Comparing threshold visual fields between the Dicon TKS 4000 automated perimeter and the Humphrey Field Analyzer

BACKGROUND

The automated perimeter is becoming the instrument of choice in the analysis of the visual field. There are a number of different perimeters in use and it can be difficult to compare results from different instruments. The purpose of this study was to compare visual field threshold measurements determined by the Humphrey Field Analyzer and the Dicon TKS 4000.

METHODS

This study provided a statistical comparison of field test results from the Dicon TKS 4000 Automated Perimeter (program number 9) and the Humphrey Field Analyzer (30-2 threshold using FastPac). Central 30 degree, 76 point full threshold fields were performed on 20 non-dilated optometry students (age range 21 to 32 years, mean age 25.1 years) using both instruments. For the purpose of statistical analysis, the visual field was divided into 24 sectors, each measuring 10 degrees by 10 degrees. The mean threshold value of each group of points was compared between instruments.

RESULTS

A difference was found between instruments (significant at P&0.0001) for 19 of 24 sectors. In addition, the Humphrey Field Analyzer threshold values were consistently higher (average difference 2.47 dB) than the Dicon values. Each subject was tested twice on each instrument; both instruments showed high test-retest correlations: 0.970 for the Humphrey and 0.971 for the Dicon. Test administration times were comparable (average 435.2 seconds for the Humphrey Field Analyzer and 430.2 seconds for the Dicon), although the Dicon averaged 41.5 fewer presentations tested per examination. Fixation losses were higher with the Dicon instrument.

CONCLUSIONS

A practitioner may be reluctant to directly compare test results between the two instruments since they produce threshold values of statistically significant differences. The Dicon TKS 4000 was subjectively preferred over the Humphrey Field Analyzer by most subjects.

A kinetic model of coronary reactive hyperemic response to transient ischemia

A kinetic model is proposed to delineate the factors that determine the coronary reactive hyperemic response (RHR) to transient ischemia. The model comprises of myocardial-interstitial (M) and vascular (V) compartments. Vasodilator metabolites (VM) are produced in the M compartment during the interval of coronary occlusion. The rate of VM production is dependent on the flow rate during the ischemic period, the ratio of excess flow above the control level (R) to the loss of flow during occlusion period (D), the amount of oxygen stored and the degree of vasodilation in the V compartment prior to occlusion. Following a complete release of occlusion, VM are transported from the M to V compartment and are washed out or degraded with time. The time course of RHR is determined by the coronary patency which is proportional to VM concentration in the V compartment. Based on a set of numerical constants, the model is tested by simulating RHR to the various occlusion manoeuvres: a pair of 10 sec occlusions separated by brief release, a 15 sec release followed by a second brief occlusion, a brief release of an occlusion followed by restricted inflow and a period of restricted inflow after occlusion. The simulated results fit the experimental R/D and RH durations data of canine hearts. Factors that determine the impairment of RH capacity in coronary stenosis are suggested in terms of the model scheme.

A model of calcium regulation in smooth muscle cell

A model of smooth muscle cell is proposed to describe the mechanisms by which intracellular calcium (Cai) is regulated when the cell is stimulated by noradrenaline (NA)/agonist (Ag) or depolarized. The intracellular Ca2+ store is comprised of two calcium pools: A-store which is sensitive to inositol 1,4,5 trisphosphate (IP3) and C-store, insensitive to IP3 but sensitive to Cai. The A-store is refilled by Ca2+ from C-store and reuptake of Cai. The uptake rate is dependent on the degree of filling in the A-store. The C-store, activated by Cai, is replenished by extracellular Ca2+ (Cao). IP3 generated by NA or Ag changes the A-store permeability, discharging its Ca2+ into the cytosol. Depolarization activates Ca2+ current through the L-type channels. The following events are simulated and compared with experimental observations: (a) Cai transient stimulated by high K+; (b) Cai transient and Ca2+ efflux induced by shortlasting and longlasting NA in the absence of Cao; (c) effect of lanthanum on NA-induced Cai and Ca2+ efflux; and (d) effects of nifedipine, ryanodine, BAY K 8644 and cyclopiazonic acid on Cai.

A model of cytosolic calcium regulation and autacoids production in vascular endothelial cell

A model of vascular endothelial cell is proposed to describe the mechanisms by which cytosolic calcium (Cai) is modulated and endothelium-derived relaxing factor (EDRF) and prostacyclin (PGI2) are released when the cell is stimulated by agonist. The intracellular Ca2+ store of the model cell is comprised of a superficial (sc) and a deep (dc) compartment. The dc Ca2+ content is refilled by the sc whose [Ca2+] is the same as extracellular Ca2+. Inositol (1,4,5)-trisphosphate (IP3) produced by agonist modifies the dc permeability which discharges its Ca2+ to the cytosol. The increase of Cai induces Ca2+ released from the sc. Ca(2+)-activated K+ current hyperpolarises the cell. The raised Cai releases PGI2 in the presence of IP3 while EDRF is released by Cai. The model explains satisfactorily the Ca2+ transient and autacoids production of the aortic endothelial cell without the need of calcium influx from extracellular space. The cytoplasmic Ca2+ oscillations observed in human endothelial cell from umbilical veins were reproduced by the model. Production of EDRF by the artery due to increase in pressure was also simulated.

Functional recovery following direct or graft repair of nerve gaps in the rat

The tibial functional index has been employed to assess functional recovery in rats following nerve repair by direct coaptation under tension or by grafting. Recovery after direct repair of small to moderate gap sizes was better than that after grafting.

Vasomotor coronary oscillations: a model to evaluate autoregulation

A simple model was proposed to characterise the oscillatory and nonoscillatory pattern of canine coronary circulation responses induced by a small dose of a vasodilator adenosine or the Ca2+ channel blocker diltiazem. This model consists of two differential equations describing the interaction of dilating (D) and constricting (C) resistance components. With the assumption that the rate constants associated with (D) were dependent on adenosine concentration and those associated with (C) were a function of Ca2+ channels, the model predicted: a) a damped oscillation of resistance to flow at low dose of adenosine, b) a predominant vasodilation at high dose of adenosine, and c) a sustained vasodilation in response to diltiazem. Parameters characterising the coronary resistance were evaluated by fitting the model results to calculated resistance from measured coronary flow and aortic pressure. As well, the model predicted accurately the peak resistance to great cardiac and coronary sinus venous flow in patients. This study indicates that the oscillation frequency of coronary resistance induced by a low dose of adenosine (0.01 mg/kg) is indicative of the uptake rate of adenosine by the heart and the coronary resistance provides considerable information on vasomotor control of the coronary circulation.

Systolic pressure gradients between the wall of the left ventricle, the left ventricular chamber, and the aorta during positive inotropic states: implications for left ventricular efficiency

To study systolic pressure gradients developed between the left ventricular wall, its chamber, and the aortic root, in one group of dogs left ventricle ventral wall intramyocardial pressure, left ventricular outflow tract pressure, and aorta pressure were compared with aortic flow as well as left ventricular dimension changes during control conditions as well as during positive intropic states induced by isoproterenol, stellate ganglion stimulation, and noradrenaline. In another group of dogs systolic pressures in the ventral wall of the left ventricle, the main portion of the left ventricular chamber, and the aorta were compared with aortic flow during similar interventions, before and after the administration of phentolamine. Pressure gradients between the wall of the left ventricle and the outflow tract of the left ventricle were minimal during control states, but during the three positive inotropic states were increased significantly. In contrast, pressure gradients between the outflow tract of the left ventricle and the aortic root were insignificant during positive inotropic states; those between the wall and main portion of the chamber were only significantly different during left stellate ganglion stimulation. The data derived from these experiments indicate that useful peak power output of the left ventricle (systolic aortic pressure X flow) is unchanged following isoproterenol infusion, but is increased by stellate ganglion stimulation and noradrenaline. The useful peak power output index (an index of left ventricular efficiency derived by dividing useful peak power output by peak intramyocardial pressure) was reduced more by isoproterenol than the other two interventions.(ABSTRACT TRUNCATED AT 250 WORDS)

Model of calcium movements in the mammalian myocardium: interval-strength relationship

A model is proposed to describe the interval-strength relationship in mammalian cardiac muscle in terms of "discrete" calcium movements associated with each cycle. The sarcoplasmic reticulum is assumed to be comprised of three functional sub-compartments: (1) The "main calcium store" which contains most of the calcium (predominantly bound) and is considered, due to its large buffering capacity, to account for the "long-term memory" lasting 7-10 beats. (2) The "releasable terminal" which contains the calcium readily available for release (all or most of it free) and accounts for the "short-term memory" which affects the subsequent beat. (3) The longitudinal network of the SR recirculating the myofibrillar calcium to the "main calcium store". The total content of calcium in the main store is determined by the transsarcolemmal influx and efflux. While influx occurs only during depolarization, efflux occurs during the whole cardiac cycle. The amount of free calcium in the main store is determined by an equilibrium equation. The release of calcium from the "releasable terminal" is governed by a "concentration-dependent" mechanism. This implies that when the concentration in the "releasable terminal" increases, the fraction released increases and the residual calcium left for the subsequent contraction decreases. The model predicts the following interval-strength relationships: steady state peak tension; changes from one steady rate to another; restitution curves; post-stimulation potentiation; paired stimulation; premature beats; post-extrasystolic potentiation following interpolated, basal or complimentary interval.

The dynamics of the coronary venous system in the dog

The coronary vascular system is characterised by a small blood-volume, high resistance arterial system, in which flow is primarily diastolic, a capillary-venule exchange system and a large blood volume, low resistance venous system with predominantly systolic flow. In order to understand the venous component we measured intramyocardial pressure (IMP), peripheral (PVP) and central (CVP) coronary venous pressure, as well as phasic coronary sinus outflow. Based on the experimental data, a model is proposed to describe the hemodynamics of the coronary venous system. This model consists of an intramyocardial vascular storage region into which the arterial blood flows during diastole. During systole, the pressure in the storage vessels increases, pushing the peripheral and epicardial venous system. Blocking the inflow tends to empty the 'storage' and the peripheral regions, reducing venous pressures. Occlusion of the outflow increases the blood volume in these regions, elevating venous pressures. The observations of peak (IMP-PVP) vs peak venous flow during vagal, right abd left stellate stimulation conform satisfactorily with the model.

Hemodynamics of coronary artery stenosis

A model is proposed to study the hemodynamics of various types of coronary stenosis. The model coronary artery is assumed to be an elastic tapered tube. A progressive degree of concentric and eccentric stenoses are studied. Measured pulsatile coronary pressure, flow, and intramyocardial pressure are used as input data to calculate the pressure and flow velocities at different locations of the artery. The simulation yields results that agree well with published data in dog experiments, and those from human stenotic coronary arteries. The present model shows that in a cardiac cycle, the overall hydraulic resistance owing to a specific stenosis tends to be flow independent at low flow rate but increases linearly with flow at higher coronary flow. This flow independent resistance increases with progressive stenosis. At low flow, the mean coronary flow in a cardiac cycle is relatively constant with stenoses up to 80%, but decreases dramatically with further increase in the degree of narrowing. At high resting flow rate, this mean flow is markedly reduced at much smaller degrees of constriction. The simulated pressure velocity relation of poststenotic dilatation indicates an additional pressure loss at the distal end of the stenosis, but the calculated resistance to flow is actually lessened. While stenosis length increases pressure loss and resistance to flow, its effect on mean flow appears disproportionally insignificant. Eccentric lesions appear to be more detrimental than concentric ones as they produce additional pressure loss and greater resistance across the coronary artery lesion.

Coronary artery compliance in the dog

Measurement of left anterior descending coronary arterial pressure, phasic coronary flow, and intramyocardial pressure in an open-chest dog provided data, which when entered into the computer model of the coronary circulation, permitted calculation of coronary artery compliance and resistance during systole and diastole. Resting in vivo compliance averaged 0.21 x 10(-3) mL/mmHg (1 mmHg = 133.322 Pa) while systolic resistance averaged 4.05 mmHg X min-1 X mL-1 and during diastole 2.06 mmHg X min-1 X mL-1. Left stellate ganglion stimulation or vasodilation caused minimal changes in compliance but glutaraldehyde applied to arterial wall caused a decrease in compliance. Sympathetic stimulation and vasodilation decreased both diastolic and systolic resistance. Transmural distribution of coronary flow was not significantly altered by the experimental changes in compliance and resistance.