What is the effect on capnography readings when the respiratory rate decreases to bradypnea (8 breaths per minute)?

From the Guidelines

A capnography reading with a respiratory rate of 8 breaths per minute is actually within the recommended range for patients with an advanced airway in place during cardiopulmonary resuscitation (CPR), as indicated by the 2010 American Heart Association guidelines 1. When the respiratory rate is within this range, the capnography waveform typically displays a normal exhalation phase with a plateau, indicating adequate time for CO2 elimination during each breath. The end-tidal CO2 (ETCO2) value would likely be within the normal range of 35-45 mmHg, reflecting adequate ventilation and CO2 elimination in the body. This respiratory pattern is not concerning and suggests that the patient is receiving adequate ventilatory support. However, it is essential to continue monitoring the patient's respiratory status and adjust the ventilatory support as needed to ensure optimal oxygenation and CO2 elimination. Some key points to consider when interpreting capnography readings include:

• The correlation between PETCO2 and cardiac output during CPR, as well as the potential effects of vasopressor therapy on PETCO2 values 1
• The use of quantitative waveform capnography to monitor CPR quality, optimize chest compressions, and detect return of spontaneous circulation (ROSC) during chest compressions or when rhythm check reveals an organized rhythm 1
• The importance of considering the clinical context and underlying cause of the patient's condition when interpreting capnography readings and making decisions about ventilatory support. In terms of specific interventions, the 2020 American Heart Association guidelines recommend providing 1 breath every 6 seconds (10 breaths per minute) with continuous chest compressions once an advanced airway is in place 1. However, the 2010 guidelines suggest that a respiratory rate of 8-10 breaths per minute may be adequate in certain situations, such as during CPR with an advanced airway in place 1. Ultimately, the decision to adjust ventilatory support should be based on the individual patient's needs and the clinical context, rather than a single capnography reading.

From the Research

Capnography Reading at a Slowed Respiratory Rate

• A slowed respiratory rate of 8bpm can affect the capnography reading, specifically the end-tidal CO2 (EtCO2) level 2.
• According to a study on the impact of ventilation rate on end-tidal carbon dioxide level during manual cardiopulmonary resuscitation, ETCO2 decays exponentially with increasing ventilation rate 2.
• However, at a slowed respiratory rate, the ETCO2 level is expected to increase due to the decreased ventilation rate 2.
• Another study compared the capabilities of capnography versus a noninvasive respiratory volume monitor to detect changes in respiratory metrics, and found that large changes in minute ventilation resulted in relatively small changes in EtCO2 3.
• The study also found that nasal cannula EtCO2 measurements were on average 4 mm Hg lower than in-line measurements 3.

Clinical Implications

• Capnography can provide earlier identification of respiratory depression than pulse oximetry, especially in patients undergoing moderate sedation 4.
• Respiratory depression identified by capnography occurred in 45% of subjects undergoing transesophageal echocardiography with moderate sedation 4.
• Nursing interventions were effective in preventing more serious cardiorespiratory compromise in patients who experienced respiratory depression 4.
• Understanding the physiological processes involved in CO2 excretion and the differences between capnography and pulse oximetry is important for safe and meaningful use of capnography in monitoring at-risk patients 5.