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Biomedical subjects

Kim H Parker

Publications and source records attributed to Kim H Parker.

8 recordsLinked to original sources
PubMed2003

Intra-aortic balloon pumping: effects on left ventricular diastolic function.

OBJECTIVE: The intra-aortic balloon pump is the most widely used form of temporary cardiac assist and often utilised in patients before and after cardiac surgery. Several effects of balloon counter-pulsation have been reported previously, but its effect on left ventricular diastolic function has not been thoroughly investigated. The aim of this study is to examine the effect of the intra-aortic balloon pump on left ventricular wall motion and transmitral flow. METHODS: We studied 20 patients in the intensive care unit, less than 36 h following cardiac surgery. We recorded left anterior descending coronary artery and transmitral E-wave flow velocities using transesophageal echocardiography pulsed Doppler. We also recorded left ventricular long axis free-wall movement using M-mode. The intra-aortic balloon pump was set to full augmentation and recordings were made at pumping cycles 1:1, 1:2, 1:3, and when the pump was on stand-by, leaving a minimum of 5 min between the pumping modes to allow the return to control conditions. In order to eliminate time effects, the sequence of recording was varied between patients using a 4 by 4 Latin-square. RESULTS: The peak diastolic left anterior descending coronary artery and transmitral E-wave flow velocities, and left ventricular free-wall early diastolic lengthening velocity increased significantly with intra-aortic balloon pumping cycles 1:1, 1:2 and 1:3 compared to their value with the pump on stand-by, all P < 0.001. The increase in peak transmitral E-wave flow velocity correlated with the increase in peak left anterior descending coronary artery diastolic flow velocity (r = 0.74, P = 0.02), and with the increase in left ventricular free-wall early diastolic lengthening velocity (r = 0.80, P < 0.001). CONCLUSION: Using the intra-aortic balloon pump post-cardiac surgery significantly increases peak diastolic left anterior descending coronary artery flow velocities and left ventricular free-wall early diastolic lengthening velocity, whose increase explains the increase in peak transmitral E-wave velocity. Although coronary flow is epicardial and mitral flow is intracardial, their close relationship suggests an improvement in left ventricular diastolic function with intra-aortic balloon pump.

Aged↗
PubMed2002

Time-domain representation of ventricular-arterial coupling as a windkessel and wave system.

The differences in shape between central aortic pressure (P(Ao)) and flow waveforms have never been explained satisfactorily in that the assumed explanation (substantial reflected waves during diastole) remains controversial. As an alternative to the widely accepted frequency-domain model of arterial hemodynamics, we propose a functional, time-domain, arterial model that combines a blood conducting system and a reservoir (i.e., Frank's hydraulic integrator, the windkessel). In 15 anesthetized dogs, we measured P(Ao), flows, and dimensions and calculated windkessel pressure (P(Wk)) and volume (V(Wk)). We found that P(Wk) is proportional to thoracic aortic volume and that the volume of the thoracic aorta comprises 45.1 +/- 2.0% (mean +/- SE) of the total V(Wk). When we subtracted P(Wk) from P(Ao), we found that the difference (excess pressure) was proportional to aortic flow, thus resolving the differences between P(Ao) and flow waveforms and implying that reflected waves were minimal. We suggest that P(Ao) is the instantaneous summation of a time-varying reservoir pressure (i.e., P(Wk)) and the effects of (primarily) forward-traveling waves in this animal model.

Animals↗
PubMed2002

Compression and expansion wavefront travel in canine ascending aortic flow: wave intensity analysis.

The forces underlying left ventricular ejection were investigated by applying a wavefront analysis to blood pressure (P) and velocity (U) waveforms measured in the ascending aorta of anesthetized dogs (n = 13). Wavefronts travel forward (to the periphery) and/or backward (to the heart) after peripheral reflection. They are characterized by the rate of pressure change they cause, i.e., the time derivative of pressure (dP/dt): compression wavefronts have dP/dt > 0: expansion wavefronts have dP/dt < 0. Wave intensity is defined as (dP/dt)(dU/dt), where dU/dt is the time derivative of U. Forward wavefronts contribute positively to wave intensity and backward wavefronts contribute negatively. Therefore, wave intensity indicates whether the effects of forward wavefronts are predominant or whether those of backward wavefronts predominate in the formation of pressure and velocity waveforms. Under control conditions, wave intensity was positive in early and late systole, indicating that forward compression and expansion wavefronts dominate aortic acceleration and deceleration, respectively. Compression wave intensity was increased during inotropic stimulation by dobutamine (10-15 microg/kg per min i.v. infusion; +161% +/- 31% mean change in peak value +/- SEM (%), P < 0.05), and was reduced during beta-blockade by propranolol (1 mg/kg i.v. injection; -58% +/- 7%, P < 0.05). Expansion wave intensity was unchanged by dobutamine and propranolol (n = 6). In a separate group of animals (n = 7), expansion wave intensity was reduced during vasodilatation by nitroglycerin (0.5mg i.v. injection and 0.02 microg/kg per min infusion; -32% +/- 12%, P < 0.05), but was unchanged during vasoconstriction by methoxamine (2 mg i.v. injection). However, methoxamine reduced compression wave intensity (-46% +/- 14%, P < 0.05). These results indicate that (1) compression and expansion wavefronts generated by the left ventricle dominate acceleration and deceleration in the ascending aorta, (2) compression wave intensity is related to the inotropic state of the left ventricle, but is reduced during vasoconstriction, and (3) expansion wave intensity is reduced during vasodilatation. This time domain analysis of traveling wavefronts readily provides information concerning the dynamics of the ventriculoarterial interaction.

Adrenergic alpha-Agonists↗
PubMed2002

Relationship between detrended fluctuation analysis and spectral analysis of heart-rate variability.

The recently-introduced technique of detrended fluctuation analysis (DFA) for heart-rate variability appears to yield improved prognostic power in cardiovascular disease through calculation of the fractal scaling exponent alpha. However, the physiological meaning of alpha remains unclear. In DFA, the signal is segmented into lengths from 4 to 64 beats. For each segmentation length (n), the individual segments are cumulated, detrended and the sum of the squares (F2) of residuals calculated. Alpha is the slope of log(F) against log(n). We show mathematical equivalence between alpha calculated by DFA and by a novel alternative method using frequency-weighted power spectra. We show F2 (and thus alpha) can be obtained from a frequency-weighted power spectrum without DFA. To do this, we cumulate and detrend the Taylor series of individual Fourier components. F2 is found to depend on the relationship between the signal period and segment length. F2 can therefore be expressed in terms of frequency-weighted power spectra. From this, the alpha coefficient of DFA can then be described in power-spectral terms, which facilitates exploration of its physiological basis. We confirm these findings using samples from 20 healthy volunteers and 40 patients with heart failure.

Fourier Analysis↗
PubMed2002

Retinal vascular tree morphology: a semi-automatic quantification.

A semi-automatic method to measure and quantify geometrical and topological properties of continuous vascular trees in clinical fundus images is described. Measurements are made from binary images obtained with a previously described segmentation process. The skeletons of the segmented trees are produced by thinning,ff branch and crossing points are identified and segments of the trees are labeled and stored as a chain code. The operator selects a tree to be measured and decides if it is an arterial or venous tree. An automatic process then measures the lengths, areas and angles of the individual segments of the tree. Geometrical data and the connectivity information between branches from continuous retinal vessel trees are tabulated. A number of geometrical properties and topological indexes are derived. Vessel diameters and branching angles are validated against manual measurements and several derived geometrical and topological properties are extracted from red-free fundus images of ten normotensive and ten age- and sex-matched hypertensive subjects and compared with previously reported results.

Adult↗
PubMed2002

Delayed metabolic and thermogenic response to a mixed meal in normoglycemic European women with previous gestational diabetes.

We assessed postprandial thermogenesis (PPT) for 3 h following a mixed meal in 29 normoglycemic European women with previous gestational diabetes (GDM), compared with 37 control women. Given the potential role of catecholamines and insulin in the regulation of PPT, we assessed insulin and catecholamine responses to the meal. There was no significant difference between the two groups in resting energy expenditure, PPT (although lower in the GDM group), or catecholamine levels. However, we observed a difference in the shape of the PPT curve between groups, and by applying a mathematical model, there was a consistent delay in PPT, insulin, and noradrenaline responses to the meal in the GDM group (T: fitted time constant, geometric mean (95% confidence interval), T(PPT) 58 (47-72) vs. 42 (37-48) min, P = 0.006; T(ins) 32 (28-37) vs. 22 (19-27) min, P = 0.002; T(NA) 30 (23-38) vs. 18 (14-23) min, P = 0.01, respectively). Fidgeting activity during the study was assessed by a novel technique and was lower in the GDM group, resting [427 (381-477) vs. 511 (466-560) kJ/min, P = 0.02] but not postprandially. These delayed PPT, insulin, and noradrenaline responses to the meal in post-GDM women represent early metabolic changes. The decrease in fidgeting activity while resting, observed in the post-GDM group, may have physiological significance for energy balance.

Adult↗