The importance of blood pressure in the emergence of arrhythmias

Diastolic (dys-)function and electrophysiology

Cross-talk between cardiac electrical and mechanical function is a bidirectional process: The origin and spread of electric excitation govern cardiac contraction and relaxation, while the mechanic environment provides feedback information to the heart's electric behavior. The latter tends to be unduly disregarded by the medical community. This article reviews experimental findings on the effects of diastolic mechanics on cardiac electrophysiology, and describes physiological correlates, clinical manifestations, and therapeutic utility of cardiac mechanic stimulation in humans.

Stretch-induced changes in heart rate and rhythm: clinical observations, experiments and mathematical models

Clinical and research data indicate that active and passive changes in the mechanical environment of the heart are capable of influencing both the initiation and the spread of cardiac excitation via pathways that are intrinsic to the heart. This direction of the cross-talk between cardiac electrical and mechanical activity is referred to as mechano-electric feedback (MEF). MEF is thought to be involved in the adjustment of heart rate to changes in mechanical load and would help to explain the precise beat-to-beat regulation of cardiac performance as it occurs even in the recently transplanted (and, thus, denervated) heart. Furthermore, there is clinical evidence that MEF may be involved in mechanical initiation of arrhythmias and fibrillation, as well as in the re-setting of disturbed heart rhythm by 'mechanical' first aid procedures. This review will outline the clinical relevance of cardiac MEF, describe cellular correlates to the responses observed in situ, and discuss the role that quantitative mathematical models may play in identifying the involvement of cardiac MEF in the regulation of heart rate and rhythm.

Atrial pressure and experimental atrial fibrillation

A possible profibrillatory effect on the atria of an elevated atrial pressure and the site of atrial stimulation was examined. In 15 anesthetized dogs, right or left atrial or biatrial pacing was applied at a high rate (300-600/min) for 5 seconds at double threshold intensity under a wide range of atrial pressures achieved by venous or arterial transfusion or bleeding. Induction of atrial fibrillation in 236 of 1,971 pacing runs was associated with a significantly higher (P < 0.001) atrial pressure (21.6 +/- 12.2 mmHg, mean +/- SD) than maintenance of sinus rhythm (16.8 +/- 11.1 mmHg in 1,735 of 1,971 pacing runs). Stimulation of the right atrium resulted in atrial fibrillation more frequently than left atrial or biatrial stimulation, with biatrial stimulation less frequent than right or left atrial stimulation. The induction of atrial fibrillation was related to the atrial pressure and to the site of stimulation but not to the pacing rate or the prepacing heart rate. The prepacing heart rate, associated with failure to induce sustained atrial fibrillation, was higher than that associated with atrial fibrillation in 12 of 15 experiments (significantly in 6) and not significantly lower in 3 of 15. Atrial fibrillation lasting 1 minute or more was more frequently associated with simultaneous stimulation of both atria than of either atrium alone. Thus, an elevated atrial pressure may facilitate the induction of atrial fibrillation. The site of stimulation also plays an important role for both the induction and maintenance of atrial fibrillation in this model.

Is ventricular wall stress rather than left ventricular hypertrophy an important contributory factor to sudden cardiac death?

Sudden cardiac death comprises a significant proportion of cardiac mortality in Western society. Left ventricular hypertrophy has been identified by many authors as a possible risk factor for sudden cardiac death, however, left ventricular hypertrophy develops in response to external stimuli on the heart as a means of normalizing wall stress. It is possible that the fundamental abnormalities in wall stress, rather than the left ventricular hypertrophy itself, pose the increased risk of sudden death. Left ventricular hypertrophy, the consequence of raised wall stress, is easy to measure and easy to study and it is understandable why this parameter should have received more attention. Wall stress by contrast is difficult to measure, and worse, is variable throughout the ventricle so that it cannot be measured in a single quantifiable figure. As a consequence, only a limited amount of attention has been paid to wall stress as a possible trigger mechanism for cardiac arrhythmia. However, there is evidence from both basic and clinical research to suggest that raised wall stress may be a risk factor for sudden cardiac death and cardiac arrhythmia. This review discusses the evidence for and against left ventricular hypertrophy and wall stress as risk factors for sudden cardiac death, and also presents recent evidence that left ventricular hypertrophy in isolation can protect the heart against the arrhythmogenic effects of raised wall stress.

Gap junction protein connexin40 is preferentially expressed in vascular endothelium and conductive bundles of rat myocardium and is increased under hypertensive conditions

Gap junction channels consisting of connexin protein mediate electrical coupling between cardiac cells. Expression of two connexins, connexin40 (Cx40) and connexin43 (Cx43), has been studied in ventricular myocytes from normal and hypertensive rats. Polyclonal affinity-purified rabbit antibodies to Cx43 and Cx40 have been used for immunohistochemical analysis on frozen sections from rat heart. These studies revealed coexpression of Cx43 and Cx40 in ventricular myocytes. In addition, Cx40 is preferentially expressed in three distinct regions: first, in the endothelial layer of the heart blood vessels but not in the smooth muscle layer of the arteries; second, in the ventricular conductive myocardium, particularly in the atrioventricular bundle and bundle branches, where Cx43 is not observed; and third, in the myocyte layers close to the ventricular cavities. These results suggest that Cx40 is preferentially expressed in the fast conducting areas of myocardial tissue. Expression of both Cx40 and Cx43 was also found in immunoblots from normal and hypertensive rat myocardiocytes. Under hypertensive conditions (ie, in spontaneous hypertensive rats and in transgenic rats that exhibit hypertension due to expression of an exogenous renin gene), we found a 3.1-fold increase in Cx40 expression, compared with normal myocardium. Furthermore, we detected a 3.3-fold decrease in Cx43 protein level in transgenic hypertensive rats. The coexpression of Cx40 and Cx43 proteins in rat myocytes, their spatial distribution, and the increased amount of Cx40 protein during cardiac hypertrophy suggest that Cx40 may be involved in mediating fast conduction under normal and pathological conditions. The increased expression of Cx40 in hypertrophic heart may be a compensatory mechanism to increase conduction velocity.

Cardiac dysrhythmias during cardiovascular autonomic reflex tests

To assess the possible dysrhythmogenic effect of cardiovascular autonomic function tests, ECG tracings of 925 consecutive subjects, taken during a battery of cardiovascular autonomic reflex tests were analyzed. The battery included the Valsalva manoeuvre, deep breathing test, orthostatic and isometric handgrip. The frequency of ventricular extrasystoles increased during or after the tests, compared with the resting phase, in 11% of healthy subjects, in 11% of diabetic subjects and in 23% of subjects with a previous myocardial infarction (p = 0.001 vs healthy subjects). In patients with previous myocardial infarction, the most dysrhythmogenic individual tests were orthostatic and isometric handgrip. In nine subjects, other cardiac rhythm disturbances were detected (including nonsustained ventricular tachycardia, conduction block, and atrial fibrillation). In all cases, the dysrhythmias were asymptomatic and resolved without medical intervention. In conclusion, we consider the cardiovascular reflex test battery safe for the patient. However, due to occasional potentially significant dysrhythmias we recommend continuous monitoring of the electrocardiogram and immediate access to resuscitation facilities during cardiovascular autonomic testing.

Pressure-related ventricular tachycardia

A case with syncope on exertion and paced heart block is presented. Non-sustained ventricular tachycardia was seen on Holter monitoring and reproduced repeatedly by either exercise or an injection of an alpha agonist, but not with provocative electrophysiology. Antihypertensive treatment using a beta-blocker with endogenous sympathomimetic activity prevented recurrences. It is suggested that this is a case of pressure-related tachycardia.