Inadequacy of pulse oximetry in the catheterization laboratory. An exploratory study monitoring respiratory status using arterial blood gases during cardiac catheterization with conscious sedation.

BACKGROUND: Benzodiazepines and opioids are commonly used for conscious sedation (CS) in cardiac catheterization laboratory (CCL) patients. Both drugs are known to predispose to hypoxemia, apnea and decreased responsiveness to PCO2 , resulting in decreased arterial pH and PO2 , as well as increased PCO2 . We want to determine the effects of CS on arterial blood gas (ABG) in CCL patient, and identify if pulse oximetry monitoring is adequate.

METHODS: We enrolled 18 subjects undergoing elective catheterization. Measurement of ABGs at one-minute intervals was done from the moment of arterial access until case end. The results of ABGs were not available to the clinician who administered sedation. Relationships of pH, PCO2 , PaO2 and SaO2 were studied by plotting time series graphs. Significant changes were defined as pH <7.30, SaO2  < 90, and PCO2  > 50 mmHg.

RESULTS: No significant change in pH, PCO2 , PaO2 and SaO2 was noted in 4/18 (22%) subjects. A significant drop in SaO2 was noted in 4/18 (22%). A significant change in PCO2 and/or pH was noted in 10/18 (55%) cases. Among the 16 (16/18) subjects receiving supplemental oxygen, 7 (7/18, 39%) had no drop in SaO2 , but developed respiratory acidosis. At the end of the case, 5/18 (28%) subjects had respiratory acidosis with normal PaO2 .

CONCLUSION: Significant hypercarbia and acidosis occurred frequently in this small study during CS in patients undergoing cardiac catheterization. Relying on pulse oximetry alone especially with patients on supplemental oxygen may lead to failure in detecting respiratory acidosis in a significant number of patients.