Baylor University Medical Center Proceedings January 2014, Volume 27, Number 1 | Page 17

90%

80

a

b

70

80%
Total number of Airway Interventions

Cumulative Percentage of Airway Interventions

100%

70%
56.3%

60%
50%
40%

31.8%

30%
20%
10%

Deep Sedation

60
Normal CO2 level
50

40

30

20

General Anesthesia
10

0%
100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5

0
0-10

0

11-20

21-30

31-37

38-42

43-50

51-60

61-70

>70

ETCO2 (mmHg)

PSI at Time of Airway Intervention

Figure 1. Measurements at the onset of airway interventions: (a) cumulative percentage of airway intervention as Patient State Index value decreases and (b) endtidal carbon dioxide level at the time of an airway intervention.

intervention occurred. If interventions were deemed to be appropriate for the blinded subjects, the intervention occurred when
the mean PSI was considered to be general anesthesia and with
significantly higher tcpCO2, indicating a prolonged hypoventilation. In addition, a larger number of airway interventions,
and more aggressive interventions, were performed in the group
where the anesthesiologist had access to PSI data.
The use of supplemental oxygen during the procedure keeps
the SpO2 in the high 90s and minimizes its usefulness as a
sensitive monitor of respiratory depression (7–9). Downs and
his team have clearly demonstrated how the administration of
supplemental oxygen can allow arterial carbon dioxide levels
to reach dangerously high levels before oxygen saturation declines significantly (8). Data from this study are consistent with
Downs’ work, as airway interventions were generally made by
anesthesiologists while the SpO2 was still within acceptable levels but the tcpCO2 was elevated. The mean tcpCO2 of 48 mm
Hg in the blinded group versus 43 mm Hg in the unblinded
group indicates the blinded group experienced significant respiratory compromise, as the physiological rate of the increase
of the partial pressure of arterial carbon dioxide is 3 to 6 mm
Hg per minute during apnea (10).
The study was designed knowing that supplemental oxygen
limits the usefulness of SpO2 and, therefore, EtCO2 and tcpCO2
were monitored to determine if the CO2 level change was predictive of respiratory distress. Stepwise logistic regression was
performed on all measures of respiratory status and sedation levels
to see if a combination of measurements in the minutes prior to
an airway intervention was predictive of the intervention. None
of the statistically significant measurements were related strongly
enough to be predictive of the need for an intervention.
All colonoscopies were performed by one of two gastroenterologists. Ideally, future studies should be done utilizing a single
gastroenterologist to ensure consistency. In addition, baseline vital
signs including baseline CO2 should be recorded immediately
prior to the start of the procedure in order to determine changes
from baseline rather than protocol-driven parame ѕ