INVESTIGATING THE ORIGIN OF CYCLIC CHANGES IN LIMB VOLUME USING MERCURY-IN-SILASTIC STRAIN-GAUGE PLETHYSMOGRAPHY IN MAN

1. Vasomotion, a phenomen frequently observed in skeletal muscle micro circulation, has been observed under physiological conditions and foun d enhanced during critical reduction of tissue perfusion due to hypoxi c hypoxia, haemorrhage and local hypotension. We used a computer assis ted non-invasive plethysmographic method to investigate periodic chang es of limb volume (volumotion), which ave previously found in critical ly ill patients. The current study was designed to investigate the ori gin of volumotion. 2. Simultaneous recordings of limb circumference of both calves were obtained. In patients with peripheral vascular disea se and patients with minor surgery the cross-correlation with spontane ous breathing was investigated. In patients who had undergone major ab dominal or vascular surgery Ive analysed for cross-correlations betwee n MSG plethysmography signals of both legs and changes in central veno us, arterial and ventilation pressures. In this group the effects of p ositive-pressure ventilation and positive end-expiratory pressure (PEE P) were also studied. 3. No ventilation-related volumotion was observe d during spontaneous breathing. During positive-pressure ventilation a ventilation-related peak was found in the Fourier transform in agreem ent with the ventilation frequency applied. The amplitude of ventilati on-related volumotion decreased significantly after a pressure cuff ap plied to the thigh was inflated above central venous pressure. We obse rved a significant increase in the amplitude of ventilation-related vo lumotion when PEEP was applied. 4. Ventilation-related volumotion show ed significant cross-correlation between both legs. Slow wave volumoti on (0.5-7 cycles min(-1)) was frequently observed, but always appeared unilaterally. Whilst we looked for correlations between slow wave vol umotion and changes in central venous pressure and arterial blood pres sure, respectively, significant crosscorrelation with the MSG plethysm ography recordings was only observed at the frequency of the positive- pressure ventilation. 5. The fact that periodic changes of limb circum ference matching the frequency of the positive-pressure ventilation we re detectable with the MSG plethysmography system demonstrates that sm all volume changes (less than 0.02 ml (100 ml tissue)(-1)) can be asse ssed using this system. As slow wave volumotion was observed unilatera lly and revealed no correlation either with breathing, ventilation or arterial and central venous pressure, we suggest that slow wave volumo tion is a local event most likely reflecting arteriolar vasomotion.