Efficacy of pulse oximetry and capnometry in postoperative ventilatory weaning
We examined the ability of capnometry and pulse oximetry to identify potential respiratory problems by comparing oxyhemoglobin saturation (O2 Sat) as measured by pulse oximetry and end-tidal CO2 (Petco2) with arterial blood gas (ABG) determinations in 40 mechanically ventilated ICU patients. Hemoglobin saturation as measured by pulse oximetry correlated significantly with Pao2 (r =.65, p <.0001); more importantly, an oximeter 02 Sat < 95% showed 100% sensitivity in identifying hypoxemia (i.e., Pao2 < 70 torr). Petco2 tended to correlate strongly with Paco2 for individual patients, but not when evaluated as a screening tool for identifying ventilatory abnormalities in the overall group (r =.52, p <.0001). A Petco2 < 26 torr identified hypocarbia (defined as Paco2 < 30 torr) with a sensitivity of 85%. However, a Petco2 of > 40 torr predicted hypercarbia (Paco2 > 45 torr) with a sensitivity of only 28%.
The efficacy of pulse oximetry and capnometry in monitoring respiratory status during postoperative ventilatory weaning was examined in a subset of 24 patients who had undergone elective cardiac surgery. All patients were weaned by intermittent mandatory ventilation, but each was assigned randomly to either a control group monitored with periodic ABG sampling or to an experimental group, monitored by following Petco2 and O2 Sat via pulse oximetry. In the experimental group, ABG values were obtained on ICU admission, but thereafter only if a) O2 Sat < 95%, b) Petco2 < 26 or > 40 torr, or c) felt to be clinically indicated by ICU staff. Patients monitored by capnometry and pulse oximetry had fewer blood gas determinations per patient (5.9 ± 2.7 [SD]) than control patients (10.5 ± 1.8, p <.0001). There was no difference in length of intubation between the two groups, and once extubated, no patient required reintubation. The noninvasive equipment alerted the staff to five episodes of hypoxemia (i.e., Pao2 < 70 torr) and six episodes of hypercarbia (Paco2 > 45 torr).
Capnometry and pulse oximetry are potentially useful techniques for monitoring respiratory status during postoperative weaning and may result in less frequent ABG sampling compared to some traditional weaning protocols. Nevertheless, while oximetry seems to be sensitive in identifying hypoxemia, capnometry appears to be relatively insensitive to hypercarbia.