Ventricular rate patterns in atrial fibrillation.

ATRIAL fibrillation has long been the intriguing subject of much speculation and investigation. In general, scientific inquiry has been addressed to two major problem areas: First, what abnormal conditions affecting the basic electrophysiologic state of the atria are responsible for the inception and perpetuation of fibrillation? Second, what factors govern the response of the atrioven-tricular transmission system and the ventricles to the showers of impulses arriving from above? A corollary question, relating to both the atrial and ventricular manifestations of the arrhythmia, asks whether the electrocardio-graphic signals are indeed completely irregular as had so long been supposed, or whether they may not in fact contain at least a modicum of intrinsic rhythmicity buried within their seemingly random temporal sequences. Recent observations on this perplexing question are described in the paper by Bootsma and associates which appears in this issue of CIRCULATION. The problem of rhythmicity in atrial fibrilla-tion appears to have commanded increasing attention over the past decade. In retrospect, this growth of interest is not surprising, being undoubtedly related to the progressive application of the electronic digital computer to existing problems in electrocardiography. These devices, especially in the form of so-called laboratory-oriented configurations, are readily capable of converting many minutes worth of the electrocardiographic signal into long strings of discrete, quantified pieces of sequential information. These digitized values may then be subjected to a battery of numerical procedures which are intended to test a variety of hypotheses. With such powerful analytic means at hand, one might expect clear and prompt answers to the question of rhythmicity in atrial fibrillation. Such has not been the case. A survey of the pertinent literature provides impressive evidence for entirely random behavior , on the one hand, and convincing support for intrinsic rhythmicity, on the other. The clinician is understandably confused by this contradictory evidence. In the case of the fibrillating atria themselves, the basic mechanism would seem most likely to be the result of a random process, and indeed a computer model of atrial fibrillation based on the random assignment of electrophysiologic parameters reproduces the rhythmic vagaries of the disorder in a highly realistic manner.' Yet, it is a common observation that the amplitude of the atrial deflections in fibrillation waxes and wanes intermittently, and during the periods of recrudescence the undulatory signal is stable enough to assign it a meaningful value of frequency. Rather interestingly, when the computer is programmed to …