Notching in ETCO2 Waveform: Causes and Clinical Significance
Notching in the ETCO2 waveform during anesthesia monitoring typically indicates incomplete exhalation, airway obstruction, or breathing circuit dysfunction, and requires immediate investigation to prevent patient harm.
Primary Causes of ETCO2 Notching
Patient-Related Causes
Spontaneous breathing efforts during mechanical ventilation create a characteristic notched or "curare cleft" appearance on the capnography waveform, representing the patient's attempt to breathe against or with the ventilator 1
Bronchospasm or airway obstruction produces notching due to uneven emptying of lung units with different time constants, where obstructed airways empty more slowly and create a secondary rise in the plateau phase 2
Chronic obstructive pulmonary disease (COPD) or severe asthma causes similar waveform distortion due to heterogeneous ventilation and prolonged expiratory phase 2
Inadequate neuromuscular blockade allows residual muscle activity that interferes with controlled ventilation, creating irregular waveform patterns 1
Equipment and Circuit-Related Causes
Partial circuit disconnection between components can create notching, particularly when the disconnection occurs on the machine side of the gas sampling port, as the sampling may continue to detect CO2 while ventilation is compromised 3
Kinked or obstructed endotracheal tube restricts gas flow and creates turbulent flow patterns that manifest as notching on the capnogram 2
Malfunctioning unidirectional valves in circle breathing systems allow rebreathing and create irregular CO2 waveforms, which should be detected during the two-bag test performed before each case 4
Inadequate fresh gas flow in certain breathing systems can lead to rebreathing and waveform abnormalities 4
Critical Differential Diagnosis
Malignant Hyperthermia Consideration
While notching itself is not a classic sign of malignant hyperthermia (MH), any unexplained change in ETCO2 patterns warrants consideration of this life-threatening condition:
Rapidly rising ETCO2 values (80-86 mmHg) that are resistant to increased minute ventilation strongly suggest MH, especially with recent exposure to triggering agents 5, 6
Progressive increase in ETCO2 accompanied by tachycardia is the cardinal early feature of MH, appearing before temperature elevation 6
Immediate discontinuation of volatile anesthetics and administration of dantrolene is required if MH is suspected, as delayed treatment increases mortality 5, 6
Systematic Approach to Investigation
Immediate Assessment Steps
Verify circuit integrity by performing visual inspection and checking all connections are secured with "push and twist" technique 4
Assess ventilator function by confirming adequate pressure generation during inspiration and proper alarm configuration 4
Evaluate patient factors including auscultation for bronchospasm, checking neuromuscular blockade depth, and assessing for spontaneous breathing efforts 4, 1
Check sampling line patency as obstruction or water accumulation can distort waveforms, though this typically affects amplitude rather than creating notching 4
Equipment Troubleshooting
Inspect the CO2 absorber for exhaustion or channeling, which can affect breathing system function 4
Verify proper vaporizer function and ensure no tilting has occurred, as this can affect gas delivery and circuit dynamics 4
Confirm correct breathing system configuration, particularly for Bain-type and circle co-axial systems where inner tube occlusion can cause problems 4
Clinical Significance and Monitoring
Continuous waveform capnography is the most reliable method for monitoring ventilation and detecting circuit problems during anesthesia 4
The waveform shape provides more diagnostic information than numeric ETCO2 values alone, as notching indicates specific pathophysiology requiring targeted intervention 2
In spontaneously breathing patients, ETCO2 monitoring may show reduced sensitivity to ventilation changes compared to intubated patients, with large changes in minute ventilation producing minimal ETCO2 changes 1
During cardiac arrest or low cardiac output states, ETCO2 values may be falsely low despite correct tube placement due to decreased pulmonary blood flow 4
Common Pitfalls to Avoid
Assuming equipment is functioning properly without systematic checking leads to delayed recognition of circuit problems 4
Failing to perform the two-bag test before each case misses opportunities to detect breathing system leaks or obstructions 4
Ignoring subtle waveform changes while focusing only on numeric values can delay diagnosis of developing problems 2
Not considering patient-ventilator dyssynchrony in patients with inadequate sedation or paralysis leads to inappropriate troubleshooting 1