Biomedical subjects
A F Rickards
Publications and source records attributed to A F Rickards.
An evaluation of two-dimensional echocardiography in the diagnosis of hypertrophic cardiomyopathy.
Various anatomical and functional features of hypertrophic cardiomyopathy are analyzed in view of the data provided by two-dimensional echocardiography. Measurement of septal thickness is crucial, and is best done by a combination of M-Mode and 2-D echo. Two types of systolic anterior movement of the mitral valve (SAM) are observed and are related to the degree of subvalvular gradient. The specificity of these patterns of SAM is analyzed. The functional anatomy of the mitral valve in relation to the presence and degree of mitral regurgitation shows that although the presence and type of SAM are important, there are other causes of mitral regurgitation in hypertrophic cardiomyopathy unrelated to SAM. We emphasize the fact the 2-D echo cannot "diagnose" hypertrophic cardiomyopathy except when cardiac hypertrophy plus SAM involving the body of the mitral valve is seen; in the remaining cases, 2-D echo confirms/suggests the clinical diagnosis.
Effect of nitroglycerin on the electrical changes of early or subendocardial ischaemia evaluated by monophasic action potential recordings.
Intracavitary recording of monophasic action potentials (MAP) is a sensitive means of detecting the electrophysiological effects of early or subendocardial ischaemia. The effects of nitroglycerin (NTG) on the MAP was evaluated during pacing-induced angina in seven patients with localised, reversible ischaemia. Recordings from the ischaemic zone demonstrated a decrease in MAP amplitude and an abnormal rate-corrected shortening of MAP repolarisation. The "control" right ventricular MAP showed only the expected rate-dependent decrease in duration throughout the pacing stress test. The ischaemic MAP were unchanged following the intracoronary administration of NTG (100 micrograms). In contrast, intravenous NTG (200 to 300 micrograms) produced a normalisation of MAP amplitude and duration in spite of continuous pacing at the angina-provoking rate. These changes were preceded by a fall in aortic pressure (from mean 123/84 to 96/62) and subsequent lowering of the rate-pressure product. The major beneficial effects of NTG on the early electrical changes of pacing-induced ischaemia are thus related to decreased oxygen demand due to reduction in cardiac preload.
The role of echocardiography in suspected bacterial endocarditis.
We evaluated the clinical application of echocardiography (M and 2D modes) in the assessment of cardiac patients with fever and an underlying valvular abnormality in whom the diagnosis of infective endocarditis was suspected. One or more of the classic clinical features of the disease were present in 50 patients (group A). Vegetations were detected by echocardiography in 17 (47%) out of the 36 patients within this group A who had positive blood cultures. Four (28.5%) of the remaining 14 patients with unequivocal endocarditis clinically and negative blood cultures had demonstrable vegetations on ultrasound. Anatomical complications resulting from the septic process (valve destruction or detachment, aortic root abscess) were visualized in 18 (36%) of the 50 patients in group A. The clinical features of endocarditis were lacking in the other 53 patients with fever and murmur (group B). This group included 12 patients with other sources of bacteraemia besides endocarditis. Unsuspected vegetations were detected only in 2 (3.7%) out of the 53 cases. Thus echocardiography is useful in confirming the clinical diagnosis of infective endocarditis, but only rarely detects vegetations in patients who lack the characteristic clinical features of endocarditis, regardless of whether they have positive negative blood cultures.
Atrioventricular discordance.
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Recordings of the local pacing-induced myocardial activation and repolarization.
We have studied the endocardial ventricular evoked response which follows delivery of a unipolar stimulus down the sensing electrode. The system uses the same lead for both pacing and sensing and permits recordings of the evoked T wave representing a dominantly local repolarization which follows a pacing-induced depolarization at the same site. In 12 animal experiments and in the course of electrophysiological investigations in 19 patients, we evaluated changes in the morphology and duration of the paced evoked response following drug interventions which alter myocardial refractoriness and repolarization time. These changes paralleled results obtained by simultaneous, paced monophasic action potential recordings, and suggest that myocardial repolarization can be accurately assessed by this new technique, which could overcome some of the difficulties in comparing 'in vivo' experiments with the clinical effects of drugs in man.
Study of electrophysiological ischemic events during coronary angioplasty.
The electrophysiological and mechanical events that follow transient therapeutic coronary artery balloon occlusion were analyzed in five patients. A marked (mean, 60 msec) decrease in the repolarization time of the left ventricular ischemic zone (assessed indirectly from endocardial monophasic action potential [MAP] recordings) ensued within 6 to 10 beats of occlusion. Abnormalities in left ventricular relaxation occurred almost simultaneously and preceded contraction abnormalities. A shift in the ST segment in the electrocardiogram (ECG) usually followed within the next 5 to 10 beats. An increase in heart rate (approximately 10 beats per minute) appeared last in the sequence of events. Angina was a variable parameter, frequently absent. Thus, intracavitary recordings of the electrical and mechanical changes are sensitive indicators of the early ischemic changes that follow coronary occlusion, and may be used to assess the effects of therapeutic interventions on events resulting from a myocardial perfusion deficit.
Coronary angioplasty and its role in the management of angina pectoris.
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Initial experience with a physiological, rate responsive pacemaker.
A new pacemaker that can adapt the heart rate in response to the patient's metabolic requirements has been developed. This pacemaker uses the QT interval as the indicator of physiological demand. Experience in five patients showed the rate response to exercise to be smooth and progressive and to return gradually to the basic paced rate after activity stopped. Physiological rate responsive pacing resulted in a 45% increase in cardiac output when compared with fixed rate pacing. Similarly, a 57% increase in maximal exercise capacity was noted when rate responsive pacing was compared with conventional pacing at 70 beats/min. This study showed that physiological rate responsive pacing using the QT interval provides a simple means of increasing the heart rate in accordance with the body's requirements.
Towards multisensor pacing.
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Study of the electrophysiological effects of early or subendocardial ischaemia with intracavitary electrodes in the dog.
The early electrophysiological patterns of regional subendocardial ischaemia were studied by using the paced endocardial evoked response and simultaneous endocardial monophasic action potential recordings in 16 experiments in open chested dogs. Ischaemia was produced by transient (1-3 min) coronary artery occlusion. Regional subendocardial isochaemia caused asynchronous activation due to differential conduction delay and shortened repolarization as evaluated by the duration of the paced evoked response from 175 +/- (SD) 18.7 ms to 167 +/- 16 ms (P less than 0.001). These changes occurred within 60 s of occlusion and reversed rapidly after release of the occlusion. In simultaneous endocardial monophasic action potentials there was a decrease in plateau amplitude and the duration of repolarization shortened from 180 +/- (SD) 21.2 ms to 167 +/- 20.4 ms (P less than 0.001). The delay in endocardial activation after 2 min ischaemia was 5.5 ms, which is considerably shorter than the conduction delay previously reported in the subepicardial layers. The calcium-channel blocking drug verapamil (infused at 0.4 mg/kg) altered the rate at which shortening of repolarization and asynchronous activation occurred during ischaemia in six experiments. These experiments suggest that intracavitary electrodes could provide earlier and more sensitive detection of regional subendocardial ischaemia and may permit the assessment of therapy on the early electrical changes in the intact heart.
The ventricular endocardial paced evoked response.
The endocardial ventricular evoked response which follows delivery of a unipolar stimulus down the sensing electrode is remarkably uniform, and is of slightly longer duration at the base than at the apex of the heart. As the same lead is used for both pacing and sensing, it is possible to record the evoked T wave representing dominantly local repolarization which follows a pacing-induced depolarization from the same site. Studies of the pacing evoked response following administration of drugs with class 3 mode of action are similar to those obtained by monophasic action potential recordings, and suggest that myocardial repolarization can be accurately assessed by this technique. The stimulus to evoked T wave timing has been used to design a pacing system which offers the advantage of physiological control of pacing rate, independent of atrial activity, using a conventional endocardial electrode lead system. The potential advantages of this system are reviewed.
The use of QT interval to determine pacing rate: early clinical experience.
Despite the current availability of physiologic pacing systems, there remain distinct disadvantages associated with their operation. Although in theory the atrial synchronous pacemaker (VDD or DDD) is the ideal system, in practice it performs satisfactorily over a very limited range of heart rates in a limited number of individuals. We describe the TX system which incorporates ventricular pacing and is driven by T wave sensing determined by the QT interval. This pacemaker can distinguish between physiologic and pathologic tachycardias while providing an almost unlimited rate response even in the presence of an unresponsive atrium.
Rate responsive pacing using the evoked QT principle. A physiological alternative to atrial synchronous pacemakers.
We have evaluated clinically a rate-responsive pacemaker which uses the evoked QT principle as indicator of physiological demand. This pacemaker is microprocessor-based and fully programmable noninvasively through radiofrequency coupling to an external microcomputer. To date this system has been implanted in 15 patients. With this QT sensing pacemaker the rate response to exercise was smooth and progressive, and gradually returned to the basic paced rate after termination of activity. Physiologic rate responsive pacing resulted in significant improvement in exercise tolerance and a 40% increase in cardiac output when compared to fixed-rate pacing in 8 patients. This initial experience confirms the possibility of obtaining a physiological response to exercise using a pacing system dependent only on a unipolar electrode which is independent of the problems of atrial activity and sensing. Rate responsive pacing might prove to be a useful alternative to atrial synchronous systems, and particularly advantageous in those patients whose sinoatrial function is abnormal or who suffer from atrial arrhythmias.
Coronary sinus potassium concentration recorded during coronary angioplasty.
Coronary sinus potassium concentration was measured continuously in two patients undergoing angioplasty of a significant stenosis of the left anterior descending coronary artery. After each coronary occlusion there was a transient rise in coronary sinus plasma potassium concentration caused by washout of potassium which had accumulated in the extracellular fluid during the short period of ischaemia. There were no significant changes in the surface electrocardiogram and the patients experienced no chest pain. Changes in coronary sinus potassium concentration provide a sensitive and early indication of myocardial ischaemia in man.
Intracardiac electrode detection of early ischaemia in man.
We have evaluated an intracardiac technique for the study of the electrophysiological patterns of early or subendocardial ischaemia in man. Simultaneous recordings of the paced endocardial evoked response and monophasic action potentials were obtained during pacing stress testing in 10 patients with reversible myocardial ischaemia. Early patterns of change occurred in both these recordings in response to regional ischaemia. Abnormal rate corrected shortening of the local repolarisation time in the paced endocardial evoked response from the left ventricular ischaemic zone diverging from control non-ischaemic values by a mean of 10.6% was paralleled by decreases in the simultaneous paced monophasic action potentials duration. A differential delay in the local activation time and conduction was also documented by the paced endocardial evoked response and monophasic action potential electrodes. Non-ischaemic control zones showed no changes in the pattern of activation and repolarisation. Disparate repolarisation times and asynchronous activation within the myocardium were thus consistently demonstrated during regional ischaemia. These changes in the endocardial paced evoked response and monophasic action potentials always preceded the appearance and regression of the clinical ischaemia. Intracavitary recordings may thus provide earlier and more sensitive detection of regional ischaemia during cardiac catheterisation or coronary artery surgery. The study of the patterns of activation and response could permit the assessment of interventions on the early electrical changes of ischaemia, and may bridge the gap between in vitro studies and the electrophysiological studies performed upon the intact heart.
High potassium concentration in a parenteral preparation of glyceryl trinitrate. Need for caution if given by intracoronary injection.
Glyceryl trinitrate (Tridil) was given by intracoronary injection to a patient who developed coronary artery spasm during angioplasty. Transient exacerbation of chest pain and electrocardiographic changes occurred. This formulation of the drug contains a high concentration of potassium ions and should not be given by this route.
The role of nitrates on regional subendocardial ischaemia--studies with endocardial monophasic action potentials during pacing-induced angina.
Intracavitary recording of monophasic action potentials (MAP) is a sensitive means of detecting the electrophysiological effects of early or subendocardial ischaemia. The effect of nitroglycerin on the MAP was evaluated during pacing-induced angina in seven patients with localized, reversible ischaemia. Recordings from the ischaemic zone demonstrated a decrease in MAP amplitude and an abnormal rate-corrected shortening of MAP repolarization. The "control" right ventricular MAP showed only the expected rate-dependent decrease in duration throughout the pacing stress test. The ischaemic MAP were unchanged following the intracoronary administration of 100 micrograms nitroglycerin. In contrast, 200-300 micrograms intravenous nitroglycerin produced a normalization of MAP amplitude and duration in spite of continuous pacing at the angina-provoking rate. These changes were preceded by a fall in aortic pressure (from mean 123/84 to 96/62) and subsequent lowering of the rate-pressure product. The major beneficial effects of nitroglycerin on the early electrical changes of pacing-induced ischaemia are thus related to decreased oxygen demand due to cardiac unloading.