CJEM Articles: Heather E. Murray

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  • A comparative evaluation of capnometry versus pulse oximetry during procedural sedation and analgesia on room air
    September 2010 12 5
    David W. Messenger, Heather E. Murray, Janet van Vlymen, Marco L.A. Sivilotti, Paul E. Dungey

    Objective: Important questions remain regarding how best to monitor patients during procedural sedation and analgesia (PSA). Capnometry can detect hypoventilation and apnea, yet it is rarely used in emergency patients. Even the routine practice of performing preoxygenation in low-risk patients is controversial, as supplementary oxygen can delay the detection of respiratory depression by pulse oximetry. The purpose of this study was to determine whether the capnometer or the pulse oximeter would first detect respiratory events in adults breathing room air.

    Methods: During a randomized clinical trial comparing fentanyl with low-dose ketamine for PSA with titrated propofol, patients were monitored using pulse oximetry and continuous oral–nasal sampled capnography. Supplemental oxygen was administered only for oxygen desaturation. Sedating physicians identified prespecified respiratory events, including hypoventilation (end-tidal carbon dioxide > 50 mm Hg, rise of 10 mm Hg from baseline or loss of waveform) and oxygen desaturation (pulse oximetry < 92%). These events and their timing were corroborated by memory data retrieved from the monitors.

    Results: Of 63 patients enrolled, 57% (36) developed brief oxygen desaturation at some point during the sedation. All responded to oxygen, stimulation or interruption of propofol. Measurements of end-tidal carbon dioxide varied substantially between and within patients before study intervention. Hypoventilation (19 patients, 30%) was only weakly associated with oxygen desaturation (crude odds ratio 1.4 [95% confidence interval 0.47 to 4.3]), and preceded oxygen desaturation in none of the 12 patients in whom both events occurred (median lag 1:50 m:ss [interquartile range 0:01 to 3:24 m:ss]).

    Conclusion: During PSA in adults breathing room air, desaturation detectable by pulse oximeter usually occurs before overt changes in capnometry are identified.

  • Can peripheral venous blood gases replace arterial blood gases in emergency department patients?
    January 2002 4 1
    Cameron K. MacGougan, George A. Wells, Heather E. Murray, Louise C.F. Rang

    Objective: To determine if peripheral venous blood gas values for pH, partial pressure of carbon dioxide (Pco2) and the resultant calculated bicarbonate (HCO3) predict arterial values accurately enough to replace them in a clinical setting.
    Methods: This prospective observational study was performed in a university tertiary care emergency department from June to December 1998. Patients requiring arterial blood gas analysis were enrolled and underwent simultaneous venous blood gas sampling. The following data were prospectively recorded: age, sex, presenting complaint, vital signs, oxygen saturation, sample times, number of attempts and indication for testing. Correlation coefficients and mean differences with 95% confidence intervals (CIs) were calculated for pH, Pco2 and HCO3. A survey of 45 academic emergency physicians was performed to determine the minimal clinically important difference for each variable.
    Results: The 218 subjects ranged in age from 15 to 90 (mean 60.4) years. The 2 blood samples were drawn within 10 minutes of each other for 205 (96%) of the 214 patients for whom data on timing were available. Pearson's product-moment correlation coefficients between arterial and venous values were as follows: pH, 0.913; Pco2, 0.921; and HCO3, 0.953. The mean differences (and 95% CIs) between arterial and venous samples were as follows: pH, 0.036 (0.030-0.042); Pco2, 6.0 (5.0-7.0) mm Hg; and HCO3, 1.5 (1.3-1.7) mEq/L. The mean differences (± 2 standard deviations) were greater than the minimum clinically important differences identified in the survey.
    Conclusions: Arterial and venous blood gas samples were strongly correlated, and there were only small differences between them. A survey of emergency physicians suggested that the differences are too large to allow for interchangeability of results; however, venous values may be valid if used in conjunction with a correction factor or for trending purposes.