5.2 Capnography, Gas Analysis & Arterial Blood Gas Interpretation

5.2 Capnography, Gas Analysis & Arterial Blood Gas Interpretation

Continuous analysis of respired gases is a fundamental patient safety standard during general anesthesia. The Certified Anesthesia Technologist must understand the physical mechanisms of carbon dioxide measurement, differentiate between side-stream and main-stream sampling technologies, rapidly diagnose circuit and patient pathologies from capnographic waveforms, and systematically interpret arterial blood gas (ABG) panels.

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Capnometry vs. Capnography

Although frequently used interchangeably in clinical jargon, capnometry and capnography describe distinct monitoring technologies:

• Capnometry: The numerical measurement and digital readout of carbon dioxide concentration (PCO₂ or fraction of CO₂) in respired gases during inspiration and expiration, recorded without a graphical waveform display.
• Capnography: The continuous, real-time graphical display of carbon dioxide concentration plotted against time (time-based capnogram) or expired lung volume (volume-based capnogram). The graphical representation is termed a capnogram.

Capnography is vastly superior to capnometry because the morphology of the waveform provides immediate diagnostic data regarding endotracheal tube placement, pulmonary airflow obstruction, neuromuscular recovery, cardiac output, and breathing circuit competence.

Qualitative vs. Quantitative CO₂ Detection

The ASATT content outline lists capnography and gas analysis as both quantitative and qualitative monitoring, and the ASATT Scope of Practice expects technologists to assess qualitative and quantitative signs of correct endotracheal tube placement. The two approaches answer different questions.

Qualitative (Colorimetric) CO₂ Detectors

A colorimetric CO₂ detector is a small disposable device placed between the tracheal tube and the resuscitation bag. It contains pH-sensitive indicator paper. Exhaled CO₂ dissolves in moisture on the paper, forms carbonic acid, and shifts the color from purple toward yellow. One widely taught scale reads purple below about 4 mmHg, tan at about 4 to 15 mmHg, and yellow at roughly 15 to 20 mmHg or more. The device shows that CO₂ is present; it gives no breath-by-breath number and no waveform.

Colorimetric detectors are useful where no capnograph is connected, such as transport or a code outside the operating room. They are rated for limited use; the Nellcor Easy Cap II, for example, is marketed for breath-to-breath response for up to 2 hours.

Quantitative Monitoring and the ASA Standard

Quantitative monitors (capnometers and capnographs, usually infrared analyzers) measure CO₂ partial pressure with every breath. The ASA Standards for Basic Anesthetic Monitoring require that correct placement of an endotracheal tube or supraglottic airway be verified by clinical assessment and by identifying CO₂ in expired gas, and that continual end-tidal CO₂ analysis from placement until removal be performed with a quantitative method such as capnography, capnometry, or mass spectroscopy, with the end-tidal CO₂ alarm audible. A colorimetric detector can support initial confirmation, but it does not satisfy the continual quantitative requirement. The 2022 ASA difficult airway guidelines likewise recommend confirming tracheal intubation with capnography or end-tidal CO₂ monitoring.

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Gas Sampling Technologies: Side-Stream vs. Main-Stream

Clinical carbon dioxide analysis relies on infrared absorption spectrophotometry (Beer-Lambert law), based on the property of asymmetrical polyatomic molecules (such as CO₂, N₂O, and halogenated volatile agents) to absorb specific wavelengths of infrared light (CO₂ exhibits a strong absorption peak at 4.26 μm). Two principal gas sampling configurations exist:

Side-Stream (Diverting) Analyzers

In a side-stream system, a small gas sample is continuously aspirated from an adapter located at the patient's airway (Y-piece or endotracheal tube connector) through a narrow-bore sampling catheter to an analyzer housed within the anesthesia workstation or patient monitor:

• Aspiration Flow Rate: Operates at sampling flow rates between 50 and 250 mL/min (standard adult anesthesia monitors typically draw 150 to 200 mL/min; dedicated neonatal monitors operate at 50 mL/min).
• Transit Delay Time: Because gas must travel through a 6- to 10-foot sampling tube, there is an inherent transit delay of 1 to 3 seconds before the waveform appears on the monitor screen.
• Moisture Management: Saturated patient exhalations contain warm water vapor that condenses inside the cold sampling line. Side-stream analyzers utilize specialized Nafion dehumidifying tubing, water traps, and hydrophobic particulate filters to remove liquid water before it reaches the optical infrared sensor.
• Waste Gas Scavenging: Because the diverted gas contains volatile anesthetics and nitrous oxide, the analyzer exhaust port must be scavenged back into the breathing circuit or directed to the Waste Anesthetic Gas Disposal (WAGD) system.

Main-Stream (Non-Diverting) Analyzers

In a main-stream system, an optical sensor containing an infrared light source and photodetector is clipped directly onto a specialized airway adapter (cuvette) positioned in-line between the endotracheal tube and the breathing circuit:

• Zero Transit Delay: Analysis occurs directly at the airway aperture, providing an immediate real-time response with no transit lag.
• No Gas Diverted: No gas is aspirated from the patient circuit, eliminating the need for scavenger return lines or moisture traps.
• Thermal Management: The sensor contains an internal heating element that keeps it slightly above body temperature to prevent water vapor from condensing onto the optical cuvette windows.

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Normal Time-Based Capnogram Morphology

A normal time-based capnogram displays carbon dioxide partial pressure on the vertical axis (0 to 50 mmHg) versus time on the horizontal axis during one complete respiratory cycle:

NORMAL TIME-BASED CAPNOGRAM WAVEFORM:

   PCO2 (mmHg)
     ^
  50 |                        Point D (PetCO2)
     |                        *
  40 |                 Phase III  ---\
     |                /               \ Phase 0
  30 |               / (Alveolar)      \
     |              / (Plateau)         \
  20 |             /                     \
     |   Phase II /                       \
  10 |   (Upstroke)                        \
     |  /                                   \
   0 +-+-------------------------------------+-----> Time
       Phase I                                Phase I
       (Baseline: Dead space gas = 0 mmHg)    (Fresh gas inhalation)

The Four Distinct Phases

1. Phase I (Inspiratory Baseline): Represents the beginning of expiration. The gas clearing the airway originates from the anatomical dead space (trachea, pharynx, and large bronchi) where no gas exchange occurs. Carbon dioxide concentration is 0 mmHg.
2. Phase II (Expiratory Upstroke): Represents the rapid transit of mixed gas from anatomical dead space and early emptying alveoli. The curve ascends sharply in an S-shaped trajectory as carbon dioxide concentration rises.
3. Phase III (Alveolar Plateau): Represents the continuous exhalation of pure alveolar gas from across all lung regions. Under normal conditions, Phase III exhibits a gentle, nearly horizontal upward slope due to progressive exhalation from slower-emptying alveoli with lower ventilation-perfusion (V/Q) ratios and higher CO₂ content.
4. Point D (End-Tidal CO₂ - PetCO₂): Point D, at the end of Phase III, represents the maximum carbon dioxide partial pressure recorded at the absolute end of exhalation, immediately before the next inspiration begins. This value is clinically recorded as the PetCO₂.
5. Phase 0 (Inspiratory Downstroke): Represents the beginning of inspiration. As fresh, carbon dioxide-free gas rushes into the airway, carbon dioxide concentration plummets precipitously back to the 0 mmHg baseline.

Capnogram Angles

• Alpha (α) Angle: The angle formed between Phase II (upstroke) and Phase III (alveolar plateau). Normally 100° to 110°. The alpha angle widens significantly (becoming obtuse) in obstructive lung disease, bronchospasm, or circuit resistance.
• Beta (β) Angle: The angle formed between Phase III (alveolar plateau) and Phase 0 (inspiratory downstroke). Normally approximately 90°. The beta angle increases (>90°) during rebreathing of carbon dioxide or when an expiratory unidirectional check valve is incompetent.

The Arterial-to-End-Tidal Gradient (PaCO₂ - PetCO₂)

In healthy, awake or anesthetized individuals with normal lungs:

• Normal Range: Arterial carbon dioxide tension (PaCO₂) is 2 to 5 mmHg HIGHER than end-tidal carbon dioxide tension (PetCO₂).
• Physiologic Mechanism: This small gradient is caused by normal alveolar dead space—alveoli that are ventilated with fresh gas but under-perfused or non-perfused with pulmonary blood (V/Q > 1). The non-perfused alveoli contain 0 mmHg CO₂, which slightly dilutes the mixed expired alveolar gas, lowering PetCO₂ below the PaCO₂ of perfusing capillary blood.
• Causes of a Widened Gradient (PaCO₂ ≫ PetCO₂): Any pathology that increases alveolar dead space or diminishes pulmonary capillary blood flow dramatically widens this gradient:
  • Pulmonary Embolism (Thrombus, Fat, or Air): Blocks pulmonary arterial flow to ventilated lung units, causing PetCO₂ to plunge while PaCO₂ escalates.
  • Hypovolemic Shock & Severe Hypotension: Decreased pulmonary perfusion pressure leaves apical lung zones unperfused.
  • Low Cardiac Output / Cardiac Arrest: Drastic drop in pulmonary blood transit.
  • High Positive End-Expiratory Pressure (PEEP): Overdistends alveoli and compresses alveolar capillary networks.
  • Chronic Obstructive Pulmonary Disease (COPD): Destroys capillary beds and creates severe V/Q mismatching.

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Pathological Capnogram Waveform Interpretation

Rapid clinical recognition of abnormal capnograms is a primary competency tested on the ASATT Cer.A.T.T. examination.

1. The Curare Cleft (Spontaneous Respiratory Effort)

CURARE CLEFT WAVEFORM:

   PCO2 (mmHg)
  40 |             ---\       /---\
  30 |            /    \     /     \
  20 |           /      \---/       \  <- Deep dip / cleft in Phase III
  10 |          /                     \
   0 +---------+-----------------------+-----> Time

• Morphology: A prominent downward dip or notch appearing in the middle or terminal third of the Phase III alveolar plateau.
• Etiology: Occurs during controlled mechanical ventilation when a patient is recovering from neuromuscular blockade. As muscle relaxation wanes, the patient's diaphragm contracts spontaneously against the positive-pressure breath delivered by the ventilator. This diaphragmatic twitch draws fresh gas into the central airways, briefly lowering the expired CO₂ concentration at the sampling port.
• Action: Alert the anesthesia provider that neuromuscular blockade is wearing off; anticipate administration of additional muscle relaxant or preparation for emergence.

2. The Shark-Fin Waveform (Expiratory Airflow Obstruction)

SHARK-FIN OBSTRUCTIVE WAVEFORM:

   PCO2 (mmHg)
  50 |                           /|
  40 |                          / |
  30 |                   /-----/  | <- Steep, upward sloping Phase III
  20 |             /----/         |    (Loss of horizontal plateau)
  10 |       /----/               | <- Prolonged, sloped Phase II
   0 +------+---------------------+-----> Time

• Morphology: Prolonged, sluggish, upward-sloping Phase II upstroke combined with an exaggerated, steeply angled Phase III alveolar plateau that never achieves a horizontal plateau, resembling the dorsal fin of a shark. The alpha angle is markedly increased.
• Etiology: Caused by expiratory airflow obstruction and asynchronous, delayed emptying of obstructed lung units. Clinical causes include:
  • Acute Bronchospasm (asthma, anaphylaxis).
  • Chronic Obstructive Pulmonary Disease (COPD).
  • Mechanical Airway Obstruction: Kinked endotracheal tube, patient biting the tube, thick bronchial secretions/mucus plugging, or a foreign body.
  • Expiratory Circuit Obstruction: Obstructed bacterial filter or jammed expiratory limb valve.
• Action: Immediately auscultate bilateral breath sounds (wheezing), check endotracheal tube depth, verify tube patency with a suction catheter, inspect breathing circuit hoses and filters, and prepare bronchodilators (albuterol inhaler via in-line adapter).

3. Elevated Baseline (Carbon Dioxide Rebreathing)

ELEVATED BASELINE (REBREATHING) WAVEFORM:

   PCO2 (mmHg)
  50 |                 Phase III  ---\
  40 |                /               \
  30 |   Phase II    /                 \
  20 |              /                   \
  10 | ------------/                     \----------- <- BASELINE > 0 mmHg
   0 +-----------------------------------------------+-----> Time

• Morphology: The capnogram fails to return to 0 mmHg during Phase I; the entire waveform is shifted upward with an inspiratory baseline elevated to 5 to 15 mmHg or higher.
• Etiology: The patient is rebreathing previously exhaled carbon dioxide. Causes include:
  • Exhausted Carbon Dioxide Absorbent: Soda lime or barium hydroxide lime granules are chemically saturated with CaCO₃, failing to scrub CO₂ from recirculated gases.
  • Incompetent Expiratory Unidirectional Valve: If the expiratory flutter valve disc is stuck open or missing, exhaled gas from the expiratory breathing tube is drawn back into the inspiratory limb during inspiration.
  • Incompetent Inspiratory Unidirectional Valve: Gas is pushed backward into the inspiratory limb during exhalation and rebreathed.
  • Channeling in Absorbent Canister: Gas flows through preferential low-resistance bypass pathways around loose absorbent granules.
  • Inadequate Fresh Gas Flow in Non-Rebreathing Systems: In Mapleson or Bain circuits, low fresh gas flow fails to flush exhaled gas out through the APL valve.
• Action: Inspect the CO₂ absorbent canister for purple ethyl violet exhaustion indicator; replace the canister if needed. Check unidirectional valve discs through transparent dome covers for condensation sticking or seating failure.

4. Sudden Loss of Waveform (Zero PetCO2)

• Morphology: PetCO2 immediately drops to 0 mmHg with complete absence of capnographic waveforms.
• Differential Diagnosis & Emergency Action:
  1. Breathing Circuit Disconnection: Most common cause. Inspect the Y-piece, ETT connector, and sampling catheter.
  2. Accidental Extubation: Tube dislodged from trachea into pharynx.
  3. Esophageal Intubation: Following intubation, an esophageal placement will show either zero CO₂ or 2 to 3 small, decaying residual gas puffs before dropping permanently to 0 mmHg. Standard protocol: "If in doubt, take it out."
  4. Complete Airway Obstruction: Massive mucus plug or completely kinked tube.
  5. Sudden Complete Cessation of Pulmonary Blood Flow: Acute massive pulmonary embolism or sudden cardiac arrest.
  6. Sampling Line Occlusion / Analyzer Failure: Water trap flooded with condensation.

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Arterial Blood Gas (ABG) Analysis

Arterial blood gas analysis is the gold standard for evaluating ventilatory efficiency, oxygenation, and systemic metabolic homeostasis.

Normal Physiological Reference Ranges

Four-Step Systematic ABG Interpretation

1. Step 1: Evaluate the pH (Acidemia vs. Alkalemia):
   • pH < 7.35 → Acidemia (acidosis process dominates).
   • pH > 7.45 → Alkalemia (alkalosis process dominates).
   • pH within 7.35–7.45 → Normal acid-base status OR fully compensated mixed disorder.
2. Step 2: Evaluate the Respiratory Component (PaCO₂):
   • If pH < 7.35 and PaCO₂ > 45 mmHg → Primary Respiratory Acidosis (hypoventilation).
   • If pH > 7.45 and PaCO₂ < 35 mmHg → Primary Respiratory Alkalosis (hyperventilation).
3. Step 3: Evaluate the Metabolic Component (HCO₃⁻ and Base Excess):
   • If pH < 7.35 and HCO₃⁻ < 22 mEq/L (BE < -2) → Primary Metabolic Acidosis.
   • If pH > 7.45 and HCO₃⁻ > 26 mEq/L (BE > +2) → Primary Metabolic Alkalosis.
4. Step 4: Assess the Degree of Compensation:
   • Uncompensated: pH is abnormal; one parameter (PaCO₂ or HCO₃⁻) is abnormal while the opposing system remains within normal limits.
   • Partially Compensated: pH remains abnormal; both PaCO₂ and HCO₃⁻ are abnormal and moving in the same direction, indicating the secondary system is actively working to normalize pH.
   • Fully Compensated: pH has returned to the normal physiological range (7.35 to 7.45), but both PaCO₂ and HCO₃⁻ remain abnormal.

Anion Gap Calculation & Metabolic Acidosis Subtypes

In patients presenting with metabolic acidosis, calculating the Serum Anion Gap (AG) distinguishes between unmeasured organic acid accumulation and hyperchloremic bicarbonate losses:

$AG = [Na^+] - \left([Cl^-] + [HCO_3^-]\right)$

• Normal Anion Gap: 8 to 12 mEq/L (unmeasured anions like albumin, phosphate, and sulfate balance unmeasured cations like K⁺, Ca²⁺, and Mg²⁺).
• High Anion Gap Metabolic Acidosis (AG > 12 mEq/L): Caused by endogenous or exogenous fixed acid accumulation (Mnemonic: MUDPILES):
  • Methanol ingestion
  • Uremia (advanced renal failure)
  • Diabetic ketoacidosis (DKA) / starvation ketoacidosis
  • Propylene glycol / Paraldehyde
  • Isoniazid / Iron
  • Lactic acidosis (hypoperfusion, shock, severe hypoxia, metformin toxicity)
  • Ethylene glycol ingestion (antifreeze)
  • Salicylate overdose (aspirin toxicity)
• Normal Anion Gap (Hyperchloremic) Metabolic Acidosis (AG = 8 to 12 mEq/L): Occurs when bicarbonate (HCO₃⁻) is lost from the body and replaced quantitatively by chloride (Cl⁻) to preserve electroneutrality:
  • Massive Normal Saline Infusion: Rapid infusion of large volumes of 0.9% NaCl (Cl⁻ concentration is 154 mEq/L, far above plasma normal of 100–106 mEq/L), causing dilutional acidosis.
  • Severe Diarrhea: Direct gastrointestinal loss of alkaline bicarbonate fluids.
  • Renal Tubular Acidosis (RTA): Failure of renal tubules to reclaim bicarbonate or secrete hydrogen ions.