Electrode for recording direction of activation, conduction velocity, and monophasic action potential of myocardium.

Conduction velocity and recovery of excitability are central facets of reentry arrhythmias, and yet there are no satisfactory techniques for the simultaneous measurement of both from the same area of myocardium. We have developed an electrode arrangement that allows the simultaneous recording of conduction velocity, repolarization of the myocardium together with an index of dispersion, and direction of activation of the myocardium. Three silver/silver chloride electrodes were arranged in an equilateral triangle with a reference electrode at the center. From this arrangement three monophasic action potentials were recorded. From the time of arrival of the wavefront of activation at each electrode the direction of activation and conduction velocity were calculated in real time by a computer. There was a good correlation for the in vivo signals from the circular electrode and the new electrode both for conduction velocity (r = 0.99, P < 0.001) and for direction of activation (r = 0.99, P < 0.001). This new mathematical method and electrode design allows the simultaneous measurement of conduction velocity and direction and monophasic action potential, and this can give a beat-by-beat indication of wavelength and dispersion of action potential duration.

Contribution to heart rate variability by mechanoelectric feedback. Stretch of the sinoatrial node reduces heart rate variability.

BACKGROUND: Heart rate variability is an important prognostic indicator for sudden death. An increased risk of sudden death and arrhythmia is associated with reduced heart rate variability in heart failure. In heart failure, there is also dilatation of the atria, which raises the prospect that there could be some physiological basis to possibly link heart rate variability with atrial dilatation. We therefore investigated whether sustained atrial stretch could modulate heart rate variability directly. METHODS AND RESULTS: Pigs were anesthetized and their hearts exposed. A specially built device stretched the sinoatrial node before and after vagal section and then after administration of propranolol. Stretch of the sinoatrial node decreases heart rate variability in the following ways: The standard deviation of the beat-to-beat interval decreases (4.2 to 2.6 ms; P = .004), and the high-frequency components are reduced (control, 6.5 +/- 2.2 ms2, during stretch, 1.4 +/- 0.3 ms2, P = .003). After section of both vagi, the high-frequency components are reduced by stretch of the sinoatrial node (2.8 +/- 0.9 ms2 for control versus 1.2 +/- 0.3 ms2 during stretch; P = .05). Similarly, after both vagal section and beta-blockade, stretch of the sinoatrial node reduces the high-frequency components (10.6 +/- 3.5 ms2 for control verses 3.0 +/- 1.5 ms2 during stretch; P = .01). CONCLUSIONS: We conclude that stretch of the sinoatrial node reduces high-frequency heart rate variability. This may account in part for the reduced heart rate variability seen in clinical conditions in which the right atrium is dilated, such as congestive cardiac failure.

Cycle length dependence of the electrophysiological effects of increased load on the myocardium.

BACKGROUND: Mechanoelectric feedback, the process by which changes in mechanical activity change the electrophysiology of the myocardium, has been linked to the genesis of arrhythmias. We investigated possible arrhythmogenic mechanisms by measuring changes in steady-state action potential duration and, more particularly, electrical restitution on a transiently applied load change, because action potential recovery may provide clues to arrhythmogenesis. METHODS AND RESULTS: Pigs were anesthetized and their hearts exposed. A snare was placed around the aorta, and the right atrium was paced. Ventricular pressure, monophasic action potential, and segment motion were recorded from the left ventricle. The action potential duration was measured before and during transient aortic occlusion. Electrical restitution curves were constructed from the records obtained during normal loading or during transient aortic occlusion. The degree of shortening of action potential duration on aortic occlusion decreased with decreases in the steady-state beat-to-beat interval (P = .0008). Control restitution curves had the typical configuration, with a rapid initial, usually monotonic, rise toward a plateau. Some curves showed a marginal "supernormal" section. Increased load reduced the action potential duration at the plateau of the restitution curve (9.4 ms, P < .0001) but increased the action potential duration at the start of the restitution curve (8.7 ms, P = .03). Increased loading increased the maximum slope of the electrical restitution curve by 32 ms/100 ms (P = .04). Increased load also increased the supernormal period of the electrical restitution curves. CONCLUSIONS: Mechanoelectric feedback produces changes in rate-dependent electrophysiology, which could favor a matrix conducive to arrhythmogenesis.

Electrical alternans and the onset of rate-induced pulsus alternans during acute regional ischaemia in the anaesthetised pig heart.

OBJECTIVES: Electrical alternans and mechanical alternans are both associated with cardiac ischaemia and in the case of electrical alternans there is a strong link with serious ventricular arrhythmia. We elected to investigate the relationship between electrical and mechanical alternans in control and acutely ischaemic myocardium in the intact porcine heart to determine the nature of their interaction and in particular to determine if abnormal mechanical events play a role in arhythmogenesis as has been suggested in non-ischaemic preparations. METHODS: We used rapid atrial pacing to induce regional mechanical alternans and pulsus alternans before and then at 5-min intervals after the onset of acute ischaemia induced by a 30-min ligation of a diagonal branch of the left anterior descending artery. Regional mechanical activity is measured with epicardial tripodal strain gauges and regional electrical activity is measured using suction-based monophasic action potential electrodes. To test whether alternate stretching of ischaemic segments during pulsus alternans contributed to electrical alternans we simulated pulsus alternans by clamping the proximal aorta on alternate beats. RESULTS: In control areas there was a constant discordant relationship between peak systolic pressure during alternans and action potential duration. In contrast, the ischaemic areas showed electromechanical alternans that was most frequently concordant. Clamping the proximal aorta on alternate beats produced an electrical alternans in control areas but not in the ischaemic area. CONCLUSIONS: Pulsus alternans during acute ischaemia is associated with electrical alternans that can be out of phase in control and ischaemic areas. This could increase electrical dispersion which may be pro-arrhythmic.

Sympathomimetic modulation of load-dependent changes in the action potential duration in the in situ porcine heart.

AIMS: Increased sympathetic stimulation is known to be arrhythmogenic. Likewise increased loading of the myocardium can directly generate arrhythmias. The interaction between the two on the electrophysiology of the myocardium has not been investigated before. We investigated the effect of dobutamine infusion on the shortening of the monophasic action potential duration secondary to increased loading. This was investigated during steady-state pacing and during an alteration in beat-to-beat interval in the form of a restitution curve. METHODS: Pigs were anaesthetised and their hearts exposed. Monophasic action potentials and segment lengths were recorded from the anterior surface of the left ventricle. The loading of the ventricle was increased by transiently occluding the aorta. Steady-state pacing and a restitution curve were performed. Recordings were taken before and during dobutamine infusion. RESULTS: At steady state, increased loading of the heart shortened the monophasic action potential duration by a mean (+/- s.e.m.) of 4.0 (+/- 0.5) ms (P < 0.001). During dobutamine infusion this shortening of the monophasic action potential increased. Shortening of the action potential duration increased with the dose of dobutamine up to 10 micrograms/kg/min after which a plateau was reached. By comparison to control, dobutamine depressed the electrical restitution curve at short test pulse intervals did not significantly alter the plateau. Increased loading elevated the initial section of the electrical restitution curve at short test pulse intervals and depressed the plateau in both the control recordings and those taken during dobutamine infusion. Increased loading increased the amplitude of the supernormal phase of the electrical restitution curve in control recordings and those taken during dobutamine infusion. Sympathetic stimulation by dobutamine during the steady state potentiates the effect of mechanoelectric feedback on the myocardium. The effect on the restitution curve varies with test pulse interval. At short test pulse intervals the effect of sympathetic stimulation dominates with only minor antagonistic modification by increased loading. However, at longer test pulse intervals the effect of mechanoelectric feedback is equal to that of sympathetic stimulation and is synergistic with it. CONCLUSIONS: The mechanically induced changes we describe in the normal pig heart in situ are relatively small. However, they are in the right direction to possibly contribute to arrhythmia under pathological conditions where mechanical as well as electrophysiological inhomogeneity is prominent.

Coronary artery fistulas presenting with bacterial endocarditis.

Coronary artery fistulas are rare congenital malformations. Two cases presenting with bacterial endocarditis are described. Both were treated successfully by grafting of the coronary artery and ligation of the fistula.

Regional alternans in relaxation and the onset of pulsus alternans in the heart of the anaesthetized pig.

1. The factors leading to the alternation in myocardial contractility believed primarily responsible for pulsus alternans are not known. We examine regional and global contraction patterns in the in situ heart at stimulation rates just below the threshold for pulsus alternans to determine if events occurring in the transition to alternans can give clues to cellular mechanisms. 2. Twelve pigs were anaesthetized, the chest wall removed and regional contraction measured in three areas of the left ventricle using tripodal strain gauges. We analysed regional and global dynamics during right atrial pacing at cycle lengths 50-150 ms greater than the threshold for pulsus alternans. 3. At pacing cycle lengths 50 ms greater than that required to produce pulsus alternans seven of twelve pigs showed alternans in the maximum rate of ventricular pressure decay but none showed alternans in the maximum rate of pressure rise. Pigs showing alternans in global relaxation were more likely to show alternans in regional contracility (P < 0.05). 4. Twenty-six of the thirty-six areas sampled showed alternans in end-diastolic length at pacing rates below the threshold for pulsus alternans. In fifteen of these areas alternation in end-diastolic length occurred in the absence of alternans in measures of contractility. 5. Alternans in global measures of relaxation may simply be a manifestation of regional alternans in contractility. It is therefore not appropriate, from global haemodynamic data, to suppose that alternans in relaxation is the primary abnormality in the generation of pulsus alternans.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional electromechanical alternans in anesthetized pig hearts: modulation by mechanoelectric feedback.

Electrical and mechanical alternans have often been found to coexist. However, the factors controlling their interdependence are not known. In this study we measure regional electrical and mechanical activity during mechanical alternans to investigate this relationship. Mechanical alternans was induced by rapid atrial pacing in 18 anesthetized, open-chest pigs. Regional segmental contraction and monophasic action potential were measured in three areas of left ventricle using epicardial tripodal strain gauges and suction electrodes. Electrical alternans always accompanied pulsus alternans. The phase of electrical alternans was not related to any measure of regional mechanical activity but did show a constant discordant relation to peak ventricular pressure. This suggested that mechanically dependent changes in action potential duration (mechanoelectric feedback) may be important in modulation electrical alternans. In support of this, pulsus alternans simulated by clamping the proximal aorta on alternate beats was associated with electrical alternans comparable to that produced with rapid atrial pacing. Mechanoelectric feedback modulates regional electrophysiology in the intact heart and may be important in the generation of electrical alternans.

Mechanically induced changes in action potential duration and left ventricular segment length in acute regional ischaemia in the in situ porcine heart.

OBJECTIVE: The electrophysiological events accompanying early ischaemia are important. The aim of this study was to investigate mechano-electric feedback in acute regional myocardial ischaemia in the intact heart in situ by measuring the change in action potential duration in response to increased ventricular loading imposed by transient aortic occlusion. METHODS: 11 landrace pigs were anaesthetised and their hearts exposed. A pneumatically operated blood pressure clamp was placed around the aorta. Monophasic action potentials and an index of segment motion were recorded from the epicardium in and around the ischaemic area produced by a snare placed around a coronary artery. Ventricular and systemic arterial pressures were measured. An initial aortic clamp was performed during which control recordings were taken. The coronary artery was then tied and the aorta clamped for 5-10 s every 5 min for the duration of the 30 min tie. Recordings were taken from the ischaemic area and non-ischaemic areas. RESULTS: Aortic clamp before ischaemia increased intraventricular diastolic and systolic pressure and reduced action potential duration in all the areas studied (p < 0.001). During acute regional myocardial ischaemia aortic clamping resulted in significantly more shortening of the action potential in the ischaemic area after 10 min of ischaemia than in the control area (5 ms v 10 ms, p = 0.008). Over the following 20 min the degree of shortening decreased. The greater shortening at 10 min could not be attributed to changes in the end diastolic segment length or peak ventricular pressure and could thus represent a change in the expression of mechano-electric feedback by ischaemic myocardium rather than a change in loading conditions. CONCLUSIONS: During the first 30 min following a coronary artery occlusion mechano-electric feedback in the ischaemic myocardium varies with time.

Medical management of endocarditis.

The treatment of infective endocarditis with antibiotics is one of the great medical success stories of this century, reducing mortality from 100% to as low as 10%. It is important for all medical and dental practitioners to be aware of endocarditis as the disease may be inadvertently caused by the treatment of other conditions.

Effects of gadolinium on length-dependent force in guinea-pig papillary muscle.

The degree to which stretch-activated channels operate during physiological length changes in multicellular heart preparations, or how much the channels could contribute to length-dependent activation, is not known. We studied the relationship between muscle length and contractile force in guinea-pig papillary muscles superfused with gadolinium chloride (10 microM), a stretch-activated channel blocker, and compared the effects to those with nifedipine (0.25 microM), a calcium channel blocker. Gadolinium reduced contractile force statistically significantly more at the longer muscle lengths than at the short muscle lengths. This did not apply with nifedipine, although a marginally greater effect at longer lengths was perceptible. The results can only partly be explained by gadolinium having a non-specific action via the calcium channel, or Na(+)-Ca2+ exchange, and are consistent with the possibility that stretch-activated channels contribute to length-dependent activation in cardiac muscle, and thus to 'Starling's Law of the Heart'.

Percutaneous transluminal coronary angioplasty of occluded coronary arteries. New angiographic features related to success.

Percutaneous transluminal coronary angioplasty of coronary artery occlusions forms a routine part of cardiological practice. However, only a few angiographic variables which might predict success have been investigated and identified. Video densitometry was used to assess structural features of the occlusion that have not been investigated previously. The sharpness and eccentricity of the tapering, the presence of any distal vessel and the presence of a reservoir or atrium in which the end of the wire could become trapped were measured. Other measurements taken from the cineangiogram included the minimum radius of curvature of the artery immediately before the occlusion, the number of bends and the distance traversed before the occlusion. The number, size, location and angle of branching of side branches from the artery in question were noted. The number of bridging collaterals and presence of distal opacification were noted. There was a significantly higher success rate in patients with less than 3 bends before the occlusion (p = 0.01) and a distance down the artery to the occlusion of less than 27 mm (p < 0.05). An atrium was present in 36% and was associated with an unsuccessful outcome (p = 0.02). Distal opacification, despite occlusion of the artery, was found to be associated with failure to cross the lesion (p < 0.05). The angle of branches of the artery was only found to be important near (within 7.5 mm) the occlusion (p < 0.05). None of the video densitometric measurements of the fine structure of the stump of the coronary artery were found to be helpful.

Characterisation of regional myocardial dynamics during mechanical alternans in heart of anaesthetised pig.

OBJECTIVE: The aim was to investigate the behaviour of regional myocardium during mechanical alternans in a multidirectional manner. METHODS: Mechanical alternans was induced in 12 anaesthetised open chested pigs by rapid atrial pacing. In contrast to previous studies, regional mechanical activity was simultaneously assessed at up to three different sites on the left ventricle using epicardial measuring devices able to provide multidirectional information on segment motion. Pressure-length loops were plotted to assess different patterns of segmental motion. The integral of pressure and length was calculated to obtain a regional work index for each beat. RESULTS: Pressure-length loops revealed profound abnormalities in segment motion and work index during regional mechanical alternans. Myocardial segments either performed alternate amounts of positive work on each beat or alternate amounts of positive and negative work on each beat. Alternating segments contracted out of phase with each other and were occasionally stretched during systole. CONCLUSIONS: The spatio-temporal heterogeneity of regional mechanical behaviour is greatly increased during mechanical alternans.

Following PTCA exercise tolerance is only moderately correlated with cinevideo-densitometric measurement of restenosis.

There is a pressing need for a non-invasive marker of restenosis following percutaneous transluminal coronary angioplasty. Computerised measurement of restenosis has not been used in conjunction with exercise test variables reflecting maximum exercise capacity. Thus we investigated thirty consecutive patients undergoing repeat coronary angiography for restenosis who had a satisfactory symptom limited exercise test. Exercise test variables namely ST segment depression, maximum exercise tolerance and rate/pressure product were correlated with the percentage stenosis measured by videodensitometry. Maximum exercise tolerance expressed in metabolic equivalents was correlated with restenosis severity (correlation coefficient -0.52, p = 0.003) as was maximum duration of exercise (correlation coefficient -0.46, p = 0.01). Maximum ST segment depression on exertion was not correlated with the degree of restenosis. In assessing patients after angioplasty exercise tolerance is related to the severity of restenosis. However the degree of variation within this relationship is such that it cannot be used confidently in the clinical setting to predict restenosis.

Infected right atrial thrombus--an important but rare complication of central venous lines.

A case of infected right atrial thrombus is described. Right atrial thrombus is difficult to diagnose and is associated with a high mortality. When it is additionally infected, antibiotics alone are not sufficient treatment and the thrombus should be surgically removed.

Efficacy of intravenous magnesium in acute myocardial infarction in reducing arrhythmias and mortality. Meta-analysis of magnesium in acute myocardial infarction.

BACKGROUND: To ascertain the effect of the intravenous administration of magnesium in acute myocardial infarction on the frequency of arrhythmias and mortality, a meta-analysis of randomized controlled trials was performed. METHODS AND RESULTS: The study included 930 patients with acute myocardial infarction admitted to primary referral hospitals. Administration of magnesium in acute myocardial infarction was associated with a 49% reduction in ventricular tachycardia and fibrillation. The incidence of cardiac arrest was reduced by 58%. The frequency of supraventricular tachycardias was also lower. Overall, there was a 54% reduction in mortality. CONCLUSIONS: Intravenous magnesium is a safe and effective method of reducing the frequency of arrhythmias and mortality in acute myocardial infarction.

Propionibacterium acnes causing an aortic root abscess.

A case of endocarditis caused by Propionibacterium acnes associated with an aortic root abscess is presented. This supports the current opinion that aortic root abscesses are not necessarily associated with microorganisms of high virulence.

A comparison of cardioversion of atrial fibrillation using oral amiodarone, intravenous amiodarone and DC cardioversion.

Fifty-two consecutive patients with atrial fibrillation underwent 86 episodes of attempted cardioversion with oral amiodarone, intravenous amiodarone or DC cardioversion. The presence of chronic obstructive pulmonary disease or a presenting heart rate of less than 110 beats per minute were associated with a favourable outcome. Conversion to sinus rhythm was achieved in 29% of the group treated with oral amiodarone, 42% of the group treated with DC cardioversion and 64% of the group given intravenous amiodarone. The overall statistical significance of this distribution on chi square testing was p < 0.032. However when only first attempts at cardioversion were analyzed there was no difference between intravenous amiodarone and DC cardioversion.