Artifacts in End-Tidal CO2 Monitoring and Capnography in Dogs and Cats

End-tidal carbon dioxide (ETCO2) monitoring and capnography are essential tools in veterinary medicine for assessing ventilation status in anesthetized and critically ill patients. However, various artifacts can interfere with accurate readings and waveform interpretation. This essay explores common capnography artifacts encountered in small animal practice, their potential causes, and practical solutions for clinicians.

Normal Capnogram Review #

Before discussing artifacts, it’s important to understand the normal capnogram waveform. A typical capnogram consists of four phases:

1. Phase I: The inspiratory baseline (normally zero)
2. Phase II: The expiratory upstroke
3. Phase III: The alveolar plateau
4. Phase IV: The inspiratory downstroke

The end-tidal CO2 value is measured at the end of Phase III, just before the downstroke. Normal ETCO2 values in healthy dogs and cats typically range from 35-45 mmHg.

Common Artifacts and Their Causes #

1. Baseline Elevation #

Differential Diagnosis: #

1. Equipment malfunction: Sensor drift or calibration error
2. Rebreathing: Inadequate fresh gas flow in a non-rebreathing system
3. Exhausted CO2 absorbent: Especially in closed or semi-closed systems
4. Dead space ventilation: Insufficient tidal volume
5. Contamination: Water vapor or secretions in the sampling line

Solutions: #

• Recalibrate the monitor according to manufacturer guidelines
• Increase fresh gas flow to prevent rebreathing if using a non-rebreathing system
• Replace CO2 absorbent when color change indicates exhaustion
• Check and adjust ventilation parameters to ensure adequate tidal volumes
• Clear or replace contaminated sampling lines

2. Sudden Zero Baseline #

Differential Diagnosis: #

1. Disconnection: Complete circuit or sampling line disconnection
2. Endotracheal tube dislodgement: Accidental extubation
3. Sampling line occlusion: Secretions or kinking blocking the line
4. Cardiopulmonary arrest: Absence of CO2 production/delivery
5. Equipment failure: Power loss or sensor malfunction

Solutions: #

• Check all connections in the breathing circuit and sampling line
• Verify endotracheal tube placement
• Flush or replace occluded sampling lines
• Assess patient for cardiopulmonary function
• Check equipment power and functionality

3. “Curare Cleft” or Respiratory Effort Artifact #

Differential Diagnosis: #

1. Inadequate neuromuscular blockade: Spontaneous respiratory efforts against ventilator
2. Inadequate anesthetic depth: Light plane causing respiratory effort
3. Pain response: Surgical stimulation causing respiratory effort
4. Ventilator asynchrony: Patient fighting mechanical ventilation
5. Hiccupping: Diaphragmatic spasms

Solutions: #

• Reassess neuromuscular blockade level if using paralytic agents
• Deepen anesthetic plane if appropriate
• Provide additional analgesia if pain is suspected
• Adjust ventilator settings to improve synchrony
• Wait for hiccups to resolve or consider anticholinergics

4. Cardiogenic Oscillations #

Differential Diagnosis: #

1. Cardiac proximity to lungs: More common in cats and deep-chested dogs
2. Severe cardiomegaly: Heart impinging on pulmonary space
3. Pericardial effusion: Causing heart to impact lung movement
4. Decreased lung compliance: Various pulmonary diseases
5. Normal variant

Solutions: #

• Generally no action required as this is a normal physiological finding in some patients
• Increase tidal volume or rate if clinically appropriate
• Note that presence may correlate with cardiac output, so changes may be clinically relevant
• Document as normal variant for the specific patient

5. “Shark Fin” Pattern #

Differential Diagnosis: #

1. Bronchospasm: Common in asthmatic patients
2. Airway obstruction: Partial endotracheal tube obstruction
3. Small endotracheal tube: Relative to patient size
4. Mucus or secretions: Partially occluding the airway
5. Pressure from pneumoperitoneum: During laparoscopic procedures

Solutions: #

• Administer bronchodilators for suspected bronchospasm
• Clear airway of obstructions or secretions
• Consider changing to a larger endotracheal tube
• Optimize ventilation parameters (I:E ratio, peak pressure)
• Decrease insufflation pressure during laparoscopy if possible

6. Biphasic Waveform #

Differential Diagnosis: #

1. Ventilator leak: Circuit or cuff leak allowing fresh gas mixing
2. Incompetent endotracheal tube cuff: Especially with short-snouted breeds
3. Esophageal intubation with gastric gas: Rare but possible
4. Sampling chamber contamination: With water or secretions
5. Dual lung populations: V/Q mismatch with different time constants

Solutions: #

• Check and correct circuit leaks
• Ensure proper endotracheal tube cuff inflation
• Verify tube placement by direct visualization and auscultation
• Replace or clean contaminated sampling lines
• Consider respiratory disease if dual lung populations suspected

7. Clipped or Attenuated Waveform #

Differential Diagnosis: #

1. Sampling line obstruction: Partial blockage by water or secretions
2. Kinked sampling line: Restricting gas flow
3. Filter obstruction: Patient-side filter becoming obstructed
4. Inappropriate scale settings: Device scale set too high

Solutions: #

• Clear or replace sampling lines
• Straighten any kinked tubing
• Replace wet or obstructed filters
• Adjust scale settings to appropriate range

Clinical Significance and Interpretation #

Recognizing capnography artifacts is crucial for accurate patient monitoring. False readings can lead to inappropriate clinical interventions, while missing true abnormalities can result in delayed recognition of patient deterioration. When interpreting capnograms:

1. Always correlate with clinical signs: Assess the patient’s overall clinical status, not just the numbers
2. Look for trends: Sudden changes often indicate artifacts, while gradual changes may reflect patient status
3. Check equipment first: Before attributing changes to patient condition, verify equipment functioning
4. Consider patient factors: Species, breed, size, and disease status affect normal readings and waveforms
5. Document baseline: Establish the patient’s normal capnogram appearance for comparison

Special Considerations for Dogs and Cats #

Several factors specific to veterinary patients influence capnography:

1. Patient size: Very small patients (cats, toy breed dogs) may produce attenuated waveforms due to lower tidal volumes. It is also more common for dead space to create elevated baseline if adding to much deadspace from adapters, sensors, etc.
2. Respiratory rate: Higher normal respiratory rates in small animals can lead to incomplete exhalation artifacts
3. Brachycephalic anatomy: Breeds with shortened airways often display unique baseline patterns
4. Specialized equipment: Use of sidestream capnography with pediatric flow rates is often necessary for accurate readings
5. Dead space: Greater significance of equipment dead space in smaller patients

Conclusion #

End-tidal CO2 monitoring and capnography provide valuable information when properly interpreted. Recognizing and addressing artifacts is essential for accurate assessment of ventilation status in anesthetized and critically ill dogs and cats. Understanding common artifacts, their causes, and resolution strategies enables veterinary clinicians to make appropriate treatment decisions based on reliable data.

References #

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