Biomedical subjects

A E Marble

Publications and source records attributed to A E Marble.

At least 19 recordsLinked to original sources

PubMed1995

Computational simulation of blood flow in human systemic circulation incorporating an external force field.

A quasi-one-dimensional non-linear mathematical model for the computation of the blood flow in the human systemic circulation is constructed. The morphology and physical modelling of the whole system (arteries, capillaries and veins) are completed by different methods for the different vessel generations. A hybrid method is used to solve the problem numerically, based on the governing equation (continuity, momentum and state equations), the input boundary conditions and the predetermined initial conditions. The two-step Lax-Wendroff finite-difference method is used to compute variables for each individual vessel, and the characteristic method is employed for the computation of internal boundary conditions of the vessel connection and the input and output system boundary conditions. Using this approach, blood flow, transmural pressure and blood velocity are computed at all vessel sites and for each time step. The pressure and flow waveforms obtained show reasonable agreement with clinical data and results reported in the literature. When an external conservative force field is applied to the system, the results computed from the model are intuitively correct. The term representing the external pressure added to the system by the muscle, which represents active control on the cardiovascular system, is also embodied in this model.

Algorithms↗

PubMed1994

Effect of intermittent delivery of warm blood cardioplegia on myocardial recovery.

Continuous warm blood cardioplegia is often temporarily interrupted during coronary artery operations to provide the surgeon with a bloodless operating field. To determine the effects of intermittent warm ischemia on myocardial recovery, we randomized 15 adult mongrel dogs to receive either multidose cold or warm blood cardioplegia during a 90-minute arrest. Myocardial metabolic and functional recovery was assessed before clamping of the aorta and after 30 and 60 minutes of reperfusion. Systolic function was well preserved, whereas diastolic function decreased slightly in both groups after arrest. Myocardial oxygen consumption increased during reperfusion after cold heart protection but was unchanged after warm blood cardioplegia. High-energy phosphates decreased significantly in both groups during reperfusion. Two conclusions were reached. (1) Myocardial functional recovery was well preserved, whereas metabolic recovery was impaired after either technique of myocardial preservation. (2) Preserved functional recovery after multidose warm blood cardioplegia suggests that repetitive episodes of ischemia may condition the myocardium, thus preventing injury during prolonged aortic cross-clamping.

Adenosine Triphosphate↗

PubMed1994

Hydrodynamic simulation of arterial networks which include compliant and rigid bypass grafts.

The conditions required to produce an ideal bypass graft have not yet been determined. In order to understand the hemodynamic impact of bypass grafts on cardiovascular function, a hydrodynamic model for the part of the human arterial network below the renal arteries has been constructed. The results from this physical model were used to validate a digital computer model of the arterial network developed by the authors, that incorporated loops which occur due to bypass grafts. The hydrodynamic model was designed to study the interaction between an arterial stenosis and bypass graft and, in particular, the effect of the compliance of the graft on their function. In the model, similarity laws have been maintained with regard to geometry, viscosity, peripheral resistance, wall elastic properties, pulse shape, and blood flow rate. Measured and predicted pressure and flow wave-forms showed an increase in their mean and peak values for the healthy leg when the bypass graft was closed. There was also a severe pressure drop across the stenosis and a marked decrease in mean and pulsatile flow in the stenotic leg. The stenosis in the hydrodynamic model produced similar behaviour to published results obtained on animals. When the graft was open, the agreement between our experimental and theoretical model was within 5% and both the pressure and flow waves were almost similar at the maxima and minima in both legs. Although the arterial system is non-linear, the non-linearities are shown to be of insignificant magnitude and therefore, we have concluded that the pressure flow relationship is essentially linear. The effect of the degree of compliance of the graft and the 'steal' phenomenon due to the graft have been investigated. It was found that there were no significant differences in the flow delivery between the compliant graft and stiff graft. Also, there was no 'steal'. Thus the compliance of the graft is not a significant factor in promoting its patency.

Arteries↗

PubMed1993

Haemodynamic model of a unilateral iliac stenosis with an aortoiliac bypass.

A hydrodynamic model for the part of the human arterial network below the renal arteries has been constructed using specially fabricated distensible tubes and a pulsatile pump to simulate an aortoiliac bypass. The experiments and the computer model indicated that no 'steal' occurred due to the insertion of the bypass graft. Also, the results showed that the length of the stenosis had a non-systematic apparent effect on the physiological significance of the obstruction and that the kinetic power represented only a small percentage of the total power. The total power efficiency of the bypass graft was unaffected by its elastic properties. The experimental investigation also indicated that the pressure drop across the stenosis was considerably larger than the drop calculated using the Poiseuille flow relationship when the stenosis was severe. Therefore, a critical arterial stenosis value cannot be defined as an obstruction of a constant percentage reduction of luminal area. It varies directly with the effective cross-sectional area and inversely with the flow rate. The value of angiography in assessing the functional significance of any arterial stenosis is therefore limited. A better method for evaluation requires quantitative measurements of local blood pressure and blood flow, not only at rest, but also under conditions creating augmented flows due to exercise.

Aorta↗

PubMed1993

Optimisation of a computer vision system for the interpretation of American Sign Language.

Presented in this paper is a simulation algorithm for the optimisation of camera position with respect to the signer, to have a full and reliable interpretation of the American Sign Language. The simulation includes a three-dimensional world point into two-dimensional image point transformation algorithm, the effect of the depth information loss and a sign projection correction test. It is concluded that the viewing camera should be positioned at any point in a specified area subtended by a solid angle of 30 degrees, where the centre of the area is located at 45 degrees in the azimuth and 45 degrees in elevation relative to the signer. The theory and the technique are tested with regard to the efficiency of interpreting American Sign Language (ASL) by two adult signers. One of the signers had been using ASL on a regular basis since infancy, and the second signer had signed for the past five years. It is demonstrated that positioning the camera anywhere in the specified area provides a 96 per cent correct interpretation of the 36 signs tested. The results also provide a preliminary indication that signer variability may not present a major problem in interpretation, and that a computer vision system which captures the optimum depth information can distinguish between signs which, to the naked eye, appear to have similar characteristics.

Algorithms↗

PubMed1992

Image processing system for interpreting motion in American Sign Language.

In this paper, an image processing algorithm is presented for the interpretation of the American Sign Language (ASL), which is one of the sign languages used by the majority of the deaf community. The process involves detection of hand motion, tracking the hand location based on the motion and classification of signs using adaptive clustering of stop positions, simple shape of the trajectory, and matching of the hand shape at the stop position.

Algorithms↗

PubMed1992

Correlation algorithm and sampling techniques for estimating the signal-to-noise ratio of the electrocardiogram.

An algorithm, based on correlation techniques, is proposed for estimating the signal-to-noise ratio of very low frequency signals contaminated by white and flicker noise. Sampling techniques based on converting a single continuous signal into two time series that satisfy the requirements of cross-correlation functions are proposed. The algorithm has been tested on simulated data and the electrocardiogram transduced from ten patients.

Algorithms↗

PubMed1992

Myocardial oxygen consumption and lactate production during antegrade warm blood cardioplegia.

UNLABELLED: Continuous warm blood cardioplegia has recently been recommended as an alternative to multidose cold blood cardioplegia for myocardial protection during coronary bypass operations. Cardioplegia may have to be interrupted in order to provide a bloodless operating field during coronary anastomosis. To determine the effects of ischemia at normothermia on myocardial oxygen consumption and lactate production we randomized 17 dogs to receive either warm blood cardioplegia (37 degrees C) or cold blood cardioplegia combined with systemic and topical cooling. After initiating arrest, cardioplegia was interrupted for periods of 1, 2, 3, 4, 5, 6, and 10 min. Myocardial oxygen debt occurred after 3.5 min of ischemia in the 9 animals receiving warm blood cardioplegia. In contrast, myocardial oxygen consumption never exceeded oxygen availability during cold blood cardioplegia (P less than 0.001). Lactate production increased linearly in both groups but was much greater in those animals receiving warm blood cardioplegia (P less than 0.001). Spontaneous electromechanical activity was much more common during warm blood cardioplegia which required frequent infusions of cardioplegia to maintain cardiac arrest (P less than 0.0003). CONCLUSIONS: (1) Oxygen debt occurred after 3.5 min of warm ischemia; (2) spontaneous electromechanical activity is more common during warm heart protection which necessitates the use of larger volumes of cardioplegia to maintain cardiac arrest.

Animals↗

PubMed1991

Oxygenated cardioplegia ameliorates the adverse effects of small amplitude electrical recording of activity on myocardial metabolic and functional recovery.

UNLABELLED: Small amplitude electrical activity has been recorded from the myocardium during cardioplegic arrest in the absence of electromechanical activity. The presence of persistent electrical activity has been associated with impaired myocardial metabolic and functional recovery. To determine whether or not oxygenated cardioplegia would provide sufficient oxygen to support the increased metabolic activity associated with persistent electrical activity during cardioplegic arrest, we randomized 14 adult mongrel dogs to receive either non-oxygenated or oxygenated cardioplegia during 90 min of ischaemia. Cardiac index (CI), left ventricular stroke work index (LVSWI) and dp/dt were measured before bypass and after 90 min of ischaemia and 45 min of reperfusion. Myocardial oxygen consumption (MVO2) and lactate extraction were measured before and after bypass. Intramyocardial voltage was monitored during cardioplegic arrest, and MVO2 was measured during cardioplegia infusion. The onset of small amplitude electrical activity was associated with a rise in intramyocardial voltage and an increase in MVO2. CI, LVSWI and dp/dt were better preserved in those animals receiving oxygenated cardioplegia. MVO2 and lactate consumption following cardioplegia arrest were also higher in this group. CONCLUSIONS: (1) small amplitude electrical activity during cardioplegic arrest is associated with a rise in MVO2. (2) Oxygenated cardioplegia increases myocardial protection by providing oxygen for the increased metabolic activity associated with the presence of this small amplitude electrical activity.

Animals↗

PubMed1991

Effect of high-volume cardioplegia on small-amplitude electrical activity during cardioplegia arrest.

UNLABELLED: The effects of high-volume cardioplegia on the presence of small-amplitude electrical activity during cardioplegia arrest were investigated in 19 mongrel dogs. The animals were randomly assigned to receive either high-volume crystalloid cardioplegia (HV-plege) or crystalloid cardioplegia guided by continuous electrical monitoring (V-plege). Cardiac index, left ventricular stroke work index dp/dt, and myocardial oxygen consumption were measured before bypass and following 90 min ischemia and 45 min reperfusion. Biopsies were taken for measurement of adenosine triphosphate (ATP) and examination of myocardial ultrastructure. Nine animals received HV-plege, while the remaining 10 animals received cardioplegia guided by voltage criteria. Small-amplitude electrical potentials were recorded within 10-15 min after the infusion of cardioplegia in all animals receiving cardioplegia guided by voltage criteria. Electrical activity, however, was immediately abolished by reinfusion of cardioplegia. HV-plege reduced the incidence of small-amplitude electrical activity during cardioplegia arrest but did not prevent electrical activity. Left ventricular function and myocardial ultrastructure were better preserved when cardioplegia was guided by electrical monitoring. ATP decreased similarly in both groups following cardioplegic arrest, but myocardial oxygen consumption was significantly higher following the arrest in the V-plege group. CONCLUSIONS: HV-plege does not prevent small-amplitude electrical activity and may have adverse effects on myocardial metabolic and functional recovery.

Adenosine Triphosphate↗

PubMed1990

Theoretical model for assessing haemodynamics in arterial networks which include bypass grafts.

The paper presents a theoretical model which can be used to simulate a vascular network which includes loops and bypass grafts, a feature not possible with previous models. Using the linearised Navier-Stokes equations, the linearised equation of a uniform thick-walled viscoelastic tube, and the equation of continuity, the model is applied to a vascular network which includes a bypass graft. This method represents each segment of an artery or graft by a four-terminal-network whose A, B, C, D parameters are functions of the frequency and physical characteristics of the segment. The model predicts the flow and pressure waveforms at any point in the human arterial network very accurately when compared with data obtained from normal patients, patients with arterial stenoses and for hypertensive patients. The model also gives results which are in close agreement with hydraulic experimental data for the input impedance of systems with bypass loops.

Arteries↗

PubMed1990

An adaptive "Wiener" filter for estimating the time-derivative of the left ventricular pressure signal.

This communication presents the development of an optimal digital differentiating filter based on the Wiener theory for estimating the peak derivative of the left ventricular pressure (LVP) signal. The magnitude coherence function is used to estimate the signal and noise spectra. The peak derivative obtained by this method is found to be within 2% of the results obtained by a seven-point second-order data fit and a DFT technique.

Algorithms↗

PubMed1990

A miniature voltmeter for monitoring small amplitude electrical activity in the hypothermic arrested myocardium.

A measurement system which is capable of detecting microfibrillation in the hypothermic, arrested heart has been developed. It is shown that this system can reliably measure microfibrillation signals as low as 10 microV in amplitude. Using this system, it has been shown that even after the heart has been rendered visibly arrested using cold potassium, it may still be producing a low level of electrical activity. It has also been shown that the presence of this microfibrillation during the period of the cold potassium administration and visible arrest, is very highly correlated with the decrease in high energy phosphates (ATP), mitochondrial injury, and a decreased cardiac function post-operatively. Using the information obtained from this first measurement system, a miniature, battery powered voltmeter has been designed and fabricated. This voltmeter has been tested using the initial measurement system as reference and produces the same information concerning the small signal electrical activity (i.e.: same waveform and RMS value). Because this voltmeter is battery powered and is, therefore, completely isolated from all electrical equipment in the operating theatre, it can be used to monitor the presence, or absence, of microfibrillation in the human heart. Of greatest importance, however, is the fact that the voltmeter can serve as a monitoring device which will indicate when the potassium should be administered in order to render the heart both mechanically and electrically arrested.

Animals↗

PubMed1990

Effect of hypothermia and cardioplegia on intramyocardial voltage and myocardial oxygen consumption.

Intramyocardial voltage and myocardial oxygen consumption were measured in the fibrillating heart between the temperatures of 37 degrees C and 25 degrees C and in the arrested heart after infusion of potassium cardioplegic solution in 10 adult mongrel dogs. Electrical activity from the myocardium was recorded using specially designed plunge electrodes, and intramyocardial voltage was monitored by an in-line voltmeter. Myocardial oxygen consumption gradually decreased from 5.8 +/- 0.6 ml O2/min at 37 degrees C to 2.3 +/- 0.5 ml O2/min at 25 degrees C. In contrast, hypothermia did not cause a similar decrease in intramyocardial voltage which remained within a range of 1.8 +/- 0.5 mV to 2.4 +/- 0.5 mV between the temperatures of 37 degrees C and 25 degrees C. The infusion of potassium cardioplegic solution resulted in a dramatic decrease in voltage to 43 +/- 5 microV, and myocardial oxygen consumption fell to 0.5 +/- 0.3 ml O2/min. Our data demonstrated that the mean voltage of the fibrillating heart remains constant between the temperatures 37 degrees C and 25 degrees C and myocardial oxygen consumption decreases with hypothermia, which suggests that voltage does not correlate with the level of myocardial oxygen consumption. Myocardial oxygen consumption and intramyocardial voltage, however, decrease dramatically when cardioplegia is instituted.

Animals↗

PubMed1989

Adaptive numerical smoothing: an efficient method of conditioning physiological signals.

The main aim of the paper is to present a method of reducing the number of necessary operations in the process of numerically smoothing digitised signals, together with increasing the quality of smoothing, using suggestions which are new in the technique of numerical smoothing. The first suggestion consists of an analytical description of the smoothing process and the representation of the input mixture and undergoing smoothing signals by a series of sinc functions. The second proposal consists of using adaptive smoothing, i.e. smoothing with varying step and with some special means for increasing the quality of smoothing. These procedures must be calculated together with the processing and this procedure causes a small delay in the output signal. The approach presented is a deterministic one, i.e. the information about all instantaneous values of the input mixture is used instead of an a priori average information about disturbances. Attention is directed towards disturbances which are associated with physiological signals such as pressure, flow and electrocardiogram waveforms. The paper contains the basic theory and some experimental results which demonstrate the potential of adaptive smoothing.

Electrocardiography↗

PubMed1989

The measurement of afterload, vascular input impedance, and power distribution in aorto-femoral bypass.

The effects of aorto-femoral bypass grafts on the vascular input impedance, and the ratio of pulsatile to total power were studied in eight dogs. Unilateral ileo-femoral stenosis was simulated and comparisons were made between the input impedance and power distribution in healthy and simulated disease situations. Input impedance magnitude spectra and phase were displayed graphically and it was shown that the presence of the simulated disease increases the ratio of pulsatile to total power as measured in the abdominal aorta from 7.5 to 14.8% (p less than 0.05). This suggests that the presence of the stenosis creates an impedance mismatch thus causing reflected waves to propagate proximally towards the heart. It was concluded that the way in which the heart transfers fluid power into the arterial bed was compromised by the presence of the ileo-femoral partial stenoses. It is further suggested that the system described in the paper makes it possible to quantitatively assess afterload, vascular input impedance and cardiovascular power distribution as a quasi-real time diagnostic procedure.

Anastomosis, Surgical↗

PubMed1989

Monitoring the voltage of the myocardium during cardioplegia arrest.

Electrical activity was monitored with specially designed plunge electrodes in 19 animals undergoing 3 h of cardioplegic arrest. Electrical activity was recorded on electromagnetic tape and intramyocardial voltage was monitored with an inline voltmeter. Haemodynamic function was assessed before bypass and following 3 h of ischaemia and 45 min of reperfusion. Intramyocardial voltage during normothermic fibrillation measured 2.4 +/- 0.4 mv. Infusion of cardioplegia initiated a complete electrical arrest in all animals and reduced intramyocardial voltage to 33 +/- 7 mu v. Small amplitude electrical activity was present in 9 of 19 animals. Intramyocardial voltage increased to 108 +/- 12 mu v with the onset of small amplitude electrical activity and spectral analysis of the wave form indicated that the fundamental frequency was in the range of 3.08 Hz. Small amplitude electrical activity during cardioplegic arrest was associated with significant post-arrest depression of left ventricular function. Our data confirms that the presence of small amplitude electrical activity impairs myocardial functional recovery and suggests that continuous intramyocardial voltage monitoring may be used to guide the administration of cardioplegia during cardioplegic arrest.

Adenine Nucleotides↗

PubMed1989

Coupling of cardiac and locomotor rhythms.

The pressure within exercising skeletal muscle rises and falls rhythmically during normal human locomotion, the peak pressure reaching levels that intermittently impede blood flow to the exercising muscle. Speculating that a reciprocal relationship between the timing of peak intramuscular and pulsatile arterial pressures should optimize blood flow through muscle and minimize cardiac load, we tested the hypothesis that heart rate becomes entrained with walking and running cadence at some locomotion speeds, by means of electrocardiography and an accelerometer to provide signals reflecting heart rate and cadence, respectively. In 18 of 25 subjects, 1:1 coupling of heart and step rates was present at one or more speeds on a motorized treadmill, generally at moderate to high exercise intensities. To determine how exercise specific this phenomenon is, and to refute the competing hypothesis that coupling is due to vertical accelerations of the heart during locomotion, we had 12 other subjects cycle on an electronically braked bicycle ergometer. Coupling was found between heart rate and pedaling frequency in 10 of them. Cardiac-locomotor coupling appears to be a normal physiological phenomenon, and its identification provides a fresh perspective from which to study endurance.

Adult↗