Rationale of venous occlusion plethysmography

Increased fatigability and impaired skeletal muscle microvascular reactivity in adults with obstructive sleep apnea: a cross-sectional study

BACKGROUND

Sympathetic nervous system hyperactivity and chronic intermittent nocturnal hypoxia in individuals with obstructive sleep apnea (OSA) predispose them to microvascular impairment, which may contribute to increased daytime muscle fatigue. This study aimed to assess microvascular reactivity of the skeletal muscle, examine fatigability, and determine the relationship between fatigability and microvascular reactivity in adults with OSA.

METHODS

Twenty-six participants were allocated into two groups-those with OSA and those without i.e. non-OSA. Each group comprised of 13 individuals who underwent an arterial occlusion test on their non-dominant leg. The percentage change of maximal hyperemic response (MHR) and the time to achieve MHR (tM) of both the total myoglobin/hemoglobin (∆[Hbtot]) and the oxygenated myoglobin/hemoglobin (∆[HbO2]) signals from near-infrared spectroscopy were calculated to examine microvascular reactivity. In addition, a 10-min walk test was performed to assess performance and perceived fatigability.

RESULTS

The OSA group demonstrated a reduced in ∆[Hbtot]MHR (150.9 ± 16.2% vs. 235.8 ± 72.7%, p = 0.006), ∆[HbO2]MHR (131.4 ± 8% vs. 161.7 ± 10.6%, p = 0.001) and increased ∆[Hbtot]tM (80.5 ± 13.1 s vs. 47.7 ± 9.9 s, p < 0.001), ∆[HbO2]tM (85.2 ± 22.4 s vs. 52.1 ± 5.9 s, p = 0.001) compared to the non-OSA group. In addition, participants in the OSA group experienced greater perceived (6 ± 1 vs. 2.8 ± 0.1, p = 0.001) and performance fatigability (1.1 ± 0.1 vs. 0.9 ± 0.1, p = 0.001) compared to adults in the non-OSA group. Moreover, both performance and perceived fatigability were significantly associated with microvascular reactivity parameters (all p < 0.05).

CONCLUSION

Microvascular dysfunction, as determined by an attenuated post-occlusive reactive hyperemia, is observed in individuals with OSA that may contribute to increased fatigability in these individuals.

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.

Measurement of peripheral blood flow in patients with peripheral artery disease: Methods and considerations

Peripheral artery disease (PAD) is a manifestation of generalized atherosclerosis which results in hemodynamic compromise of oxygen and substrate delivery to the lower extremity skeletal muscles. Hemodynamic assessments are vital in PAD diagnosis and in the evaluation of strategies aimed at treating claudication (i.e. exercise training, revascularization, and pharmacological agents). Venous occlusion plethysmography (VOP) is a century-old, non-invasive technique used to quantify limb blood flow and has been used to evaluate hemodynamic compromise in patients with PAD. However, the literature suggests a wide array of methodological variability in the measurement and analysis of limb blood flow using VOP. In this manuscript, we overview the clinical application of VOP measurement, and secondly we review the methodological variation that occurs during the measurement and analysis of VOP in healthy individuals and in patients with claudication.

Can venous occlusion plethysmography be used to measure high rates of arterial inflow?

To investigate whether venous occlusion plethysmography (VOP) may be used to measure high rates of arterial inflow associated with exercise, venous occlusions were performed at rest, and following dynamic handgrip exercise at 15, 30, 45, and 60% of maximum voluntary contraction (MVC) in seven healthy males. The effect of including more than one cardiac cycle in the calculation of blood flow was assessed by comparing the cumulative blood flow over one, two, three, or four cardiac cycles. The inclusion of more than one cardiac cycle at 30 and 60% MVC, and more than two cardiac cycles at 15 and 45% MVC resulted in a lower blood flow compared to using only the first cardiac cycle (P < 0.05). Despite the small time interval over which arterial inflow was measured (~1 s), this did not affect the reproducibility of the technique. Reproducibility (coefficient of variation for arterial inflow over three trials) tended to be poorer at the higher workloads, although this was not significant (12.7 +/- 6.6, 16.2 +/- 7.3, and 22.9 +/- 9.9% for the 15, 30, and 45% MVC workloads; P = 0.102). There was also a tendency for greater reproducibility with the inclusion of more cardiac cycles at the highest workload, but this did not reach significance (P = 0.070). In conclusion, when calculated over the first cardiac cycle only during venous occlusion, high rates of forearm blood flow can be measured using VOP, and this can be achieved without a significant decrease in the reproducibility of the measurement.

Plethysmography without venous occlusion for measuring forearm blood flow: comparison with venous occlusive method

Limb blood flow is widely used as an indicator of the human vascular properties. There are only few non-invasive methods for its measurement such as venous occlusion plethysmography. However, several authors have questioned its validity. The problems appear to be related to the process of venous occlusion. We developed two methods to measure forearm blood flow by plethysmography without venous occlusion in combination with Doppler velocimetry (without imaging). Method 1: the gradient of a tangent drawn on the latter part of the down stroke of the plethysmographic volume pulse is an approximation of venous blood flow in the absence of diastolic blood flow. At equilibrium, it equals the average arterial flow in a cardiac cycle. The Doppler velocity waveform recorded simultaneously allows improvement of this approximation when there is diastolic blood flow. Method 2: the volume pulse detected by a plethysmograph calibrated in absolute volume is used to calibrate the velocity waveform recorded simultaneously to produce an approximation of arterial volumetric flow waveform. Bland-Altman analysis shows both methods have good correlation and agreement with venous occlusion plethysmography at rest. Method 1: mean difference (blood flow measured by venous occlusion minus calculated flow) = 0.10 ml/pulse (+/-0.18), limits of agreement = -0.41 and 0.61 ml/pulse. Method 2: mean difference = -0.041 ml/pulse (+/-0.15), limits of agreement = -0.45 and 0.37 ml/pulse. During hyperaemia, venous occlusion plethysmography grossly underestimated relative to the new methods. The new methods are not dependent on venous occlusion and produce consistent results with or without hyperaemia.

Intermittent venous compression, and the duration of hyperaemia in the common femoral artery

External compression of limbs to below-diastolic pressure (venous compression) has been shown to produce a short-lived hyperaemia in supply arteries. Intermittent pneumatic compression is currently under investigation therefore as a treatment for peripheral arterial disease. The optimal timing of the compression will depend on the duration of hyperaemia produced by a particular duration of compression, and the purpose of this work was to test that link. Nineteen healthy volunteers underwent intermittent compression of one leg with two compression cycles - one compressing for 10 s each time, the other for 1 min. Blood flow velocities in the common femoral artery was shown to increase on release of the compression by 38% (inter-quartile range 27-56%) for the sequence with short duration compression, and by 57% (inter-quartile range 37-87%) for the longer sequence (difference, P = 0.005, Wilcoxon). The hyperaemia duration above the baseline level was 37 s (inter-quartile range 32-49 s) for the short sequence, and 54 s (inter-quartile range 37-76 s) for the longer sequence (difference, P = 0.001, Wilcoxon). The magnitude of the change in the compression duration was not equalled by the difference in hyperaemia duration, suggesting that the physiological mechanism behind the hyperaemia is unlikely to be due solely to simple accumulation of metabolites, and a myogenic mechanism remains possible. Therapies for peripheral arterial disease need not employ long duration compression, as a greater percentage of time will be spent in hyperaemia with short duration intermittent compression.

Noninvasive measurement of regional blood flow in man

Regional blood flow in man is ideally measured by techniques that are noninvasive, accurate, and can measure flow repetitively with comparative ease. Although numerous noninvasive techniques are available, no single method records blood flow accurately in every location. The neophyte investigator is often faced with a confusing array of methods and can spend considerable time searching for the ideal one. This paper presents current methods available to the clinical or metabolic researcher and comments on the strengths and limitations of each method. It is hoped that this will allow more rapid selection of a flow measurement method that is tailored to each individual's need.

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.

Non-invasive measurement of limb and digit blood flow

There are many clinical applications for non-invasive measurements of limb and digit blood flow, but often plethysmographs are inconvenient to use. Most experimental work has been carried out on patients with vascular disease. The variation of blood flow with various environmental and physiological factors has been investigated. Little information is available on the effects of injury or surgical trauma on blood flow in limbs. Plethysmographic methods measure arterial flow or pulse volume by several techniques including volumetric displacement, electrical impedance, gravimetry and the mercury in rubber strain gauge. Calorimetric, Doppler and isotope clearance techniques have also been applied to the measurement of blood flow in digits. None of these methods fulfils all the criteria for the requirements of an ideal device and some are too complicated, uncomfortable or cumbersome for widespread use. The results from different techniques are compared and the reasons for variability are discussed. Possible mechanisms for the effects of trauma on limb blood flow are suggested. Experimental comparisons of different plethysmographs are described and the requirements for future development of these devices are defined.

The evaluation of blood flow measurement using gravimetric plethysmograph

The gravimetric plethysmograph is a simple, robust and inexpensive device which measures blood flow in the limb in terms of the increase in weight which follows temporary venous occlusion. Validation of the instrument using an artificial circuit shows that it is accurate and gives reproducible results.

The mean resting blood flow in the legs of 18 healthy volunteers was 3·95 ml 100 ml−1 tissue min−1 and this measurement was not reproducible. A 3-min period of arterial occlusion resulted in an immediate hyperaemic response of a mean 43·5 ml 100 ml−1 min−1, and this result was much more reproducible (r = 0·18).

The mean resting blood flow of 45 patients with occlusive arterial disease of the legs was 2·1 ml 100 ml−1 min−1, and this flow rate was variable (r = 0·71). Their immediate hyperaemic response of 5·18 ml 100 ml−1 min−1 was much less than that in normal volunteers, but was quite reproducible (r = 0·93). Following arterial reconstruction, their mean immediate hyperaemic response rose to 10·35 ml 100 ml−1 min−1, which was a significant increase compared to their preoperative values (P &lt; 0·0005, Student's t test) and was associated with concomitant improvement in symptoms, treadmill performance and ankle/brachial systolic pressure indices. Measurement of the immediate hyperaemic response to temporary arterial occlusion with the gravimetric plethysmograph may assist in the evaluation of patients with occlusive arterial disease of the legs, and in their medical and surgical management.

Gravimetric plethysmography for measurement of blood flow in the leg

The gravimetric plethysmograph measures blood flow in the limb in terms of the increase in weight which follows temporary venous occlusion. Following validation of the device, blood flow in the leg was evaluated in normal subjects and in patients with intermittent claudication. Measurement of the immediate hyperaemic response to a three-minute period of arterial occlusion proved to be very reproducible in patients with occlusive arterial disease of the legs.

Sequence of return of neurological function and criteria for safe ambulation following subarachnoid block (spinal anaesthetic)

Twenty-three adult men were studied during and after subarachnoid block anaesthesia for elective surgery. Measurements were obtained of mean arterial pressure and pulse, both supine and after standing for five minutes, core body (tympanic) and peripheral skin (toe) temperatures and blood flow in the leg. Time of measurements included one hour after the injection of tetracaine and after regression of the block. Results obtained indicate that the sequence of return of neurological activity following tetracaine subarachnoid block is sympathetic nervous system activity, pinprick sensation, somatic motor function followed by proprioception in the feet. This progression provides the basis for recommended criteria which indicate when it is safe for patients who have been subarachnoid block anaesthesia to become ambulatory. These criteria include: (1) return of pinprick sensation in the peri-anal area (sacral 4--5); (2) plantar flexion of the foot (while supine) at pre-anaesthetic levels of strength; and (3) return of proprioception in the big toe, always provided that the patient is not hypovolaemic or sedated.

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.

Forearm vasodilatation following release of venous congestion

1. The volume rate of forearm blood flow was measured with a mercury-in-rubber strain gauge, or with a water-filled plethysmograph, from 1 sec after termination of a 2-3 min period of venous congestion.2. When congesting pressure had been less than 18 mm Hg, average post-congestion flow (five subjects) was constant during approx. 10 sec and not significantly different from resting flow.3. When congesting pressure had been 30 mm Hg, average post-congestion flow (eight subjects) was 26% higher than resting, during 3-4 sec after release of congestion, but rose to 273% of resting during 4-6 sec after release of congestion.4. In other studies forearm vascular resistance had been found normal or increased during such venous congestion, and theoretical studies here indicated that passive mechanical factors could not account for the delayed occurrence of high post-congestion flow.5. It appears, therefore, that the forearm vascular bed dilates actively shortly after release of substantial venous congestion. It would seem more likely that a myogenic mechanism, rather than a metabolic one, is responsible.