Pulse Oximetry: Clinical Considerations

In addition to SpO₂, pulse oximeters provide an indication of tissue perfusion (pulse amplitude) and measure heart rate. Because SpO₂ is normally close to 100%, only gross abnormalities are detectable in most anesthetized patients. Depending on a particular patient’s oxygen–hemoglobin dissociation curve, an SpO₂ of 90% may indicate a PaO₂ of less than 65 mm Hg. This compares with clinically detectable cyanosis, which requires 5 g of desaturated hemoglobin and usually corresponds to an SpO₂ of less than 80%. Bronchial intubation usually goes undetected by pulse oximetry in the absence of lung disease or low fraction of inspired oxygen concentrations (FIO₂).

Methemoglobinemia: Methemoglobin has the same absorption coefficient at both red and infrared wavelengths. The resulting 1:1 absorption ratio corresponds to a saturation reading of 85%.

Causes of pulse oximetry artifact: Excessive ambient light, motion, methylene blue dye, venous pulsations in a dependent limb, low perfusion, malpositioned sensor and leakage of light from the light-emitting diode to the sensor (bypassing the arterial bed).

Extensions of Pulse Oximetry Technology

Mixed venous blood oxygen saturation: Requires the placement of a pulmonary artery (PA) catheter containing fiberoptic sensors that continuously determine SVO₂. A variation involves placing the sensor in the internal jugular vein, which provides measurements of jugular bulb oxygen saturation to assess cerebral oxygen delivery.

Noninvasive brain oximetry: Monitors regional oxygen saturation (rSO₂) of hemoglobin in the brain. A forehead sensor emits light of specific wavelengths and measures the light reflected back to the sensor. In contrast to pulse oximetry, brain oximetry measures venous, capillary, and arterial saturation, thereby providing an average oxygen saturation of all regional hemoglobin (∼70%). Cardiac arrest, cerebral embolization, deep hypothermia, or severe hypoxia can cause a dramatic decrease in rSO₂.

Capnography

Capnographs rely on the absorption of infrared light by CO₂. Determination of end-tidal CO₂ (ETCO₂) concentration to confirm adequate ventilation is mandatory during all anesthetic procedures.

Capnography: Nondiverting (Flowthrough)

Nondiverting (mainstream) capnographs measure CO₂ passing through an adaptor placed in the breathing circuit. Infrared light transmission through the gas is measured and CO₂ concentration is determined by the monitor.

Capnography: Diverting (Aspiration)

Diverting capnographs continuously suction gas from the circuit into a sample cell in the monitor. CO₂ concentration is determined by comparing infrared absorption in the cell with a chamber devoid of CO₂.

Aspiration rates: High aspiration rates (up to 250 mL/min) and low-deadspace sampling tubing usually increase sensitivity and decrease lag time. If tidal volumes (VT) are small (e.g., pediatric patients), however, a high rate of aspiration may entrain fresh gas from the circuit and dilute ETCO₂ measurement. Low aspiration rates (less than 50 mL/min) can underestimate ETCO₂ during rapid ventilation.

Water precipitation: Diverting units are prone to water precipitation in the aspiration tube and sampling cell that can cause obstruction of the sampling line and erroneous readings.

Expiratory valve malfunction is detected by the presence of CO₂ in inspired gas.

Capnography: Clinical Considerations

Capnographs reliably indicate esophageal intubation but do not reliably detect bronchial intubation. Sudden cessation of CO₂ during the expiratory phase may indicate a circuit disconnection. A marked rise in ETCO₂ may be caused by the increased metabolic rate associated with malignant hyperthermia.

The PaCO₂-ETCO₂ gradient (usually 2–5 mm Hg) reflects alveolar dead space (alveoli that are ventilated but not perfused). Capnographs display a CO₂ waveform that allows recognition of a variety of conditions:

A. Normal capnograph with the three phases of expiration: phase I—dead space; phase II—dead space and alveolar gas; phase III—alveolar gas plateau.

B. Capnograph of severe chronic obstructive pulmonary disease. No plateau is reached before the next inspiration. The ETCO₂ and arterial CO₂ gradient is increased.

C. Depression during phase III indicates spontaneous respiratory effort.

D. Failure of the inspired CO₂ to return to zero may be attributable to an incompetent expiratory valve or exhausted CO₂ absorbent.

E. Persistence of exhaled gas during the inspiratory cycle signals the presence of an incompetent inspira-tory valve.

Anesthetic Gas Analysis

Anesthetic gas analysis is essential during any inhalational anesthetic use. There are no contraindications.

Infrared absorption: Relies on a variety of techniques similar those used in capnography. Based on the Beer-Lambert law; the absorption of infrared light passing through a solvent (inspired or expired gas) is proportional to the amount of the unknown gas. O₂ and N₂ do not absorb infrared light and must be measured by other means.

Piezoelectric analysis: Uses oscillating quartz crystals, one of which is covered in lipid. Volatile anesthetics dissolve in the lipid layer, and their concentration is determined by the change in oscillation frequency. This method cannot distinguish different anesthetic agents.

Oxygen Analysis

Galvanic cell: Galvanic cell hydroxyl ions are formed at the gold cathode and react with the lead anode. An electrical current is produced that is proportional to the amount of oxygen being measured.

Paramagnetic analysis: Oxygen is a nonpolar gas that is paramagnetic and expands when placed in a magnetic field. By switching the field on and off, the volume change can be used to measure O₂ content.

Polarographic electrode: A semipermeable membrane separates a gold cathode and silver anode. A voltage is applied, and hydroxide ions are formed from O₂; the resultant current is proportional to the amount of O₂.

Spirometry

Anesthesia machines can measure and manage airway pressures, volume, and flow; calculate resistance and compliance; and then display the relationships of these variables as flow–volume or pressure–volume loops.

Low and high peak inspiratory pressure: Indicate circuit disconnect or airway obstruction, respectively.

Minute ventilation: Obtained by measuring VT and breathing frequency.

Spirometric loops and waveforms are altered by certain disease processes and events (e.g., obstruction, bronchial intubation).

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