What effect does bradypnea have on capnography readings?

From the Guidelines

The waveform would slow and rise above 45 mmHg. When the respiratory rate decreases to 8 breaths per minute, the capnography waveform would show fewer cycles per minute, reflecting the slowed breathing pattern. With slower breathing, carbon dioxide has more time to accumulate in the lungs between breaths, causing the end-tidal CO2 (ETCO2) levels to rise above the normal range of 35-45 mmHg, as indicated by studies on capnography in critical care settings 1. This occurs because the decreased ventilation leads to hypoventilation, where CO2 elimination is reduced relative to CO2 production. The slower respiratory rate means each breath is further apart on the time axis of the capnography display, and the plateau phase (representing alveolar gas exchange) would likely be extended.

This pattern is clinically significant as it may indicate respiratory depression, which could be caused by sedative medications, neurological injury, or other conditions that suppress the respiratory drive. Monitoring this change in capnography is crucial for patient safety, especially in sedated patients or those with compromised respiratory function. The importance of monitoring and interpreting capnography waveforms accurately is highlighted in guidelines for cardiopulmonary resuscitation and emergency cardiovascular care 1, emphasizing the need for healthcare providers to be vigilant about changes in respiratory rate and their impact on capnography readings.

Key points to consider in this scenario include:

• The normal range for end-tidal CO2 (ETCO2) is between 35-45 mmHg 1.
• A decrease in respiratory rate can lead to an increase in ETCO2 levels due to reduced CO2 elimination.
• Monitoring capnography is essential for detecting changes in respiratory function and guiding patient care, particularly in critical care settings where patients may be at risk for respiratory depression or other complications 1.
• The correlation between PETCO2 and cardiac output during CPR, as well as the effects of various interventions on PETCO2 levels, underscores the complexity of interpreting capnography data in the context of cardiac arrest and resuscitation efforts 1.

From the Research

Capnography Reading with Slowed Respiratory Rate

• A slowed respiratory rate of 8bpm would affect the capnography reading, with the waveform slowing down as well.
• According to the principles of capnography, a decrease in respiratory rate would lead to a longer expiratory time, resulting in a higher end-tidal CO2 value 2.
• The normal end-tidal CO2 value is typically around 35-45 mmHg, so a slowed respiratory rate could cause the waveform to rise above 45 mmHg 3.
• Therefore, the correct answer is that the waveform would slow and rise above 45.

Relevant Studies

• A study published in the Journal of Clinical and Diagnostic Research found that cutaneous capnography can be used to optimize non-invasive ventilation pressures in patients with chronic hypercapnic respiratory failure due to hypoventilation, with a mean baseline cutaneous carbon dioxide of 45.4 ± 6.5 mmHg 3.
• Another study published in the Scandinavian Journal of Clinical and Laboratory Investigation compared combined oximetry and cutaneous capnography with arterial blood gas analysis, finding a good agreement between the two methods 4.
• The use of capnography in monitoring ventilation is also discussed in a video review published in The New England Journal of Medicine, which explains the principles of capnography and how to interpret the information it provides 2.