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Biomedical subjects

T Cochrane

Publications and source records attributed to T Cochrane.

At least 19 recordsLinked to original sources
PubMed1992

Continuity equation and Gorlin formula compared with directly observed orifice area in native and prosthetic aortic valves.

Orifice areas calculated by the continuity and Gorlin equations have been shown to correlate well in vivo. The continuity equation, however, gives underestimates compared with the Gorlin formula and it is not clear which is the more accurate. Both equations have therefore been tested against maximal orifice area measured by planimetry in eight prepared native aortic valves and four bioprostheses. A computer controlled, ventricular flow simulator (cycled at 70 beats/min) was used at five different stroke volumes that gave cardiac outputs of 2.8 to 7.0 l/min. The mean difference between measured and estimated orifice area was zero for the continuity equation, but -0.14 cm2 for the conventional Gorlin formula. Thus the Gorlin formula tended to give overestimates compared with both measured area and area estimated by the continuity equation, probably because of the effect of pressure recovery. When predictive equations derived from these data were tested, residual standard deviations were around 0.3 cm2 at all stroke volumes for the continuity equation, around 0.2 cm2 for the invasive Gorlin formula, and between 0.2 and 0.4 cm2 for the modified Gorlin formula. These results suggest that estimates of orifice area in an individual valve as judged by any of the equations tested should be seen as a guide to rather than as a precise measure of actual orific area.

Anthropometry↗
PubMed1991

Simple model of circulatory system dynamics including heart valve mechanics.

This paper describes an extension of the Windkessel model of circulatory system dynamics, which takes into account the opening action of the mitral and aortic valves, including stenotic and regurgitant orifices. The starting point for the model is the ventricular emptying/filling curve which is taken from a quasi-physiological ventricular flow relationship which incorporates variation of systolic and diastolic intervals with cycle rate. The valves are assumed to open with a linear rise in area up to maximum orifice, followed by a period at maximum orifice and then a linear fall-off in area to the closed position (which may allow regurgitation). Flow through the valves is assumed to be governed by the Gorlin equation. Peripheral resistance and compliance are considered as fixed parameters of the arterial system. The model is useful in helping to understand the complex interaction between valvular mechanics and the rest of the cardiovascular system. Applications of the model are illustrated by considering isolated aortic stenosis, isolated aortic regurgitation, cardiac adaptation in the presence of these two abnormalities and the effects of variation of peripheral resistance on pressures within the cardiovascular system.

Adaptation, Physiological↗
PubMed1991

Validation of the orifice formula for estimating effective heart valve opening area.

Interest in the Gorlin formula for estimating heart valve effective orifice area (EOA) has recently been rekindled and the formula itself has been challenged. In this validation study, explanted native heart valves, unimplanted mechanical prostheses, unimplanted bioprostheses and explanted bioprostheses have been tested in vitro in a pulsatile flow simulator. Pressures have been measured 30 mm upstream and 100 mm downstream from the plane of the valve sewing ring (to give pressure drop, pd in kPa). Flow (Q in 1 min-1) has been measured directly by electromagnetic flowmeter and orifice areas have either been taken from manufacturer supplied data (mechanical valves) or have been digitised from video images at maximum orifice (biological valves). The formula EOA = Q/(6.96 x pd 1/2) - 0.7 fitted the data with good correlation, r = 0.96 (n = 179). The orifice assumption on which this formula is based (cf. Gorlin formula) is confirmed though it is recommended that the formula should be modified to account for (i) the pressure recovery phenomenon and (ii) the fact that forward flow through a valve only occurs over a portion of the cycle in pulsatile flow. Heart rates used in the study ranged from 40 to 140 min-1, stroke volumes ranged from 20 to 114.3 ml, cardiac outputs from 2.0 to 8.0 1 min-1 and peripheral resistance from 0.1 to 1.6 kPa 1-1 min (1 - 12 mmHg l-1 min). Application of the formula was independent of the flow conditions.

Adult↗
PubMed1990

Ventricular stroke work loss: validation of a method of quantifying the severity of aortic stenosis and derivation of an orifice formula.

Because aortic stenosis results in the loss of left ventricular stroke work (due to resistance to flow through the valve and turbulence in the aorta), the percentage of stroke work that is lost may reflect the severity of stenosis. This index can be calculated from pressure data alone. The relation between percent stroke work loss and anatomic aortic valve orifice area (measured by planimetry from videotape) was investigated in a pulsatile flow model. Thirteen valves were studied (nine human aortic valves obtained at necropsy and four bioprosthetic valves) at stroke volumes of 40 to 100 ml, giving 57 data points. Valve area ranged from 0.3 to 2.8 cm2 and mean systolic pressure gradient from 3 to 84 mm Hg. Percent stroke work loss, calculated as mean systolic pressure gradient divided by mean ventricular systolic pressure x 100%, ranged from 7 to 68%. It was closely related to anatomic orifice area with an inverse exponential relation and was not significantly related to flow (r = -0.15). An orifice formula was derived that predicted anatomic orifice area with a 95% confidence interval of +/- 0.5 cm2 (orifice area [cm2] = 4.82 [2.39 x log percent stroke work loss], r = -0.94, SEE = 0.029). These results support the clinical use of percent stroke work loss as an easily obtained index of the severity of aortic stenosis.

Aortic Valve↗
PubMed1989

Efficacy of aortic balloon valvoplasty: direct measurement of orificial area in a model with pulsatile flow.

The efficacy of balloon valvoplasty of calcific aortic stenosis remains controversial. We studied, therefore, 5 human aortic valves obtained at necropsy in a positive-displacement pulse duplicator which delivered stroke volumes of 40-100 ml with a quasiphysiological waveform of flow. All valves had three leaflets without commissural fusion and were preserved in antibiotic solution before study. Orificial area was planimetered from videotape of opening of the valve and varied with flow in all cases. Valvoplasty with a 20 mm diameter balloon had no effect on the orifice of the normal valve but increased the orifice of 2 mildly calcified valves from 0.70-1.77 cm2 (range) at baseline to 1.06-1.95 cm2. In 2 valves with severe calcification of the leaflets, the orifice was increased from 0.31-0.82 cm2 to 0.73-1.07 cm2. Dual balloon valvoplasty achieved a variable but small further increase in orificial area. No valve showed tears of the leaflets or fracture of calcific deposits after valvoplasty. We conclude that balloon valvoplasty can acutely increase orificial area, independently of any change in stroke volume. In valves without commissural fusion, its mechanism appears to be an increase in the pliability of the leaflets which does not require macroscopic fracture of calcific deposits.

Adult↗
PubMed1989

Doppler spectral waveform generation in vitro: an aid to diagnosis of vascular disease.

This paper describes a microcomputer controlled pump which generates pulsatile flows similar to those found in the human peripheral circulation. Continuous wave Doppler ultrasound was used to investigate the flows generated by the pump and the behaviour of diagnostic indices derived from the spectra was examined. Sonograms were recorded from elastic and rigid tubes with various degrees of axisymmetric constriction. Heart rate, stroke volume, severity of vessel constriction, vessel wall elasticity, distal resistance, and systemic peripheral impedance were varied in turn and the resulting Doppler spectra compared. Indices considered were the pulsatility index, spectral broadening index and peck Doppler frequency. In general, the indices lacked the sensitivity to detect low to moderate levels of disease.

Humans↗
PubMed1989

The Gorlin formula validated against directly observed orifice area in porcine mitral bioprostheses.

To assess the effect of fluid flow on orifice area and to test the Gorlin formula, six Carpentier-Edwards mitral valve prostheses were studied in a positive displacement pulse duplicator at 20 different rate-stroke volume combinations. Peak transvalvular velocity (V max) was measured by continuous wave Doppler ultrasound, and orifice area was determined from hard copy of video images. Orifice area was directly related to mean flow (Q), although cusp opening behavior was asymmetric and complex and varied among the individual valves. There was a strong correlation between measured orifice area (OA) and the modified Gorlin relation, Q/V max (r = 0.88; p less than 0.00001) given by the regression formula OA = 0.18 x Q/V max - 0.15. There was also a good correlation between measured orifice area and the conventional Gorlin relation, Q/root mean pressure drop. The derived empiric Gorlin constant did not vary significantly with flow.

Bioprosthesis↗
PubMed1989

The effect of flow on Doppler estimates of bioprosthetic mitral valve function in vitro.

In order to assess the effect of flow on peak transmitral velocity and pressure half time and to test the Hatle orifice area formula, four Carpentier-Edwards mitral prostheses were studied in a positive displacement flow simulator at 20 different stroke volume/rate combinations using a constant left ventricular filling curve. Peak transmitral velocity was directly related to mean flow and differed significantly between the individual values (F79 = 38; p less than 0.00001). There was, however, no significant difference in pressure half time between the four values (F79 = 0.04; p = 0.99). Pressure half time was directly related to diastolic time interval (r = 0.98, y = 0.41x - 17.84; p less than 0.00001), but was independent of stroke volume. There was only a moderate inverse correlation between pressure half time (T1/2) and orifice area (r = 0.56, y = 0.67 + 69/T1/2; p less than 0.0001). These results are consistent with the suggestion that pressure half time is more closely related to the shape of the left ventricular filling curve than to orifice area in normally functioning bioprostheses.

Bioprosthesis↗
PubMed1988

Reduced hyperaemic response under the diabetic neuropathic foot.

Diabetic neuropathic ulcers typically occur at high pressure sites. Microvascular blood flow has been assessed on the plantar surface of the foot in three matched groups each of 12 subjects free from macrovascular disease: (a) patients with diabetic neuropathy with abnormal foot pressures and previous neuropathic ulceration; (b) non-neuropathic diabetic patients; (c) non-diabetic control subjects. Resting flow was measured at the highest pressure point under the metatarsal heads (defined by pedobarograph) using laser doppler flowmetry, and the hyperaemic response was assessed at the same site following 3 min standing. Peak flow was significantly reduced in neuropathic patients (2.3 +/- 1.4 (SD) volts) compared with control subjects (4.0 +/- 2.0 volts; p less than 0.03). The time for blood flow return to baseline was significantly prolonged in neuropaths (159 +/- 72 s) compared with normal subjects (93 +/- 18 s; p less than 0.01), with a significant delay also seen in non-neuropaths (151 +/- 38 s; p less than 0.0001 compared with normal).

Adult↗
PubMed1987

Rapid autonomic tone regulation of atrioventricular nodal conduction in man.

The changes in P-P intervals and atrioventricular nodal (AVN) conduction during the Valsalva maneuver were studied in 17 patients. In spite of a significant decrease in the sinus P-P interval during phase II of the maneuver (733 +/- 143 to 520 +/- 86 msec, p less than 0.005) and prolongation during phase IV (884 +/- 171 msec, p less than 0.01), there was no change in the AH interval (control: 78 +/- 15: phase II: 76 +/- 15: phase IV: 72 +/- 14 msec, N.S.). In six patients consecutive P-P intervals during phase II were recorded in solid-state memory and were used to trigger pacing of the high right atrium at rest. This showed a significant increase in the AH interval (75 +/- 10 to 123 +/- 45 msec, p less than 0.05). Valsalva maneuver during constant rate atrial pacing resulted in a significant decrease in the AH interval during phase II (115 +/- 36 to 80 +/- 15 msec, p less than 0.001). During phase IV there was prolongation of the AH interval (156 +/- 58 msec) but in 11 patients (61%) a variable degree of Wenckebach periodicity appeared. Thus autonomic tone modulates the changes in AVN conduction induced during physiologic heart rate variation, resulting in maintenance of adequate 1:1 AVN conduction.

Adolescent↗
PubMed1987

A one-dimensional model of atrioventricular nodal conduction.

A computer model of the cardiac conduction process and an integral equation mathematical model have been employed to study the functions of the atrioventricular (AV) node. Special attention has been paid to the dependence of conduction delay on cycle length. The models have been used to evaluate the question as to whether simple cycle length dependences of AV nodal conduction could cause the oscillations of cycle length which are sometimes observed in AV reentry tachycardia. The models consider the AV node as a linear structure in which the depolarisation wavefront radiates in one dimension only. A model representing the node by two parallel linear structures has also been examined. Some results obtained do not conform with those of natural circumstance and clinical experiments, suggesting that the above hypotheses of AV nodal function are too simple and restricted to explain the natural processes.

Atrioventricular Node↗
PubMed1987

Comparison of laser Doppler and Doppler ultrasound in lower limb vascular diagnosis.

This study represents a preliminary investigation into the use of laser Doppler flowmetry in lower limb vascular diagnosis. The method is compared with the accepted method of continuous wave Doppler ultrasound. Fifty-six patients with symptoms of lower limb vascular disease were investigated consecutively and independently using both techniques. In approximately two-thirds of patients the two methods showed broad agreement in their classification of disease severity (correlation coefficient = 0.63, P less than 0.001, n = 41). However, about 30% of those studied had impaired microvascular responses on laser Doppler evaluation with no detectable major vessel disease on continuous wave ultrasound investigation. The results indicate that more information is obtained about a subject's vascular status when results from both methods are combined. Thus laser Doppler flowmetry may be useful adjunct to Doppler ultrasound and segmental pressure measurements in the vascular laboratory.

Adult↗
PubMed1986

Computer simulation of cardiac rhythm and artificial pacemakers using a ten-element heart model.

This paper describes a universal simulation system which enables simulation models of cardiac rhythm disturbances and of artificial pacemaker actions to be constructed. Three types of heart element are introduced according to the way in which excitation impulses are propagated or produced. Cycle rate dependence of repolarization periods and of impulse transmission speeds is incorporated. Random and predetermined deviations from natural behavior are also allowed for in the model. Using the universal system, a model of simple configuration has been developed. There are five physiological and five pathological heart components joined together by uni- and bidirectional connectors. Additional bidirectional channels allow a variety of pacemakers to be "implanted" in the simulated heart.

Arrhythmias, Cardiac↗
PubMed1986

Laser Doppler flowmetry: in the assessment of peripheral vascular disorders? A preliminary evaluation.

Skin is a representative microvascular bed providing easy access for the study of capillary haemodynamics. In this preliminary evaluation we have used laser Doppler flowmetry to measure the response of skin capillary blood flow to local thermal stimulation. The purpose of our study was to see if such measurements can be used to derive useful clinical information. Thirty subjects, 15 male and 15 female, were selected to establish "normal' patterns of behaviour. All normal subjects studied showed similar patterns of response. There was a slight downward trend in red-blood-cell flux with age. Periods of vasomotor activity were present at rates ranging from 4-17 cycles/min. The mean temperature rise in the heated skin area was 4 degrees C. Blood flow over this range increased by a factor of 10 or more. Clinical cases studied were strictly limited to three categories of disease with known or suspected microvascular changes: algodystrophy, lower-limb ischaemia and diabetic neuropathy. All the clinical cases studied showed deviations from the normal pattern. Patients with algodystrophy had impaired peripheral vascular function in the affected area. Patients with lower-limb ischaemic problems all had poor peripheral perfusion. In the limited number of diabetic neuropathic feet studied two patterns emerged: patients with no evidence of ulceration displayed striking vasomotor activity; those with more severe problems had reduced red-blood-cell flux and little evidence of vasomotor control.

Adolescent↗
PubMed1985

Shell computer model of cardiac electropotential changes.

A discrete process computer model has been developed to simulate the electropotential changes of heart musculature and the operation of the cardiac conduction system. The model is implemented on an ICL-4/72 computer and is oriented to cardiac rhythm studies, allowing practically all rhythm pathologies, including pacemaker applications, to be simulated. The paper describes in detail the principles on which the model is based, compares the model with other models of the same system and shows concisely some results of simulation experiments in the form of computer generated ECG records.

Atrioventricular Node↗
PubMed1985

A report generator package for routine laboratory tests in a hospital cardiology department.

A simple report generating package is described. The package is designed for hospital computing personnel who wish to develop fast, easy access, easy-to-use report generating programs for use by non-computer specialists. Use of the system is illustrated by examples from three sections within a hospital cardiology department: nuclear cardiology, M-mode echocardiography and reporting of 24-hour ambulatory electrocardiogram tapes.

Cardiology↗