8.1 Noninvasive Monitoring: Pulse Oximetry, Capnography Waveforms & Temperature

8.1 Noninvasive Monitoring: Pulse Oximetry, Capnography Waveforms & Temperature

Noninvasive physiological monitoring constitutes the primary line of defense in modern anesthesia practice. A thorough understanding of the optical physics of pulse oximetry, infrared gas analysis in capnography, and thermoregulatory physiology enables the CRNA to rapidly distinguish between true patient emergencies, equipment malfunctions, and physiological artifacts.

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1. Pulse Oximetry Physics & Spectrophotometry

Pulse oximetry provides continuous, noninvasive estimation of arterial hemoglobin oxygen saturation ($SpO_2$) and peripheral pulse rate by combining two physical principles: spectrophotometry (light absorption by hemoglobin species) and optical plethysmography (detection of pulsatile vascular volume changes).

+-------------------------------------------------------------------------+
|                       PULSE OXIMETRY DUAL-WAVELENGTH OPTICS              |
+------------------------------------+------------------------------------+
| Red Light (660 nm)                 | Infrared Light (940 nm)            |
+------------------------------------+------------------------------------+
| • Preferentially absorbed by       | • Preferentially absorbed by       |
|   Deoxygenated Hemoglobin (DeoxyHb)|   Oxygenated Hemoglobin (OxyHb)    |
| • DeoxyHb extinction is ~10x       | • OxyHb extinction is higher at    |
|   higher than OxyHb at 660 nm      |   940 nm than DeoxyHb              |
+------------------------------------+------------------------------------+

The Beer-Lambert Law

Pulse oximeter microprocessors apply the combined Beer-Lambert Law to quantify solute concentration based on light transmission:

$I = I_0 \cdot e^{-\alpha \cdot C \cdot d}$

Where:

• $I =$ Transmitted light intensity
• $I_0 =$ Incident light intensity
• $\alpha =$ Extinction coefficient (molar absorptivity of the specific hemoglobin species at a chosen wavelength)
• $C =$ Hemoglobin concentration
• $d =$ Optical path length (tissue and vascular bed thickness)

Optical Plethysmography & The R Value

The pulse oximeter probe houses two light-emitting diodes (LEDs)—emitting at $660 \text{ nm}$ (visible red) and $940 \text{ nm}$ (near-infrared)—and a single silicon photodiode receiver. The sensor samples light thousands of times per second and divides the transmitted light into two distinct components:

1. Pulsatile (AC) Component: Light absorbed by expanding arterial blood during ventricular systole.
2. Non-Pulsatile (DC) Component: Baseline light absorbed by non-pulsatile venous blood, capillary blood, muscle, bone, and connective tissue.

By dividing the AC signal by the DC signal at both wavelengths, the microprocessor eliminates background tissue absorption and calculates the Red-to-Infrared Absorption Ratio ($R$):

$R = \frac{\left(\frac{AC_{660}}{DC_{660}}\right)}{\left(\frac{AC_{940}}{DC_{940}}\right)}$

+-------------------------------------------------------------------------+
|                 R VALUE TO SpO₂ CALIBRATION RELATIONSHIP                |
+-------------------+-----------------------------------------------------+
| R Ratio Value     | Calibrated Saturation Displayed                     |
+-------------------+-----------------------------------------------------+
| R = 0.4           | SpO₂ = 100% (Infrared absorption dominates)         |
| R = 1.0           | SpO₂ = 85% (Red and Infrared absorption are equal)  |
| R = 2.0           | SpO₂ = 0% (Red absorption dominates)                |
+-------------------+-----------------------------------------------------+

Isobestic Points

An isobestic point is a specific optical wavelength where two distinct chemical substances possess identical molar extinction coefficients (absorb light equally regardless of oxygenation state).

• Primary Isobestic Point for Hemoglobin: $805 \text{ nm}$ (also $590 \text{ nm}$). At $805 \text{ nm}$, deoxygenated hemoglobin and oxyhemoglobin absorb light identically.
• Significance in Co-Oximetry: Light absorption at an isobestic wavelength depends strictly on total hemoglobin concentration, independent of oxygen saturation.

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2. Dyshemoglobinemias, Intravenous Dyes & Measurement Artifacts

Standard clinical pulse oximeters assume only two circulating hemoglobin species: functional oxyhemoglobin ($HbO_2$) and deoxygenated hemoglobin ($Hb$). When abnormal dyshemoglobins or optical dyes are present, standard dual-wavelength pulse oximetry becomes inaccurate, requiring multi-wavelength fractional co-oximetry for accurate measurement.

+-------------------------------------------------------------------------+
|                    DYSHEMOGLOBIN & DYE INTERFERENCE SUMMARY             |
+------------------+----------------------------------+-------------------+
| Interfering Agent| Optical Mechanism                | SpO₂ Effect       |
+------------------+----------------------------------+-------------------+
| Carboxyhemoglobin| Absorbs 660 nm identically to    | Falsely elevated  |
| (CO-Hb)          | OxyHb; negligible at 940 nm      | (reads ~100%)     |
| Methemoglobin    | High, equal absorption at both   | Fixed at ~85%     |
| (Met-Hb)         | 660 nm and 940 nm (R = 1.0)      | (over/under reads)|
| Methylene Blue   | Massive absorption at 660 nm     | Precipitous drop  |
| Dye              | mimics pure DeoxyHb              | to ~65% for 1-2 m |
| Indigo Carmine   | Mild optical absorption at 660 nm| Slight false drop |
| Indocyanine Green| Moderate absorption at 800-900 nm| Slight false drop |
+------------------+----------------------------------+-------------------+

Carboxyhemoglobin (CO-Hb)

• Etiology: Smoke inhalation, structure fires, carbon monoxide poisoning, tobacco smoking, or volatile anesthetic degradation in desiccated carbon dioxide absorbents (most pronounced with desflurane).
• Optical Trap: CO-Hb has an extinction coefficient at $660 \text{ nm}$ virtually identical to oxyhemoglobin ($HbO_2$). Standard pulse oximeters interpret CO-Hb as $HbO_2$.
• Clinical Impact: A patient with $40%$ CO-Hb and $60%$ true oxyhemoglobin will display an SpO₂ of $98 - 100%$, masking severe, life-threatening arterial hypoxemia and cellular asphyxiation. Fractional saturation via co-oximetry is mandatory.

Methemoglobin (Met-Hb)

• Pathophysiology: Iron in heme is oxidized from the ferrous state ($Fe^{2+}$) to the ferric state ($Fe^{3+}$), which cannot bind oxygen. It also creates a severe leftward shift of remaining normal heme groups.
• Etiologies: Topical local anesthetics (benzocaine sprays like Hurricaine, prilocaine/EMLA), intravenous lidocaine, sodium nitroprusside infusions, dapsone, sulfonamides, nitric oxide, and aniline dyes.
• Optical Trap: Methemoglobin absorbs heavily and equally at both $660 \text{ nm}$ and $940 \text{ nm}$, driving the $R$ ratio toward $1.0$. As a result, the displayed SpO₂ locks at $\approx 85%$ regardless of the patient's true arterial $PaO_2$.
  • If true $SaO_2$ is $99%$, the monitor reads falsely low ($85%$).
  • If true $SaO_2$ is $50%$, the monitor reads falsely high ($85%$).
• Treatment: Methylene Blue $1 - 2 \text{ mg/kg}$ IV administered over 5 minutes. Methylene blue is reduced by erythrocyte NADPH-methemoglobin reductase into leukomethylene blue, which acts as an electron donor to reduce $Fe^{3+}$ back to $Fe^{2+}$.
• Critical Caution: In patients with Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency, methylene blue is ineffective (lacks sufficient NADPH) and may induce severe, life-threatening acute hemolytic anemia; ascorbic acid (vitamin C) is used alternatively.

Intravenous Dyes & Optical Artifacts

1. Methylene Blue: Absorbs strongly at $660 \text{ nm}$, driving the red-to-infrared ratio up and causing a transient, steep plunge in displayed SpO₂ (often down to $60 - 65%$) lasting $1 - 2 \text{ minutes}$ before returning to baseline.
2. Nail Polish & Ambient Light: Black, blue, and dark green nail polishes absorb $660 \text{ nm}$ light and cause falsely low SpO₂ readings (red nail polish has minimal effect). Ambient fluorescent lights and infrared heating lamps can scatter optical sensors; covering the probe with an opaque shield resolves optical ambient interference.
3. Low Perfusion States: Severe hypothermia, peripheral vasoconstriction, cardiopulmonary bypass, and profound shock attenuate the pulsatile AC signal, resulting in loss of tracking or inaccurate saturation display.

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3. Capnography Fundamentals & Normal Waveform Morphology

Capnography is the continuous measurement and graphic display of carbon dioxide tension in respiratory gases over time. It is the gold standard monitor for verifying endotracheal tube placement, detecting circuit disconnections, and assessing the adequacy of pulmonary ventilation and perfusion.

                    [NORMAL TIME CAPNOGRAM (PHASES I - 0)]

      CO₂ (mmHg)
        50 |
           |                    Phase III (Alveolar Plateau)
        40 |                     /-----------------------D (ETCO₂)
           |                    /  (Alpha Angle: 100-110°)
        30 |                   /                         |
           |       Phase II   /                          |  Phase 0
        20 |      (Expiratory/                           | (Inspiratory
           |       Upstroke)/                            |  Downstroke)
        10 |               /                             |
           |    Phase I   /                              |
         0 +-------------+-------------------------------+-----
                  Inspiratory Baseline                 Time (sec)

The Four Phases of the Time Capnogram

Arterial-to-End-Tidal $CO_2$ Gradient ($PaCO_2 - P_{ET}CO_2$)

• In healthy individuals under general anesthesia, $PaCO_2$ exceeds $P_{ET}CO_2$ by $2 - 5 \text{ mmHg}$ due to normal physiological alveolar dead space ($V/Q > 1$).
• Factors that Widen the Gradient ($PaCO_2 \gg P_{ET}CO_2$):
  • Increased alveolar dead space (pulmonary embolism, hypovolemia, excessive PEEP, upright/sitting position).
  • Decreased cardiac output or low pulmonary blood flow.
  • Chronic obstructive pulmonary disease (COPD) with severe $V/Q$ mismatch.

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4. Diagnostic Angles & Capnogram Waveform Pathologies

+-------------------------------------------------------------------------+
|                      CAPNOGRAPHY DIAGNOSTIC ANGLES                      |
+-------------------+--------------------+--------------------------------+
| Angle             | Normal Value       | Diagnostic Elevation Pathology |
+-------------------+--------------------+--------------------------------+
| **Alpha (α)**     | 100° to 110°       | Bronchospasm, COPD, kinked ETT |
| (Between II & III)|                    | (Expiratory airflow resistance)|
| **Beta (β)**      | ~90° (Right angle) | Rebreathing of CO₂, incompetent|
| (Between III & 0) |                    | expiratory valve, exhausted lime|
+-------------------+--------------------+--------------------------------+

1. Bronchospasm ("Shark Fin")     2. Curare Cleft (Muscle Relaxant Wearing Off)
        /------------------D                 /----------\_/-------D
       /                                    /
      /  (Alpha Angle > 110°)              /    (Diaphragmatic effort notch)
     /                                    /
----+                                ----+

3. CO₂ Rebreathing / Valve Incompetence   4. Cardiac Oscillations
          /----------------D                     /--v--v--v--v----D
         /                                      /
        /   (Beta Angle > 90°)                 /    (Pulsating heart recoil)
  _    /                                      /
   \--+ (Elevated Phase I Baseline > 0)  ----+

Classic Capnography Pathologies & NCE Board Recognition

1. Obstructive Airflow / Shark-Fin Pattern:
   • Morphology: Prolonged, sluggish Phase II upstroke, steeply ascending Phase III slope, and an abnormally widened Alpha angle ($>110^\circ$).
   • Etiologies: Bronchospasm (asthma), COPD, partially obstructed/kinked endotracheal tube, foreign body, or secretions in the airway adapter.
2. Curare Cleft:
   • Morphology: A discrete downward notch or dip appearing in the middle-to-late portion of the Phase III alveolar plateau.
   • Etiology: Patient attempting spontaneous breathing against the mechanical ventilator; uncoordinated diaphragmatic contraction draws $CO_2$-free fresh gas across the sensor.
   • Action: Deepen anesthesia or administer additional neuromuscular blocking agent.
3. Cardiac Oscillations:
   • Morphology: Small, regular, rhythmic undulating ripples on the terminal Phase III plateau and Phase 0 downstroke.
   • Etiology: Mechanical pulsation of the contracting heart against adjacent left lower lobe pulmonary parenchyma in relaxed, thin patients with low respiratory rates.
4. $CO_2$ Rebreathing (Elevated Baseline):
   • Morphology: Phase I baseline fails to return to zero (elevated baseline $>0 \text{ mmHg}$) and the Beta angle increases ($>90^\circ$).
   • Etiologies: Incompetent expiratory unidirectional valve in the circle system, exhausted soda lime / carbon dioxide absorbent, or insufficient fresh gas flow in non-rebreathing Mapleson systems.
5. Esophageal Intubation:
   • Morphology: Small, transient $CO_2$ waveforms during the first $1 - 3$ mechanical breaths (residual stomach gas from bag-mask ventilation) that rapidly decay to zero within $4 - 6$ breaths, followed by a flatline.
6. Sudden Loss of $ETCO_2$ to Zero:
   • Etiologies: Circuit disconnection, accidental tracheal extubation, complete airway obstruction/plug, ventilator power failure, or cardiac arrest.
7. Sudden Exponential Plunge in $ETCO_2$ (without reaching zero):
   • Etiologies: Massive pulmonary thromboembolism, acute air embolism, profound acute systemic hypotension, or severe acute hemorrhage (loss of pulmonary capillary perfusion creating massive alveolar dead space).

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5. Temperature Regulation & Core vs. Peripheral Sites

Under general and neuraxial anesthesia, behavioral thermoregulation is abolished and physiological thermoregulatory thresholds are profoundly blunted (interthreshold range widens from normal $\pm 0.2^\circ\text{C}$ to $\approx \pm 2.0 - 4.0^\circ\text{C}$).

+-------------------------------------------------------------------------+
|                       TEMPERATURE MONITORING SITES                      |
+------------------------------------+------------------------------------+
| Core Temperature Sites             | Intermediate & Peripheral Sites    |
| (Rapid response, high accuracy)    | (Subject to thermal lag/errors)    |
+------------------------------------+------------------------------------+
| 1. Distal Esophagus (Lower 1/3)    | 1. Urinary Bladder (accurate only  |
|    - Placed retrocardiac in lower  |    with high urine output)         |
|      fourth of esophagus           | 2. Rectum (slow response; fecal lag|
| 2. Tympanic Membrane (Thermocouple)| 3. Axilla (reflects skin/periphery)|
|    - Direct contact with membrane  | 4. Forehead Liquid Crystal Skin    |
| 3. Pulmonary Artery Catheter       |    - Correlates poorly with core   |
|    - Gold standard core reference  |      temperature during rapid drops|
| 4. Nasopharynx (Posterior wall)    |                                    |
|    - Reflects brain core temp      |                                    |
+------------------------------------+------------------------------------+

NCE Clinical Pearl — Distal Esophageal Probe Placement: To record true core body temperature without false cooling from cold inhaled respiratory gases, the esophageal temperature probe must be positioned in the distal third (lower fourth) of the esophagus, where it sits directly posterior to the left atrium. Placement depth is approximately $38 - 42 \text{ cm}$ from the incisors in an adult, confirmed at the point of maximal heart sounds on an esophageal stethoscope.

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6. Perioperative Hypothermia: Phases, Mechanisms & Clinical Sequelae

Perioperative hypothermia is defined as a core body temperature $<36.0^\circ\text{C}$ ($<96.8^\circ\text{F}$). Anesthetic drugs induce peripheral vasodilation and suppress hypothalamic vasoconstriction thresholds.

+-------------------------------------------------------------------------+
|                   THREE PHASES OF PERIOPERATIVE HYPOTHERMIA             |
+-------------------------------------------------------------------------+
|                                                                         |
|   Phase 1: Internal Redistribution Hypothermia (Hour 1)                 |
|   • Core temp drops precipitously by 1.0°C to 1.5°C                     |
|   • Anesthesia-induced vasodilation allows warm core blood to flow to   |
|     cooler peripheral tissues (responsible for >80% of initial drop)   |
|                                                                         |
|   Phase 2: Linear Heat Loss (Hours 2 to 3)                              |
|   • Slower, steady, linear temperature decline                          |
|   • Environmental heat loss (radiation, convection) exceeds metabolic   |
|     heat production                                                     |
|                                                                         |
|   Phase 3: Thermal Plateau (Hours 3 to 4+)                              |
|   • Core temperature stabilizes                                         |
|   • Metabolic heat production reaches equilibrium with heat loss, or    |
|     anesthetic-blunted peripheral vasoconstriction finally triggers     |
+-------------------------------------------------------------------------+

Mechanisms of Operating Room Heat Loss

Adverse Consequences of Perioperative Hypothermia

1. Post-Anesthetic Shivering & Oxygen Consumption Surge: Involuntary uncoordinated thermogenic muscle contractions increase total metabolic rate and oxygen consumption ($VO_2$) by up to $400%$ (4-fold). In patients with coronary artery disease, this metabolic surge triggers subendocardial ischemia, lactic acidosis, and hypoxemia.
   • Treatment: Meperidine (Demerol) $12.5 - 25 \text{ mg}$ IV (acts via $\kappa$-opioid receptor stimulation to lower shivering threshold), dexmedetomidine, or clonidine.
2. Coagulopathy & Increased Hemorrhage: Lowers platelet activation/aggregation and inhibits the enzymatic activity of the coagulation cascade. Every $1.0^\circ\text{C}$ drop in core temperature increases surgical blood loss by $\approx 15 - 20%$.
   • NCE Diagnostic Trap: Laboratory coagulation studies (PT/INR, aPTT) are routinely warmed to $37^\circ\text{C}$ in the hospital laboratory, falsely masking in vivo hypothermic coagulopathy!
3. Delayed Drug Clearance & Emergence: Hepatic clearance of opioids, volatile anesthetics, and neuromuscular blockers is severely reduced. Minimum Alveolar Concentration (MAC) of volatile anesthetics decreases by $\approx 5%$ for every $1.0^\circ\text{C}$ decrease in core temperature.
4. Surgical Site Infections (SSI): Hypothermia-induced vasoconstriction impairs tissue perfusion and subcutaneous oxygen delivery, suppressing neutrophil oxidative killing and increasing wound infection rates 3-fold.