Measurement of digital blood flow using the laser Doppler,impedance and strain-gauge methods

Digital volume changes and blood flow have been measured with impedance and strain-gauge plethysmography as well as with laser Doppler flowmetry. A good agreement was found between the impedance and strain-gauge flow measurements with a correlation coefficient of 0·905. The laser Doppler method recorded minor changes in finger skin blood flow following changes in posture from 30 cm below heart level to 60 cm above heart level. This result can be explained as a consequence of the limited penetration depth of laser light into the skin or as a sign of autoregulation of skin blood flow. In these experiments the total blood flow to the finger underwent major changes.     

Corrected formula for estimating peripheral blood flow by impedance plethysmography

In this paper, the limitations of the conventional formula for the computation of peripheral blood flow from impedance plethysmograms are highlighted, and a correction to the formula is suggested. A conductivity cell experiment is described to show the dependence of the value of the blood flow index (BFI), obtained from the conventional formula, on the mean resistivity of the cell. It is also shown that the value of the corrected BFI is independent of the mean resistivity. Anomalies observed in the amplitude of systolic waves in impedance plethysmograms of patients with oedema are explained.     

Analytical study of electrical impedance plethysmography in peripheral blood flow

The impedance Z, the derivative of Z with respect to time t, dZ/dt, and blood flow F have been measured simultaneously in the central caudal artery of a series of anaesthetised rats. The results of the experiments were expressed in the relationship derived from the Windkessel theory where f(t)=adz(t)/dt+βz(t), in which α and β are constants and f(t) and z(t) express the pulsating fractions in the blood flow F and the impedance Z, respectively. The correlation between the peak-to-peak values of f(t) and of adz(t)/dt+βz(t) was 0·96. The correlation between the mean flow f_o and the peak-to-peak value of f(t) was 0·74. Furthermore, even when the mean flow was zero, adz(t) still existed. The theoretical model was tested against volume flow measurements made using an electromagnetic flowmeter and was found to correlate well. Theoretical estimates of vessel calibre also accord with measured values.     

Comparison of volume changes in the forearm assessed by impedance and water-displacement plethysmography

The ability of impedance plethysmography to measure changes in volume of the forearm in normal subjects is assessed by comparison with water-displacement plethysmography. The forearms were subjected to venous occlusion and simultaneous assessment of absolute volume changes of the same section of arm were made using the two techniques. The linear correlation between the two measurements had a coefficient of 0·93 but the impedance values were lower than those from water displacement by a factor of 0·41. These results are discussed by comparison with those from similar measurements described in the literature.     

Effect of electrode size on the contributions of intracranial and extracranial blood flow to the cerebral electrical impedance plethysmogram

The contribution of scalp blood flow to pulses obtained by cerebral electrical impedance plethysmography (rheoencephalography) has been investigated using a four-electrode (quadripolar) montage with brass disc electrodes 1 cm, 2 cm and 4 cm in diameter. Three states of scalp perfusion were produced by the application of a tourniquet around the occipto-frontal circumference of the head, and blood flow in the scalp was monitored by means of reflection photoplethysmography and an electrical impedance technique. When scalp perfusion was occluded, cerebral impedance pulse amplitude was reduced to 80% of that obtained during normal scalp flow; in the presence of maximal scalp flow as a result of the reactive hyperaemia induced by release of the tourniquet, cerebral impedance pulse amplitude was 30% greater than during normal scalp flow. Altering electrode diameter did not affect the contribution of scalp flow to cerebral impedance pulse size. The effect of changes in scalp flow did, however, increase when a bipolar electrode montage was used instead of a quadripolar montage.     

Use of digital techniques to process cerebral electrical impedance signals in the newborn

Electrical impedance measurements can provide much useful information on cerebral haemodynamics in ill newborn infants. However, the analysis of the pusatile impedance signal presents a number of problems. These are due to the electrical noise generated by other monitoring equipment in the neonatal intensive care unit, the presence of a strong breathing component within the impedance signal and frequent movement artefacts. Algorithms based on techniques such as digital filtering, coherent averaging and template matching have been developed to deal with these problems. The continuous analysis of the pulsatile impedance waveforms to be carried out automatically.     

Effects of posture,hypotension and locally applied vasoconstriction on the middle ear microcirculation in anaesthetized humans

Studies by laser-Doppler flowmetry of middle ear microcirculation changes induced by physical and chemical stimuli in the animal have only recently been made. This prospective study, performed in humans, was designed to compare the effects of a postural manoeuvre (headup tilt 30°), hypotension and locally applied vasoconstriction on middle ear blood flow during anaesthesia. Circulatory changes provoked by a headup tilt of 30°, and successive intravenous boluses of potent vasodilators, were compared with circulatory changes provoked by locally applied adrenaline, in ten healthy patients in good physical states undergoing middle ear surgical repair. Heart rate and direct arterial pressure were continuously recorded via a radial artery cannula. Middle ear blood flow was continuously recorded via a laser-Doppler probe placed on the promontorium cavi tympani. Metabolic parameters (partial pressure of O₂ and CO₂ in arterial blood, pH, arterial lactate concentrations) and arterial concentrations of propofol were measured just before and just after the experiment. Headup tilt did not modify heart rate, mean arterial pressure or middle ear blood flow. Vasodilators (nicardipine, nitroprusside, nitroglycerin) provoked a fall in arterial pressure (P<0.0001,P<0.0001,P<0.019, respectively), but did not induce any significant variations in heart rate; variations occurred in middle ear blood flow (P>0.05, not significant) which were different according to patients and agents. Locally applied adrenaline provoked a fall in the middle ear blood flow (P<0.0012), with no effect on heart rate and arterial pressure. There were no significant changes in metabolic values, or propofol serum concentrations. The behaviour of the middle ear blood flow submitted to hypotension, posture, or to vasoconstriction could be related to counteracting regulatory responses and/or to direct vascular effects.     

Axon reflexes in human cold exposed fingers

Exposure of fingers to severe cold induces cold induced vasodilatation (CIVD). The mechanism of CIVD is still debated. The original theory states that an axon reflex causes CIVD. To test this hypothesis, axon reflexes were evoked by electrical stimulation of the middle fingers of hands immersed in water at either 5°C or 35°C. Axon reflexes were pronounced in the middle finger of the hand in warm water, but absent from the hand in cold water, even though the stimulation was rated as “rather painful” to “painful”. These results showed that axon reflexes do not occur in a cold-exposed hand and thus are unlikely to explain the CIVD phenomenon. Accepted: 6 September 1999     

In vitro comparison of different signal processing algorithms used in laser Doppler flowmetry

The paper reports the results of investigations comparing the relative in vitro responses of different signal processing algorithms commonly employed in laser Doppler flowmetry (LDF). A versatile laser Doppler system is described which enabled complex signal processing to be implemented relatively simply using digital analysis. The flexibility of the system allowed a variety of processing algorithms to be studied by simply characterising the algorithm of interest under software control using a personal computer. An in vitro physical model is also presented which was used to maintain reproducible fluid flows. Flows of particles were studied in a physical model using both a near-infra-red (NIR) diode and an He/Ne laser source. The results show that frequency-weighted algorithms are responsive to both particle velocity and concentration, whereas non-weighted algorithms respond to concentration only. The linearity of the velocity response is critically dependent on both the dimensions of the in vitro model and the frequency bandwidth of the signalprocessing algorithm.     

Impedance plethysmography in the diagnosis of arterial and venous disease

The objective of this paper is to review the theoretical basis and clinical application of electrical impedance plethysmography in the noninvasive evaluation of peripheral arterial and venous disease. Theoretical, experimental and clinical studies have now demonstrated a direct relationship between electrical impedance changes and limb volume changes. Potential sources of error have also been identified. This has led to the development of clinical tests based on impedance plethysmography for the detection of peripheral arterial disease, venous insufficiency and venous outflow obstruction. Impedance plethysmography, using the method of venous occlusion, is presently the most commonly employed noninvasive method for the detection of deep venous thrombosis.     

Effects of early aging and cerebral hypoperfusion on spreading depression in rats

Cortical spreading depression (CSD) is a feature of stroke pathophysiology. As stroke incidence increases with age, we have examined the effects of early aging and chronic cerebral hypoperfusion on CSD in rats.Three groups were studied: Young, 2-month-old animals; Middle-aged-2VO, subjected to 8 months of bilateral carotid occlusion from 2-month-of-age; and Middle-aged-SHAM, sham-operated. At 2- and 10-month-of-age for the Young and Middle-aged groups, recurrent CSD were induced under halothane anesthesia, by sustained application of 1 M KCl to the cortex for 2 h. Propagating CSD (i.e., cortical EEG, direct current potential) and associated laser Doppler blood flow changes were recorded anteriorly.Susceptibility to CSD and event duration were both decreased by early aging (frequency: 21 ± 0.5 and 6 ± 0.5 CSD/h; duration: 139 ± 7 and 63 ± 8 s; in Young and Middle-aged-SHAM, respectively). There was also a tendency for CSD-associated hyperemia to be reduced in the Middle-aged-2VO group (8.9 ± 2.1 vs. 32.8 ± 12.6% × min in Young). These data suggest reduced sensitivity of the cortex to CSD elicitation with early aging, and a less responsive cerebrovascular system with chronic hypoperfusion.     

Continuous determination of cardiac output during exercise by the use of impedance plethysmography

A computer-based system has been developed that enables the continuous measurement of cardiac output at rest as well as during exercise. The respiratory and motion artefacts appearing in the first derivative of the transthoracic impedance change (dZ/dt) were eliminated by adopting an ensemble averaging technique. A sufficiently high correlation was observed between cardiac outputs determined by the impedance and CO₂ rebreathing methods. The system may facilitate the physiological investigation of cardiac function during exercise.     

Vibration plethysmography: A method for studying the visco-elastic properties of finger arteries

Vibration plethysmography records changes in vascular volume produced by fast vibrations of cuff pressure. From these, waveforms of dynamic vascular compliance (DVC) are obtained. A total of 46 recordings of DVC, photo-electric plethysmogram (PG), cuff pressure (CP), and indirect blood pressure (BP) are performed on two adjacent fingers (third and fourth) in 23 healthy subjects. The shape and polarity of the DVC waveform markedly depends upon CP or transmural pressure (TP) (TP=BP-CP). The correlation coefficient between DVC and PG waveforms is nearly −1 at negative mean TP, near zero at zero TP, and approaches +1 at positive TP. For CP moving between systolic and diastolic BP, the DVC waveform shows a diastolic peak, with its maximum close to the zero value of instantaneous TP. xy-diagrams of PG against TP and of DVC against TP plotted for the diastolic phase of single pulses reveal a close coincidence of the DVC peak with the maximum slope of the PG/TP curve. A similar relationship appears when slow changes in PG and the amplitude of PG pulse waves are plotted against mean TP.     

Laser-Doppler measurements of concentration and velocity of moving blood cells in rat cerebral circulation

In brain cortex all capillaries are perfused with plasma at anyone time while the flow of blood cells is heterogenous. Increased blood flow is associated with increased number of moving erythrocytes in the microcirculation, while capillary recruitment in its classical anatomical sense appears not to exist in the brain. Modulation of the concentration of flowing erythrocytes may influence the oxygen supply to the tissue. Therefore, we examined the possibility that laser-Doppler flowmetry (LDF) could be used to quantify changes in the microvascular concentration of moving blood cells (CMBC) and blood cell velocity (< v >) by comparing LDF measurements with electromagnetic flow measurements in vitro, and confocal laser-scanning microscopy in vivo in the brain of anaesthetized male Wistar rats. In vitro measurements showed that CMBC was affected by changes in haematocrit, while < v > correlated almost linearly with blood cell velocity measured electromagnetically within a relevant physiological range. In vivo studies during hypercapnia (Paco ₂ from 39 ± 4 to 66 ± 5 mmHg) with confocal laser scanning microscopy disclosed a 39 ± 10% increase of cortical capillary erythrocytes, while CMBC measured with LDF increased by 37 ± 5%. Erythrocyte flow velocity in brain cortex capillaries increased by 65 ± 17% with confocal microscopy as compared to 72 ± 8% with LDF. Local electrical stimulation of cerebellar cortex, and application of adenosine or sodium-nitroprusside, increased CMBC and < v > simultaneously, while during hypercapnia the < v > increase preceded the CMBC increase by 30 s. The CMBC rise rapidly reached a steady state in response to all types of stimulation, while < v > continued to increase during the major part, or the entire stimulation period. In conclusion, our data support the hypothesis that LDF may be useful for haemodynamic studies of brain microcirculation.     

Discrimination of capillary and arterio-venular blood flow in skin by laser Doppler flowmetry

The scattering and absorption of light by tissue and blood is wavelength dependent; the tissue penetration of green light (λ=543·5nm) is about 60 per cent of that of red light (λ=632·8 nm) but the absorption of green light by blood is about 20 times greater than for red light. The effect of this difference has been studied by observing the responses of skin blood flow to heat and weal, measured by laser Doppler flowmetry at the two wavelengths. By using time autocorrelation function analysis (ACF) of the scattered light measured, low and high frequency components have been associated with capillary and larger vessel flow, respectively. The comparison of ACF from scattered green and red light has shown that measurements cannot be interpreted by only considering light penetration depth through a homogeneous tissue. Light absorption and multiple scattering by blood at the individual microvessel level, blood rheology and vessel morphology are parameters which are considered for greater attention.     

Cutaneous vasomotor adjustments during arm-cranking in individuals with paraplegia

Skin blood flow (SKBF) was evaluated during arm-cranking exercise in able-bodied control subjects (AB; n=6) and in individuals with low- (LP; T10–T12 lesions; n=6) and high-level paraplegia (HP; T5–T9 lesions; n=6), using laser Doppler flowmetry (LDF). During moderate exercise SKBF decreased to [mean (SD)] 82 (15)% of the pre-exercise resting level in AB, whereas it increased to 158 (52)% in LP and to 112 (51)% in HP (the LP:AB difference, P < 0.05). During intense exercise SKBF increased to 366 (180)% of the resting level in AB, whereas it increased only moderately [147 (68)%] in both paraplegic groups (the paraplegic:AB difference, P < 0.05). The paraplegics developed a higher esophageal and leg skin temperature, which was attributed to the lack of active vasodilation and evaporative cooling over the legs. The results indicate that individuals with paraplegia suffer from impaired cutaneous vasoconstriction at the onset of arm exercise, and possess only a limited vasodilatory capability in the paralyzed regions. During intense exercise, thermoregulation depends critically on active cutaneous vasodilation and skin cooling. Accepted: 25 August 2000     

Measurement of forearm blood flow by venous occlusion plethysmography: Influence of hand blood flow during sustained and intermittent isometric exercise

Summary The requirement for using an arterial occlusion cuff at the wrist when measuring forearm blood flows by plethysmography was tested on a total of 8 subjects at rest and during and after sustained and intermittent isometric exercise. The contribution of the venous effluent from the hand to the forearm flow during exercise was challenged by immersing the arm in water at 20, 34, and 40 C. Occlusion of the circulation to the hand reduced the blood flow through the resting forearm at all water temperatures. There was an inverse relationship between the temperature of the water and the proportion in the reduction of forearm blood flow upon inflation of the wrist-cuff, ranging from 45 to 19% at 20 to 40 C, respectively. However, during sustained isometric exercise at 10% of the subjects maximum voluntary contraction (MVC) there was no reduction in the measured forearm flow when an arterial occlusion cuff was inflated around the wrist. Similarly, there was no alteration in the blood flow measured 2 s after each of a series of intermittent isometric contractions exerted at 20% or 60% MVC for 2 s whether or not circulation to the hand was occluded nor of the post-exercise hyperemia following 1 min of sustained contraction at 40% MVC. These results indicate that a wrist-cuff is not required for accurate measurement of forearm blood flows during or after isometric exercise.This work was supported by N.I.H. training grant HLO 7050-03, H.E.W. contract 210-77-0044 and Air Force grant AFOSR-76-3084 B     

Field calculations in the head of a newborn infant and their application to the interpretation of transcephalic impedance measurements

A spherical model, of five regions, has been applied to the head of a typical premature baby for the purpose of computing the potential and current density fields, with a constant total current applied via small electrodes at opposite poles. The results have been compared within vivo transcephalic impedance measurements, and the model has been used to determine the magnitudes of the impedance effects to be expected in various clinical situations. The model predictions are found to agree well with impedance measurements as a function of the distance between the potential and current electrodes. It is calculated that a large intracranial haemorrhage would produce an increase in the transcephalic impedance of the order of 1 Ω. The model predicts a decrease of up to 20% in the impedance for a change in head circumference of 2 cm due to increased cerebrospinal fluid content. This confirms the conclusion ofReigel et al. (1977) that transcephalic impedance is a sensitive indicator of developing hydrocephalus.     

Impedance plethysmography: the origin of electrical impedance changes measured in the human calf

Electrical impedance plethysmography of the lower leg is now a widely used test for detection of deep vein thrombosis. The origin of the impedance signal is difficult to evaluate in the living subject, and experimental animals have important anatomic differences. A controlled study on human cadavers was therefore undertaken. Conductive and nonconductive fluids were injected into the lower legs of cadavers, while electrical impedance changes were recorded utilising a 4-electrode technique. X-ray studies confirmed the localisation of the injections. Results from ten cadavers showed that significant impedance changes occurred only in response to injections of saline in the region between the electrodes. Injections of nonconductive silicone oil caused a small increase in the measured impedance. It is concluded that electrical impedance plethysmography reflects changes in conductivity confined to the region between the electrodes; and that the ratio of deep to superficial impedance sensitivity is a function of the electrode spacing.