Hyperventilation Reduces Transcutaneous Oxygen Tension and Skin Blood Flow

Abstract

Transcutaneous oxygen tension (Ptco2) is often used to monitor neonates and infants in special care units and the operating room. The transcutaneous index (TCI = Ptco2/arterial oxygen tension [Pao2]) is known to depend both on age and on cardiac index but is assumed to be independent of other physiologic variables. In this study we have shown that TCI also depends upon arterial carbon dioxide tension (PaCO2). Five young pigs were anesthetized and paralyzed and their lungs mechanically ventilated while they were monitored with Ptco2 electrodes and serial arterial blood gas analyses. For a 45° C Ptco2 sensor, the mean TCI during normocapnia was 0.78, whereas during hyperventilation (PaCO2 = 20 mmHg) the mean TCI was reduced 65%, to 0.27. The corresponding TCI values for a 43° C sensor were 0.33 and 0.065, representing an 80% decrease in TCI during hyperventilation. Hypoventilation had little effect upon TCI as long as hypoxemia was avoided. Twelve awake adult volunteers with radial artery cannulas were monitored with Ptco2 sensors at several body sites and two sensor temperatures. For a 44° C sensor on the chest, the mean TCI decreased from 0.77 at normocapnia to 0.60 at a PaCO2 of 17 mmHg, a 22% change. For the same sensor on the foot, TCI decreased from 0.63 to 0.32, a 49% change. For a 42° C sensor under the same conditions, the corresponding TCI decreases were 51 and 64%. Six of the volunteers were also monitored with laser-Doppler skin blood flow probes located on the chest, hand, and foot. The skin blood flow measured on the chest decreased by an average of 8% during hyperventilation; blood flow on the hand (thenar eminence) decreased by 60%; and blood flow on the foot decreased by 51%. The TCI is significantly reduced in both swine and adult humans during hyperventilation. This effect is greater on the extremities and at lower sensor temperatures. Laser-Doppler velocimetry measurements indicate a corresponding reduction in cutaneous blood flow during hyperventilation. Transcutaneous oxygen should be used with caution as a predictor of Pao2 during spontaneous or controlled hyperventilation.