Long-term Registration of Cutaneous Microcirculation During General Anesthesia

The temporal dynamics of the systemic arterial pressure can be monitored noninvasively from the skin of the earlobe or forehead by photoplethysmography under the provision that the active control of the microcirculatory perfusion is eliminated. Using this approach, we have been able to detect a highly stable blood pressure rhythm in the range of 0.15 Hz during psychophysical relaxation or sleep. The aim of the present study was to investigate the occurrence and behavior of blood pressure rhythms below 0.2 Hz during general anesthesia. In 30 patients (ASA groups I-II) undergoing basic surgical procedures, photoplethysmographic recordings from the earlobe were made during the whole time of anesthesia. The recorded signals were divided into segments of 200 s of duration, the temporal structure of which was analyzed by fast Fourier transform. Different characteristic patterns of rhythmical behavior were detected: (1) absence of activity below 0.2 Hz ('low-frequency range'); (2) slow sinusoidal rhythmicity below 0.05 Hz; (3) 'chaotic' behavior, i.e. multiple incoherent fluctuations without stationary periods or amplitudes; (4) short-term rhythmical activity at about 0.15 Hz, and (5) long-term rhythmical activity at about 0.15 Hz. In patients sufficiently sedated to eliminate low-frequency activity, rhythmicity could sometimes be triggered by certain surgical stimuli, the response to which was suppressed by injection of opioids. The data presented strongly suggest that rhythmical perfusion patterns of the cutaneous microcirculation could serve as an indicator for the depth of anesthesia.