotracheal tube into the pharynx or esophagus, sudden severe hypotension, massive pulmonary embolism, a cardiac arrest, or a disconnection or disruption of sampling lines.

c. Gradual reductions in ETCO may reflect decreases in PaCO₂ that occur when there exists an imbalance between minute ventilation and metabolic rate (CO₂ production), as commonly occurs during anesthesia at a fixed minute ventilation.

D. Interpretation of Inspired and Expired Anesthetic Gas Concentrations. Monitoring the concentration of expired anesthetic gases (minimum alveolar concentration) assists in titrating those gases to the clinical circumstances of the patient.

E. Indications. Capnography is the standard of care for monitoring the adequacy of ventilation in patients receiving general anesthesia and during procedures performed while the patient is under moderate or deep sedation.

F. Contraindications. There are no contraindications to the use of capnography, provided that the data obtained are evaluated in the context of the patient’s clinical circumstances.

G. Common Problems and Limitations

1. The sampling lines or water traps of expired gas analyzers may become occluded with condensed water vapor during prolonged use.

2. Although capnography provides a quantitative measurement of ETCO₂, it is not as accurate as blood gas analysis for the assessment of the partial pressure of arterial carbon dioxide. (A gradient exists between the partial pressure of arterial carbon dioxide and ETCO₂, and this gradient increases as the dead-space volume increases.)

IV. INVASIVE MONITORING OF SYSTEMIC BLOOD PRESSURE

A. Principles of Operation. Indwelling arterial cannulation permits the opportunity to monitor arterial blood pressure continuously and to have vascular access for arterial blood sampling.

B. Proper Use and Interpretation (Table 25-2)

1. The radial artery remains the most popular site for cannulation because of its accessibility and the presence of a collateral blood supply. (The prognostic value of the Allen test in assessing the adequacy of the collateral circulation has not been confirmed.)

2. Ultrasound imaging with Doppler color flowmetry can provide valuable further assistance when the pulse is difficult to locate or the caliber of the vessel appears to be small.

3. Arterial blood pressure transduction systems must be “zeroed” before use. The transducer is positioned at the same level as the right atrium, and the stopcock is opened to the atmosphere so that pressure-sensing crystal senses only atmospheric pressure. For neurosurgical procedures in which the patient may be positioned in an upright or beach-chair position, it is common practice to zero the transducer at the level of the circle of Willis so that the arterial pressure tracing provides a reading that is adjusted for the height of the fluid column between the heart and the brain.

4. Abnormal radial artery blood flow after catheter removal occurs frequently. (Blood flow usually normalizes in 3–70 days.)

C. Indications. The standards for basic monitoring stipulate that arterial blood pressure shall be determined (intermittent and noninvasive, continuous) and recorded at least every 5 minutes. This standard is usually met by intermittent, noninvasive blood pressure monitoring. However, continuous monitoring may be indicated by patient comorbidities or by the nature of the surgery to be performed (Table 25-3).

D. Contraindications. Arterial cannulation is regarded as an invasive procedure with documented morbidity (ischemia after radial artery cannulation). Patients with compromised collateral arterial supply (Raynaud’s phenomenon, thromboangiitis obliterans) are at increased risk for ischemic complications.

E. Common Problems and Limitations. The fidelity of the transducer system is optimized when catheters and tubing are stiff, the mass of the fluid is small, the number of stopcocks is limited, and the connecting tubing is not excessive. In clinical practice, underdamped transducer systems tend to overestimate the systolic pressure by 15 to 30 mm Hg and amplify artifacts

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