Research Article: The relationship between minute ventilation and end tidal CO2 in intubated and spontaneously breathing patients undergoing procedural sedation

Date Published: June 29, 2017

Publisher: Public Library of Science

Author(s): Jaideep H. Mehta, George W. Williams, Brian C. Harvey, Navneet K. Grewal, Edward E. George, Yu Ru Kou.

http://doi.org/10.1371/journal.pone.0180187

Abstract

Monitoring respiratory status using end tidal CO2 (EtCO2), which reliably reflects arterial PaCO2 in intubated patients under general anesthesia, has often proven both inaccurate and inadequate when monitoring non-intubated and spontaneously breathing patients. This is particularly important in patients undergoing procedural sedation (e.g., endoscopy, colonoscopy). This can be undertaken in the operating theater, but is also often delivered outside the operating room by non-anesthesia providers. In this study we evaluated the ability for conventional EtCO2 monitoring to reflect changes in ventilation in non-intubated surgical patients undergoing monitored anesthesia care and compared and contrasted these findings to both intubated patients under general anesthesia and spontaneously breathing volunteers.

Minute Ventilation (MV), tidal volume (TV), and respiratory rate (RR) were continuously collected from an impedance-based Respiratory Volume Monitor (RVM) simultaneously with capnography data in 160 patients from three patient groups: non-intubated surgical patients managed using spinal anesthesia and Procedural Sedation (n = 58); intubated surgical patients under General Anesthesia (n = 54); and spontaneously breathing Awake Volunteers (n = 48). EtCO2 instrument sensitivity was calculated for each patient as the slope of a Deming regression between corresponding measurements of EtCO2 and MV and expressed as angle from the x-axis (θ). All data are presented as mean ± SD unless otherwise indicated.

While, as expected, EtCO2 and MV measurements were negatively correlated in most patients, we found gross systematic differences across the three cohorts. In the General Anesthesia patients, small changes in MV resulted in large changes in EtCO2 (high sensitivity, θ = -83.6 ± 9.9°). In contrast, in the Awake Volunteers patients, large changes in MV resulted in insignificant changes in EtCO2 (low sensitivity, θ = -24.7 ± 19.7°, p < 0.0001 vs General Anesthesia). In the Procedural Sedation patients, EtCO2 sensitivity showed a bimodal distribution, with an approximately even split between patients showing high EtCO2 instrument sensitivity, similar to those under General Anesthesia, and patients with low EtCO2 instrument sensitivity, similar to the Awake Volunteers. When monitoring non-intubated patients undergoing procedural sedation, EtCO2 often provides inadequate instrument sensitivity when detecting changes in ventilation. This suggests that augmenting standard patient care with EtCO2 monitoring is a less than optimal solution for detecting changes in respiratory status in non-intubated patients. Instead, adding direct monitoring of MV with an RVM may be preferable for continuous assessment of adequacy of ventilation in non-intubated patients.

Partial Text

Whereas it is standard practice to both control and monitor ventilation during general anesthesia, it is equally important to monitor ventilation in non-intubated patients undergoing procedural sedation. End tidal CO2 (EtCO2) monitoring with capnography has become the standard of care in intubated patients for both confirming endotracheal tube placement and monitoring adequacy of ventilation [1,2]. Capnography with an endotracheal tube in place is considered a reliable method to non-invasively reflect arterial PaCO2 [3,4], however, measuring EtCO2 in spontaneously breathing patients can be inaccurate in certain settings, particularly during procedural sedation [5] and post-operatively in the post-anesthesia care unit [6–8]. Variables such as sensor positioning, changes in respiratory patterns, and changes in oxygen supplementation often distort EtCO2 measurements in non-intubated patients, rendering them unreliable. As a result, it is common for healthcare providers to overlook or discount information obtained from the capnography waveform [9].

Data were collected from 160 patients across the three cohorts (Table 2). Height and weight were not significantly different across the three cohorts (p = 0.12 and p = 0.17, respectively), however, BMI and age were significantly lower in the Awake Volunteers group compared to the Procedural Sedation and General Anesthesia groups (BMI: p < 0.02; age: p < 0.0001 for both comparisons). Procedural Sedation and General Anesthesia patients tended to have more comorbidities than the Awake Volunteers. All patients in the Procedural Sedation and General Anesthesia groups received supplemental oxygen and both groups had a similar average FiO2 delivered throughout the procedure (p = 0.86). In this study, we assessed and quantified the ability of capnography to measure and reflect real-time changes in respiratory status, specifically ventilation (MV), in non-intubated patients undergoing procedural sedation. First, we quantified EtCO2 instrument sensitivity for each patient as the slope of a Deming regression between corresponding measurements of EtCO2 and MV. Next, we compared the instrument sensitivity between patients under Procedural Sedation and two control groups: General Anesthesia and Awake Volunteers. In the intubated patient under General Anesthesia, we found a strong relationship between MV and EtCO2 (median EtCO2 instrument sensitivity of -85.1°). This EtCO2 instrument sensitivity was better than the clinically-relevant EtCO2 instrument sensitivity of -76°, confirming EtCO2 measurements in intubated patients could adequately reflect changes in MV. In contrast, in the non-intubated patients (i.e., both the Procedural Sedation and Awake Volunteer groups), the relationship between MV and EtCO2 is much weaker (median EtCO2 instrument sensitivities of -38.1° and -20.2°, respectively) and better than the clinically-relevant instrument sensitivity of -76° in only 23% (24/106) of the non-intubated patients. This finding indicates that the EtCO2 instrument sensitivity in non-intubated and spontaneously breathing individuals may not be adequate for detecting meaningful changes in MV in over three-fourth of patients. While EtCO2 is a useful indicator of respiratory status in patients under General Anesthesia, its sensitivity to changes in ventilation is greatly reduced in non-intubated patients. Therefore, agumenting standard patient care with EtCO2 monitoring is a suboptimal solution for monitoring respiratory status in non-intubated patients undergoing Procedural Sedation. The addition of direct monitoring of MV with an RVM may be preferable for primary continuous assessment of adequate ventilation of non-intubated patients undergoing procedural sedation.   Source: http://doi.org/10.1371/journal.pone.0180187

 

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