Objective: To determine the error of measurement in pulse oximetry with a decreased arterial perfusion and to identify a systolic pressure threshold for (1) initial detection and (2) a reliable reading of oxygen saturation.
Design: An experimental clinical prospective study. The study was approved by the local ethics committee.
Setting: Eighteen bed intensive care unit at a University hospital.
Patients and participants: Twenty-five adult mechanically ventilated and critically ill patients in the ICU during a 3-month period.
Interventions: A blood pressure cuff at the upper arm (same side as an arterial catheter already in place) was inflated to decrease the arterial pulsatile flow. The cuff was deflated stepwise and the resulting oxygen saturation was measured simultaneously. The error of measurement [delta S = SpO2 (baseline)-SpO2 (indicated)] was calculated for each 5 mmHg of blood pressure (BP).
Measurements and results: Twenty-five patients (9 female, 16 male, 48 +/- 15.9 years old) with a mean SpO2 of 98.3 +/- 1.5% and a BP of 129 +/- 18.4 mmHg participated. The mean systolic BP to obtain initial readings with pulse oximetry was 45.8 +/- 17.7 (range, 25-101) mmHg (35% of the baseline pressure) resulting in lower readings of pulse oximetry (mean -11.5 +/- 13.6%, range -45 to +4%). With a systolic BP > 80 mmHg the mean bias was within the manufacturers limits of +/-2%.
Conclusions: Pulse oximetry is reliable with a systolic blood pressure > 80 mmHg. The lower the BP, the lower the pulse oximetry readings leading to a bias of up to -45%.