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Biomedical subjects

Y C Chiu

Publications and source records attributed to Y C Chiu.

At least 19 recordsLinked to original sources
PubMed2000

Effects of 9 weeks of hindlimb unloading on motor performances in adult rats.

It has been reported that abnormal steps associated with an ankle hyper-extension during walking were observed in adult rats after 2 weeks of hindlimb suspension (Canu and Falempin, 1997 & 1998). But such phenomena were normalized after 7 days of reambulation recovery. Canu and Falempin (1996) suggested that the spinal cord has a capacity to generate a well-organized pattern of locomotion even after a period of muscle disuse. There are, however, no reports about the effects of more prolonged suspension on motor performances. In the present study, 7 weeks old male rats were hindlimb-unloaded by tail suspension for 9 consecutive weeks and landing performances in response to drop from head-down, head-up, or supine position were investigated during 8 weeks of recovery. Posture maintenance during right-left translation was also checked.

Animals↗
PubMed1992

Power-afterload relation in the failing human ventricle.

Animal studies have shown that the power output of the left ventricle is related to afterload by a bell-shaped curve. Furthermore, the normal ventricle operates at the maximum power point, whereas the diseased ventricle operates off the maximum power point because of increased afterload. We studied this relation in eight patients with dilated cardiomyopathy. A high-fidelity catheter with micromanometer pressure and electromagnetic velocity transducers was used to measure blood pressure and flow velocity in the ascending aorta. The latter was converted into volumetric flow by calibrating with the simultaneously determined thermodilution cardiac output. Ventricular power was calculated by dividing the integral of the aortic blood pressure-flow product by the duration of the cardiac cycle. Intravenous nitroprusside was used to alter afterload and the power-afterload relation was obtained by plotting power against mean aortic blood pressure. In all patients, as blood pressure was lowered initially, the power output of the left ventricle increased. As the dose of nitroprusside was increased further, the total power either plateaued or actually decreased. The averaged power and the mean blood pressure at control were 1.00 +/- 0.62 W and 92 +/- 9.3 mm Hg, respectively. The averaged maximum increase in power with nitroprusside was 22%, to 1.22 +/- 0.73 W, and this occurred at a mean pressure of 80 +/- 8.8 mm Hg. This study showed that the power-afterload relation in the human ventricle has a maximum power point at some intermediate level of afterload, similar to that found in animal studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1992

Changes in valvular resistance, power dissipation and myocardial reserve with aortic valvuloplasty.

Balloon aortic valvuloplasty results in small changes in valve area with great symptomatic improvement in some patients, while others have little relief with greater increases in valve area. Alternative indices to valve area may help explain this clinical discrepancy. A calculation of valve area does not provide a means of assessing the load imposed by a stenotic valve, while the complementary index valve resistance, defined as the quotient of mean pressure difference divided by flow, allows many other hemodynamic calculations and may provide an additional measure of the hemodynamic importance of valvular obstructions. To assess the value of these calculations, we studied hemodynamic changes in thirty elderly patients undergoing valvuloplasty for aortic stenosis. The valve area, as calculated by the Gorlin formula, increased by an average 67% (0.59 cm2 to 0.95 cm2), while hemodynamic resistance decreased by an average 52% (453 to 207 dyne.sec.cm5). The values of resistance were used to predict pressure gradients and work loads at different cardiac outputs. The increase in myocardial reserve with valvuloplasty was calculated as the increase in cardiac output that could be achieved at the pre-valvuloplasty value of either total ventricular pressure or ventricular work. These calculations assumed that valvular resistance did not change with cardiac output and that peripheral resistance varied inversely to cardiac output so as to maintain a constant aortic (systemic) pressure. The increase in myocardial reserve was 18% when ventricular work rate was the limiting factor, and 103% when pressure was limiting. The increase in reserve may be closer to the higher value since the myocardial work rate is probably not limited by myocardial energy in the absence of coronary artery disease. Four patients who did not do well clinically were characterized by small increases in reserve, either because of inadequate dilatation of the valve or because the original stenosis was not severe. Valve resistance, myocardial reserve, and ventricular work may be calculated using standard hemodynamic measurements. In conjunction with aortic valve area, these indices provide significant complimentary information and may further elucidate the hemodynamic consequences of valvular obstruction.

Aged↗
PubMed1991

Determination of pulse wave velocities with computerized algorithms.

Careful determination of pulse wave velocity is important in the study of arterial viscoelastic properties, wave reflections, and ventricular-arterial interactions. In spite of its increasingly widespread use, there is as yet no standardized method for its determination. Most studies have manually identified the transit time of the pressure wave front as it travels over a known distance in the arterial system, but the issues of accuracy and reproducibility have not been addressed. This study was designed to investigate the efficacy of four computerized algorithms in the determination of pulse wave velocities in invasive as well as in noninvasive pressure determinations. The four methods were the identification of: (1) the point of minimum diastolic pressure, (2) the point at which the first derivative of pressure is maximum, (3) the point at which the second derivative of pressure is maximum, and (4) the point yielded by the intersection of a line tangent to the initial systolic upstroke of the pressure tracing and a horizontal line through the minimum point. High-fidelity aortic pressure recordings were obtained in 26 patients with a multi-sensor micromanometer catheter. Noninvasive brachial and radial pressure waveforms were recorded in 11 volunteers with external piezoelectric transducers. The results show that the first derivative method consistently provided results that were different from the other methods for both the invasive and noninvasive methods because of changes in the structure of the upstroke as the arterial pulse propagates distally. Although the minimum method worked well for the invasive determinations, it was erratic with the noninvasive determinations, probably because of the higher amount of noise and reflection in the latter. Among the four algorithms, the second derivative and the intersecting tangents methods worked well with both invasive and noninvasive determinations with mean variation coefficients of less than 7% and correlation coefficients between the methods of greater than 0.90 for all data. In conclusion, computerized algorithms allow accurate determination of pulse wave velocity in invasively and noninvasively measured arterial pressure waveforms.

Adult↗
PubMed1990

Hemodynamic resistance as a measure of functional impairment in aortic valvular stenosis.

Calculated valve area depicts anatomical stenosis but does not quantify hemodynamic impairment. We propose that hemodynamic resistance, defined as the mean pressure gradient across the valve divided by mean flow rate during systolic ejection, gives a better indication of hemodynamic obstruction. This index was compared with Gorlin valve area in 40 patients with aortic stenosis. Calculated area ranged from 0.22 to 1.26 cm2, and mean transvalvular resistance ranged from 117 to 1,244 dyne.sec.cm-5. In general, resistance varied inversely with calculated area, but there was substantial variation about the mean relation. All of the variation could be accounted for by variations in the pressure gradients at each value of calculated area. Resistance was higher in proportion to area when flow and pressure gradient were high. Analysis of five published studies of a total of 83 valves showed that calculated area changed at least three times more than resistance when pressure gradient was varied. The utility of resistance as an index of stenosis is demonstrated by example calculations that show how during exercise a stenotic valve increases the ventricular work rate out of proportion to the work done on the peripheral resistance. These calculations are possible because hemodynamic resistance defines functional impairment in units commonly used for quantification of opposition to flow. Furthermore, resistance appears to be less dependent than area on conditions of measurement and does not require an empirical constant.

Aortic Valve↗
PubMed1989

An improved catheter design for crossing stenosed aortic valves.

Retrograde catheterization of the left ventricle in patients with aortic valve stenosis is commonly performed using a pigtail or coronary arterial catheter. This approach, although usually ultimately successful, may be highly tedious. To overcome the limitations of these conventional catheter shapes, we have designed a catheter specifically for crossing stenotic aortic valves. In a group of 17 patients, mean aortic valve area 0.75 cm2, the average time to cross the aortic valve was 2 minutes 45 seconds and the median time 30-40 seconds. Once the proper size catheter was selected, the average time to cross was 59 seconds. The success of this catheter is based on the operator's ability to "scan" or interrogate the surface of the valve thoroughly by manipulating the catheter and a moveable core straight tip guidewire.

Aged↗
PubMed1989

Comparison of the effects of different inotropic interventions on force, velocity, and power in rabbit myocardium.

To gain some insight into inotropic mechanisms, we compared the effects of several classes of inotropic interventions on the isometric twitch and force-velocity properties of isolated rabbit myocardium. Postextrasystolic potentiation was chosen as one of the interventions in the belief that its onset is so rapid that it would be unlikely to cause substantial chemical changes in the contractile proteins and that most of the effects would be due to changes in the level of activation. The effects of a digitalis analogue (acetylstrophanthidin), an adrenergic agent (isoproterenol), and a methylxanthine (caffeine) were then compared with those of postextrasystolic potentiation. The conditions were chosen so that each agent caused a twofold increase in twitch force. Acetylstrophanthidin and postextrasystolic potentiation caused twitch force to increase with only a slight (11%) decrease in time to peak force. Isoproterenol caused the peak of the twitch to occur substantially (40%) earlier with marked abbreviation of the twitch. Caffeine had the opposite effect: time to peak force was delayed (by 60%), and the twitch was markedly prolonged. In contrast to the marked differences in the time course of the twitch, there was no significant difference between the instantaneous force-velocity curves obtained with the different interventions. All four interventions caused maximum velocity to increase slightly (1-9%) and maximum power to increase only slightly more than twitch force (5-21%). All of the changes observed can be accounted for by changes in activation, either by an increase in the amount of calcium released into the myofilament space or by a change in the sensitivity of the myofilaments to calcium. There was no need to postulate direct changes in the contractile machinery to account for these results.

Animals↗
PubMed1987

Force, velocity, and power changes during normal and potentiated contractions of cat papillary muscle.

The instantaneous force-velocity relations of cat papillary muscles were studied at different times in the twitch in normal and in postextrasystolic potentiated contractions. Fourteen to sixteen different loads were used to define each of the force-velocity curves. The curves were fitted by a least-squares procedure to the hyperbolic (Hill equation). The hyperbolae were extrapolated to obtain maximum velocity and isometric force and interpolated to obtain maximum power. All three of these values rose more quickly than developed force. Maximum velocity reached 77% of its peak at the earliest time studied, 20-25% of the time to peak force. Developed force achieved 22% of its final value at this time, while extrapolated isometric force and maximum power both reached 44% of their peak values. Postextrasystolic potentiation sufficient to produce a 1.5 to twofold increase in peak developed force produced less than a 20% increase in extrapolated maximum velocity. The results can be interpreted in terms of a model in which the maximum velocity of the contractile elements remains constant during the twitch. Variation in maximum velocity is attributed to a small internal load, equivalent to 6% of twitch force. Since maximum velocity is relatively constant, it does not give a good measure of changes in the force-velocity curves. By contrast, the extrapolated isometric force and maximum power are much more sensitive to changes in the force-velocity curves, and they vary in proportion to each other. The advantage of using interpolated maximum power rather than isometric force to define changes in the curves is that it can be normalized to muscle mass.

Animals↗
PubMed1986

Myocardial infarction with topical cocaine anesthesia for nasal surgery.

Cocaine, the active alkaloid in coca leaf, is widely used as local anesthetic for otolaryngologic procedures. Our patient suffered an acute nontransmural myocardial infarction following clinical use of cocaine as topical anesthesia for nasal surgery, the first such case to be documented, to our knowledge. Although evidence documenting its cardiovascular toxicity is listed in contemporary pharmacologic literature, clinical cardiac injury has been reported to date only with the recreational use of cocaine. Authentic documentation of drug composition when received through the intervention of illicit vendors is always difficult. The literature is reviewed, justifying the use of cocaine as the most popular topical anesthetic in otolaryngologic practice. However, we hope that awareness of this possible complication will create a resurgence of research interest in topical cocaine anesthesia.

Adult↗
PubMed1985

System for automatic activation of skinned muscle fibers.

A system is described for automatically changing the solutions surrounding skinned muscle fibers at a controlled temperature. The system consists of a complex muscle chamber and solenoid valve-controlled vacuum-driven syringe pumps. The chamber has a muscle trough, a conduit for circulating coolant, and channels for precooling solutions. After traveling through the cooling channels, the solutions are injected into one end of the muscle trough and removed by suction at the other. The volume of the trough (approximately 40 microliter) is kept as low as possible to minimize the amount of solution required for complete solution exchange (approximately 400 microliter). Solution changes are complete within 400 ms. The timing of the sequence of solution changes is precisely controlled by digital timers that activate the solenoid valves of the pumps. By exposing the fibers briefly to a high concentration of free calcium and then changing to a buffered calcium solution, it is possible to achieve steady tension levels in 1-2 s, even when partially activating the fibers. Such rapid partial activation is possible because the exposure to free calcium can be precisely timed. The timers also allow the solution changes to be synchronized with other perturbations.

Animals↗