Mechanism of depolarization in the ischaemic dog heart: discrepancy between T-Q potentials and potassium accumulation

1. To study the origin of ischaemic myocardial depolarization, the diastolic surface potential - T-Q depression-was correlated with subepicardial extracellular K+ accumulation during serial episodes of widespread ischaemia in open-chest dogs, and in isolated, blood-perfused canine hearts. Placement of the reference electrode on a small island of non-ischaemic myocardium simplified the interpretation of the T-Q potentials. 2. In some experiments, changes in resting potential in the ischaemic zone were recorded using a 'contact' monophasic action potential (MAP) electrode. The change in MAP resting potential was linearly related to T-Q depression for a wide range of experimental conditions (R greater than 0.98). T-Q depression is therefore a linear index of depolarization in superficial myocardial cells. 3. The validity of T-Q depression as a 'measure' of local cellular depolarization was further tested by infiltration of isotonic KCl into the superficial myocardium subjacent to the ischaemic zone electrode. Resulting T-Q depression was 2- to 3-fold larger than the maximum values obtained in ischaemia; and the ratio of T-Q depression to the amplitude of the accompanying monophasic potential was consistent with the assumption that KCl had fully depolarized the underlying myocardium (delta Vm = 89 mV). KCl prevented (i.e. occluded) further changes in the T-Q potential during ischaemia. KCl did not have these effects if it was introduced at sites more remote from the electrode (greater than 4 mm). 4. Ischaemic T-Q depression was drastically accelerated by increasing the heart rate from 90 to 180 beats/min and was further accelerated by arterial infusion of CaCl2. These effects were most striking during the first minute of ischaemia. 5. In contrast, the above manoeuvres produced little acceleration of subepicardial K+ accumulation. After CaCl2 infusion, large ischaemic potentials, severe conduction impairment, and arrhythmias could be observed when K+ activity was almost normal (aK = 4.0-4.5 mM). 6. T-Q depression was larger in vivo than in isolated hearts, both absolutely and relative to K+ accumulation. 7. Based on the reproducible amplitude of ischaemic epicardial potential-estimates of cellular depolarization (delta Vm) could be obtained, which were compared with the concurrent change in K+ electrode potential (delta EK) for each experimental condition. 8. Estimated depolarization was nearly identical to delta EK in isolated hearts under basal conditions. However, depolarization significantly exceeded delta EK during rapid pacing, CaCl2 infusion, or during paced occlusions performed in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

Health beliefs, satisfaction and compliance

Relationships were studied between measures of parents' Health Beliefs, satisfaction, and compliance in 174 children suffering from asthma. It was found that while concurrent measures of Health Belief Model (HBM) variables and compliance were significantly correlated, measures of HBM variables did not predict future compliance. Satisfaction measures, however, not only correlated with concurrent measures of compliance but also predicted future compliance. It is concluded that these findings are consistent with the view that in this sample health beliefs were reactive to compliance behaviours and that they did not have a casual effect on compliance.

Rate dependence of ischaemic myocardial depolarisation: evidence for a novel membrane current

Depolarisation of ischaemic myocardial cells is at least partly due to loss of cellular potassium. Whereas most manifestations of ischaemia vary with heart rate potassium loss, however, reportedly does not. Cellular depolarisation was therefore correlated with extracellular potassium activity during serial coronary artery occlusions in Langendorff perfused canine hearts. Occlusions in sinus rhythm (92(11) beats X min-1) were alternated with rapidly paced occlusions (180 beats X min-1). For each occlusion cellular depolarisation was estimated from TQ depression and compared with the simultaneous increase in potassium electrode potential, delta EK. Although potassium accumulation accounted for most of the estimated depolarisation at slow heart rates, a potassium independent mechanism predominated during rapid pacing. The potassium independent mechanism was especially important in the first minute of ischaemia when pacing increased depolarisation by 324%, with little increase in delta EK. It appears that ischaemia induces a rate sensitive depolarising membrane current, which worsens conduction and promotes arrhythmias.

Effects of intervention on medication compliance in children with asthma

We have assessed the effects of intervention on medication compliance in asthmatic children. The intervention comprised both written information about the medications and behavioural strategies effected by the physician. Children were assigned at random to either control (received no intervention) or test (received the intervention) groups. Compliance was assessed by questionnaire. The mean compliance for the test (78.0%; n = 93) and for the control (54.5%; n = 103) groups differed significantly (P less than 0.001; Mann-Whitney U-test). The test group had a better knowledge of asthma and of the medications, and was more satisfied with the physician and with the regimen than was the control group. These variables were also related to good compliance. This study demonstrates that a programme of intervention can significantly improve medication compliance and can be accompanied by increases in the knowledge of, and satisfaction with, treatment.

Medication compliance in children with asthma

A questionnaire to assess compliance with prescribed therapy was completed by 200 children with asthma. The validity of the questionnaire was assessed objectively by measuring plasma theophylline concentrations in 37 subjects and by weighing metered dose aerosol canisters before and after use in 19 subjects. The average compliance was 67.9%. The close agreement between answers to the questionnaire and the objective measurements of compliance indicated that most participants recalled the drug regimens accurately. Good compliance was related to whether Australia was the parents' country of origin, to knowledge of the disorder and to comprehension of medication but was not related to perception of the severity of the illness. This study demonstrates that compliance is relatively poor even in a clinic population which attends regularly and appears well-motivated.

A comparison of ultrasonography and radiography in the localization of renal calculi: experimental and operative experience

The capabilities of radiography (mammography film) and ultrasonography (7 MHz. probe) were investigated in cadaver kidneys. Stones of varying size and composition were studied. Clinically, 11 patients then were studied with intraoperative radiography and ultrasonography. Both techniques appeared to localize calculi more than 3 mm. with high consistency in cadaver kidneys and in patients. Real-time B scan ultrasonography is a rapid and accurate technique that has the ability to identify lucent calculi. When used in association with intraoperative radiography, intraoperative ultrasonography can be a great aid in the surgical removal of renal calculi.