Capnography Waveform Differences Between Hypoventilation and Hyperventilation
Capnography waveforms show distinct patterns in hypoventilation versus hyperventilation, with hypoventilation characterized by elevated end-tidal CO2 (ETCO2) values >50 mmHg or an increase >10 mmHg from baseline, while hyperventilation shows decreased ETCO2 values typically below 35 mmHg. 1
Hypoventilation Waveform Characteristics
ETCO2 Values
- Elevated ETCO2 readings >50 mmHg indicate significant hypoventilation requiring intervention 2, 1
- An absolute increase from baseline ETCO2 >10 mmHg is clinically significant 1
- Normal ETCO2 range is 35-40 mmHg in healthy individuals 1
Waveform Appearance
- Increased amplitude of the waveform (taller peaks)
- Slower respiratory rate with maintained or increased ETCO2
- Prolonged expiratory phase (plateau phase)
- May show "shark fin" appearance in severe cases with gradual rise and no clear plateau
Clinical Significance
- Hypoventilation is detected by capnography before oxygen desaturation occurs, providing earlier warning of respiratory compromise 2, 1
- All patients with respiratory depression demonstrate either ETCO2 >50 mmHg, an absent waveform, or an absolute change from baseline in ETCO2 >10 mmHg 2
- Pulse oximetry alone would identify only one-third of patients with respiratory depression 2
Hyperventilation Waveform Characteristics
ETCO2 Values
- Decreased ETCO2 readings below 35 mmHg 2
- In primary hyperventilation, Pa,CO2 typically remains depressed during exercise 2
Waveform Appearance
- Decreased amplitude of the waveform (shorter peaks)
- Increased respiratory rate with decreased ETCO2
- Shortened expiratory phase
- More frequent waveforms due to increased respiratory rate
Clinical Significance
- Primary hyperventilation shows marked hyperventilation at rest with abnormally large increases in ventilation and an elevated ventilation-CO2 production (VE-VCO2) slope 2
- Secondary hyperventilation may result from hypoxemia-induced stimulation of peripheral chemoreceptors 2
- Capnography helps prevent hyperventilation, which can worsen outcomes in shock by reducing cerebral blood flow 1
Monitoring Considerations
Technical Factors Affecting Readings
- High oxygen flows can dilute expired CO2, causing artificially lower ETCO2 readings 3
- Mask designs may either cause CO2 rebreathing or inadequately capture expired CO2 3
- ETCO2 is not an accurate estimate of PaCO2 during mouth breathing or with low tidal volume and fast respiratory rates 2
Clinical Applications
- Capnography provides real-time information about three critical physiological processes: ventilation, perfusion, and metabolism 1
- In procedural sedation, average ETCO2 typically increases from 36 to 42 mmHg, which may be clinically insignificant in isolation 1
- Capnography should be used in conjunction with clinical assessment, not as a replacement 4
Management Implications
For Hypoventilation
- When observing rising capnography in a conscious patient, assess respiratory status, encourage deep breathing, consider supplemental oxygen, and reduce sedative medications 1
- Prepare for potential airway intervention if ETCO2 continues to rise despite interventions 1
For Hyperventilation
- Identify and address underlying causes (anxiety, pain, metabolic acidosis)
- Consider breathing techniques to slow respiratory rate if appropriate
- Monitor for signs of respiratory alkalosis
Limitations of Capnography
- Less reliable in patients with severe respiratory failure due to increased V/Q mismatch 5
- May be least useful in the sickest patients with widened P(a-ET) gradient 5
- Accuracy decreases with low tidal volumes or equipment malfunction 4
- Should always be used in conjunction with clinical assessment and other monitoring parameters 4
Capnography remains an essential monitoring tool that provides early detection of ventilatory abnormalities before they manifest as oxygen desaturation, making it invaluable for patient safety during procedural sedation and in critically ill patients.