A critique of the plethysmographic method of measuring blood flow in the extremities of man

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 2: physiological measurements

In this, the second of four historical reviews on human thermoregulation during exercise, we examine the research techniques developed by our forebears. We emphasise calorimetry and thermometry, and measurements of vasomotor and sudomotor function. Since its first human use (1899), direct calorimetry has provided the foundation for modern respirometric methods for quantifying metabolic rate, and remains the most precise index of whole-body heat exchange and storage. Its alternative, biophysical modelling, relies upon many, often dubious assumptions. Thermometry, used for >300 y to assess deep-body temperatures, provides only an instantaneous snapshot of the thermal status of tissues in contact with any thermometer. Seemingly unbeknownst to some, thermal time delays at some surrogate sites preclude valid measurements during non-steady state conditions. To assess cutaneous blood flow, immersion plethysmography was introduced (1875), followed by strain-gauge plethysmography (1949) and then laser-Doppler velocimetry (1964). Those techniques allow only local flow measurements, which may not reflect whole-body blood flows. Sudomotor function has been estimated from body-mass losses since the 1600s, but using mass losses to assess evaporation rates requires precise measures of non-evaporated sweat, which are rarely obtained. Hygrometric methods provide data for local sweat rates, but not local evaporation rates, and most local sweat rates cannot be extrapolated to reflect whole-body sweating. The objective of these methodological overviews and critiques is to provide a deeper understanding of how modern measurement techniques were developed, their underlying assumptions, and the strengths and weaknesses of the measurements used for humans exercising and working in thermally challenging conditions.

Simplified determination of arterial insufficiency; plethysmographic observation of reactive hyperemia following fifteen minute arterial occlusion at the ankle

Reactive hyperemia following 15 minute arterial occlusion at the ankle was studied in plethysmographic tracings using the venous occlusion technic. The resulting curves present an essentially different pattern in normal subjects and patients with various types of peripheral arterial disease. Several characteristic features can be distinguished, notably the level of highest blood flow after release of arterial occlusion, the time required until the highest blood flow is reached, and the degree of secondary vasoconstriction. Evaluation of these factors permits an estimate of the therapeutic value of lumbar sympathectomy.

Effects of ankle-to-knee external pressures on skin blood perfusion under and distal to compression

OBJECTIVE

To compare the effects of select leg compression pressures on blood perfusion in the skin overlying bone and the skin distal to the leg compression.

DESIGN

Blood perfusion of skin overlying the tibia and the foot dorsum was simultaneously measured in 12 healthy subjects using laser Doppler. Each subject's calf was compressed from ankle to knee with an air cast that applied external pressure, ranging from 0 to 40 mm Hg in 10 mm Hg increments. All measurements were initiated after a 20-minute rest interval and were recorded with each subject in a supine position.

SETTING

University research center.

RESULTS

The main findings show that compression causes a significant decrease in skin blood perfusion (SBF) at both sites, with greater reductions in SBF occurring with increased compression pressure. Greater SBF reductions occurred at the foot than at the directly compressed tibia site. Thus, at the highest pressure (40 mm Hg), the foot SBF reduction (61.6% +/- 13.7%) was greater than at the directly compressed tibial site (33.3% +/- 0.3%), both of which were significantly less than baseline (P <.01).

CONCLUSIONS

The present findings and considerable related evidence indicate that lower extremity external compression, within the range of commonly used therapeutic levels, reduces SBF by an amount that depends on the magnitude of the compression pressures. The quantitative difference between the effect of compression on SBF within the compression area, and distal to it, is likely a result of the way that the compression affects arteriolar and venous resistance to blood flow at the 2 sites. From a clinical perspective, it would seem prudent to anticipate that sub-bandage and distal SBF would be negatively affected by compression, with greater effects likely in tissue distal to compression.

Blood flow in diabetics with foot lesions due to 'small vessel disease'

Blood flow was measured in the feet and toes of 23 diabetics, 7 controls and 6 non-diabetic neuropathic controls, using venous occlusion plethysmography. All of the diabetics showed a characteristic flow abnormality with mild hyperperfusion of the foot at rest but impaired peak flow following arterial occlusion. When the diabetics were subdivided into those with 'small vessel disease', those with neuropathic ulceration and those with neuropathy but no ulceration, the groups had remarkably comparable blood flows, except that peak great toe blood flow was rather lower in small vessel disease. In small vessel disease, the combination of high resting blood flow and elevated foot venous oxygen saturation suggests that the hyperperfusion is due to arteriovenous shunting. It seems likely that the toe lesions ascribed to small vessel disease were in fact manifestations of severe diabetic neuropathy. The term small vessel disease should be avoided in the context of diabetic foot lesions.

Plethysmography: history and recent advances

A short historical note on plethysmography is given and the development of the modern computerized pneumoplethysmograph is described. The computer-aided pneumoplethysmogram (CAP), with proper programming, makes possible numerous computations that give new, useful, and rapid information about the patient. New applications of the CAP will quickly develop and the data collected will be quickly analyzed and displayed.

Comparative plethysmography; evidence for a hydrostatic effect on foot blood flow

Blood flow in the right foot of 11 subjects was measured simultaneously by a strain gauge placed around the mid metatarsal circumference of the foot and a water displacement plethysmograph in which the foot was resting. A close linear correlation (r = 0.88) between the results of the two methods existed over a wide range of blood flows. It was apparent however that blood flow at the mid metatarsal region of the foot was only about 30% of the total foot blood flow measured by the plethysmograph. The likely cause of this finding is the varying proportion of bone to soft tissue along the length of the foot. It was observed that the strain gauge estimates of blood flow increased two to three fold when the plethysmograph was emptied, an effect that was abolished by refilling the plethysmograph. These changes were highly statistically significant (P less than 0.01) in all ten subjects in whom this comparison was made. The application of progressively increasing hydrostatic pressure in a further 4 subjects demonstrated that the reduction in blood flow was proportional to the pressure applied. Explanations for this effect based upon small temperature and pressure changes altering strain gauge performance are excluded. Three mechanisms are proposed, based upon an increase in venous leakage, a reduction in arterial inflow and the consequence of increased capillary filling occurring as a result of hydrostatic pressure within the plethysmograph.

Radionuclide assessment of peripheral intravascular capacity: a technique to measure intravascular volume changes in the capacitance circulation in man

Changes in the capacitance vasculature influence venous return and cardiac performance, so an understanding of the effects of pathophysiologic states on the human capacitance vasculature is necessary to understand integrated cardiovascular function in man. Techniques available to assess the capacitance vasculature in man, however, have limitations. We performed radionuclide imaging of the calf or forearm in 51 patients whose erythrocytes had been labeled in vivo with technetium-99m, basing our approach on the principle that counts from the radiolabeled intravascular space are proportional to blood volume. Two-minute or 15 second count acquisitions were obtained from the calf in 42 patients. Counts obtained at rest demonstrated little variation. With veno-occlusion at 15 and 30 mm Hg, counts increased 8 +/- 1% (+/- SEM) (p less than 0.001) and 28 +/- 2% (p less than 0.001), respectively. After 0.4 mg of sublingual nitroglycerin, counts increased 9 +/- 1% (p less than 0.001). With leg elevation, counts decreased 34 +/- 4% (p less than 0.001). Response patterns were similar with 2-minute and 15-second acquisitions. In nine patients who underwent forearm imaging (2-minute acquisitions), counts increased 14 +/- 2% (p less than 0.001) and 26 +/- 4% (p less than 0.001) at 15- and 30-mm Hg veno-occlusion and 15 +/- 3% (p less than 0.001) after nitroglycerin. Volume displacements, recorded simultaneously with a fluid-filled plethysmograph about the contralateral forearm, correlated linearly in all nine patients. Thus, gamma camera imaging of the radiolabeled peripheral intravascular space provides a quantitative and reliable assessment of peripheral vascular capacity in man. The technique could be used in conjunction with gated cardiac imaging in order to assess the interactions of peripheral vascular capacity and ventricular performance.

A new method for determining the blood quantity in the eye in a unit of time

The principles of rheography with "infinite time constant" are utilized. After applying a suction cup electrode mechanism, by gradual increases of the vacuum, ocular pressure rises above venous pressure, and without modification of arterial inflow, the venous outflow is blocked; in consequence an increase of blood accumulation in the interior of the eye occurs. The electrical impedance variation, as a result of the latter, gives the recorded tracing a "slow" rising curve, that maintains the same pulse morphology. After being analyzed and elaborated on with certain mathematical formulae, this curve makes it possible to calculate the blood flow in the eye in a unit of time. Statistics on normal patients of various ages are reported.

A Hydrostatic Pressure Stocking for the Treatment of Ulcers due to Chronic Venous Insufficiency

Chronic venous insufficiency may be complicated by ulceration of the skin of the involved extremity. The ulcers appear to be caused by the prolonged elevation of venous pressure in the leg and the resultant edema. A device is described that counterbalances exactly the elevated pressure in the veins of the leg, regardless of the position of the patient. This hydrostatic pressure stocking prevents formation of edema without interfering with the flow of blood through the tissues. The use of this device in the treatment of ulcers due to chronic venous insufficiency is also described.

Effect of cooling and of smoking tobacco upon the blood flow of reactive hyperemia of the foot

Tobacco smoking and body cooling have been shown to decrease the "resting" blood flow to the foot. The present study was designed to determine whether either of these 2 stimuli also affects the increased blood flow that follows a period of ischemia (reactive hyperemia). Blood flows to the foot were measured in subjects with and without vascular disease by the venous occlusion, water plethysmographic method in a warm and a cool environment before and during smoking. Patients with bilaterally and unilaterally sympathectomized limbs were also studied to determine the mechanism of the observed responses.

Rationale of venous occlusion plethysmography

The experimental evidence reported here supports the validity of venous occlusion plethysmography.