Relative refractory period

However, there is a period of time following the absolute refractory period during which a premature electrical stimulus can be conducted, and is often conducted abnormally. This period of time is called the relative refractory period (Fig. 6-2).2 The relative refractory period corresponds roughly to the latter half of phase 3 repolarization on the action potential and to the latter half of the T wave on the ECG. If a new... 

O Ventricular tachycardia is usually initiated by a precisely timed VPD, occurring during the relative refractory period, which provokes reentry within ventricular tissue. 

Distinguish between the absolute refractory period and the relative refractory period... 

The effective refractory period is followed by a relative refractory period that lasts for the remaining 50 msec of the ventricular action potential. During this period, action potentials may be generated however, the myocardium is more difficult than normal to excite. 

Phase 3 The L-type Ca2+ channels close and K+ efflux now causes repolarization as seen before. The relative refractory period (RRP) occurs during phases 3 and 4. 

FIGURE 23-1 T The cardiac action potential recorded from a Purkinje cell. The effective refractory period is the time during which the cell cannot be depolarized, and the relative refractory period is the time in which a supranormal stimulus is required to depolarize the cell. Action potential phases [0-4] and the ionic basis for each phase are discussed in the text. From Keefe DLD, Kates RE, Harrison DC. New antiarrhythmic drugs their place in therapy. Drugs. 1981 22 363 with permission.]... 

Class III anti-arrhythmic drugs usually prolong the eardiae aetion, for instance amiodarone and bretylium tosylate. The latter depresses automaticity and increases the threshold with respect to fibrillation-inducing electrical stimulation in either normal or infarcted n ocardiunt It has also been found to enhance the functional refractory period thereby shortening the relative refractory period, thus the overall effect being to discourage re-entry. 

Besides the existence of a threshold for stimulation, excitable systems are characterized by the existence of a refractory period during which the response to further stimulation is reduced or even totally absent (Fitzhugh, 1961). As shown in fig. 5.33, the cAMP signalling system of D. discoideum shares this property. Represented in this figure is the ratio Aic/Aiii of the maxima of two successive peaks of intracellular cAMP, as a function of the time interval separating the second stimulus from the maximum of the first response, which defines time zero. For 4 min, no cAMP synthesis can be elicited by the second stimulus. This phase defines an absolute refractory period. Thereafter, the response to the second stimulus increases gradually, until the second maximum reaches the value of the first, after some 15 min. This second phase defines a relative refractory period. The two types of refractory period are known in other excitable systems, for example in nerve cells (Fitzhugh, 1961). 

Fig. 5.33. Absolute and relative refractory periods for relay of cAMP signals. The ratio of the maximum of the second peak of intracellular cAMP...

When applying these results to intercellular communication in cellular slime moulds, we need to remember that the amoebae relaying cAMP signals during aggregation are excitable. These cells therefore possess an absolute and a relative refractory period, but only the latter period characterizes the response of the system when self-amplification in cAMP synthesis is suppressed under conditions where the level of... 

The depolarization and repolarization phases of action events can occur quickly over intervals of tens of microseconds, although the actual durations depend very much on die cell type. During the time when the cell is depolarized, it cannot be restimulated to another action event. This interval is known as the cell s absolute refractory period. The cell s relative refractory period is the interval... 

Atrial Refractory Period The atrial refractory period (ARP) (expressed in milliseconds) is the interval following a paced or sensed atrial event during which the pacemaker ceases to be responsive to detectable atrial signals. It contains two parts the absolute refractory and relative refractory periods (Fig. 3.16). 

Fig. 3.16 Example of AAI pacing from Figure 3.1 with refiactory periods added. The ARP has an initial absolute refractory period (abs). No sensed events are noted in the marker channel during the absolute refractory period. During the remaining portion of the ARP, the relative refractory period (rel), sensed events may be noted, but the LRL will not be reset.

All signals are ignored during the absolute refractory period, and noise sampling occurs within the relative refractory period. The purpose of the ARP is to prevent oversensing of the output pulse stimulus afterpotential, the atrial electrogram produced by the atrial stimulus, or ventricular depolarization. 

Undersensing. Inappropriate delivery of pacing stimuli when the pacemaker system fails to sense P-waves or QRS complexes defines undersensing. Delivery of stimuli can be harmful if they occur during the atrial and ventricular relative refractory periods that are predisposed to tachyarrhythmia induction. Of particular concern is the induction of ventricular tachycardia or fibrillation when ventricular pacing occurs on the terminal portion of the T-wave ( R-on-T ), especially in the critical care setting where concomitant ischemia, metabolic and electrolyte abnormalities are frequently present. 

Synchronizing the electric charge with the R wave ensures that the current won t be delivered on the vulnerable T wave and disrupts repolarization. This reduces the risk that the current will strike during the relative refractory period of a cardiac cycle and induce VF. 

Anderson, R. J. Relative refractory period as a measure of peripheral nerve neurotoxicity. Toxicol Appl Pharmacol 1983, 71, 391-397. 

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