Review: Pulse oximetry is better than capnometry for blood gas monitoring in critical care
ACP J Club. 1992 July-Aug;117:21. doi:10.7326/ACPJC-1992-117-1-021
Technology Subcommittee of the Working Group on Critical Care, Ontario Ministry of Health. Non-invasive blood gas monitoring: a review for use in the adult critical care unit. Can Med Assoc J. 1992 Mar 1;146:703-12.
To evaluate the accuracy, reliability, clinical effectiveness, and economic effect of bedside pulse oximetry and capnometry in adult critical care patients.
MEDLINE was searched for articles published between January 1985 and January 1991, using the key words oximetry, carbon dioxide/analysis, and evaluation studies. Reference lists were scanned for relevant articles.
Articles in English reporting original studies were included if they evaluated the use of oximetry or capnometry in critically ill adult patients. 7 studies evaluated oximetry, 3 evaluated capnometry, and 1 evaluated both.
Information was extracted about the clinical setting, patients, and performance of the devices in comparison with direct laboratory arterial blood gas measurement of oxygen saturation (SaO2) or carbon dioxide tension (PaCO2).
9 of 11 studies described the patients and the setting adequately. Diagnostic accuracy could be assessed in 9 studies, but the reliability of the devices, the effects on patient outcome, and the costs were inadequately evaluated. The diagnostic accuracy of pulse oximetry (SpO2) in 6 studies yielding 1447 paired measurements (SaO2 range 51% to 100%) was clinically acceptable. In 5 studies, the bias (mean difference between SaO2 and SpO2) ranged from -1.4% to +1.9%. Oximetry was less accurate in hypoxemic patients with an SaO2 of ≤ 90% (mean difference 5.1%, SD 2.7%). Dark skin, but not hyperbilirubinemia, caused an overestimate of SaO2 (mean difference 3.3%, SD 2.7%). Extreme body temperature and blood pressure also affected accuracy.
The diagnostic accuracy of capnometers measuring the PaCO2-PETCO2 (end-tidal CO2 tension) difference was evaluated in 3 studies including a total of 153 paired measurements (PaCO2 range 19 to 72 mm Hg). The bias (PaCO2-PETCO2) ranged from 2.4 to 18.0. Changes in the gradient were not measured accurately by capnometers. Only 18% of the changes in PETCO2 were within 20% of the changes in PaCO2. In 1 study only 25 (27%) of 91 episodes of hypercarbia were identified by capnometry.
Pulse oximetry is accurate and useful when continuous monitoring of oxygenation is required, except in specified circumstances. Routine bedside use of capnometry is inaccurate under most conditions.
Source of funding: Not stated.
Address for article reprint: Dr. D.J. Leasa, Chest Disease Unit/Critical Care Unit, University Hospital, 339 Windermere Road, London, Ontario N6A 5A5, Canada.
The authors provide a valuable review on the applicability of pulse oximetry (SpO2) and capnography in critically ill patients. The studies show that SpO2 correlates well with arterial O2 saturation in stable critically ill patients. When SpO2 > exceeds 91% in white and 94% in black patients, arterial oxygen tension is adequate (PaCO2 ≥ 60 mm Hg) in 92% of white and 85% of black patients. SpO2 is less accurate, however, during episodes of hemodynamic instability, agitation, and hypothermia, when direct measurement of arterial blood gases is needed.
The findings that end-tidal PCO2 (PETCO2) did not correlate well with arterial PCO2 (PaCO2) in patients with acute hemodynamic and pulmonary disorders is not surprising. Nevertheless, valuable physiologic information may be derived from bedside capnography. Simultaneous measurements of end-tidal and arterial PCO2 can provide estimates of the proportion of dead space ventilation according to the modified Bohr equation. Increases in dead space ventilation may prompt recognition of pulmonary embolism (1) or alert the caregiver to excessive airway pressure during mechanical ventilation. Moreover, during low flow states of circulatory shock and cardiac arrest, PETCO2 becomes a good estimate of cardiac output and may therefore be used to guide resuscitation interventions (2, 3).
In summary, pulse oximetry has emerged as a reasonably accurate noninvasive monitor of arterial oxygenation. In contrast, capnography does not substitute for arterial PCO2 in the critically ill patient. PETCO2, alone or in combination with arterial PCO2 however, provides relevant information on systemic perfusion and dead space ventilation. The ultimate effects of these monitoring techniques on patient management, morbidity, and mortality are yet to be established.
Raúl J. Gazmuri, MD
North Chicago VA Medical CenterChicago, Illinois, USA
Raúl J. Gazmuri, MD
North Chicago VA Medical Center
Chicago, Illinois, USA