Скачать или смотреть Absolute Refractory Period & Relative Refractory Period 🫀CVS Physiology🫀 Dr G Bhanu Prakash

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Absolute & Relative Refractory Periods | USMLE Step 1 | CVS Physiology | Dr G Bhanu Prakash

This high-yield lecture on Absolute and Relative Refractory Periods in Cardiovascular Physiology is crafted specifically for USMLE Step 1 students aiming to master the electrophysiological properties of cardiac action potentials. These concepts are essential for understanding cardiac excitability, arrhythmias, and the effects of antiarrhythmic drugs, making them a common focus in Step 1 exam questions.

We begin by reviewing the fast-response action potential seen in cardiac myocytes and Purkinje fibers, which consists of five phases (0 to 4). During this action potential, the absolute refractory period (ARP) and the relative refractory period (RRP) play key roles in determining the timing of safe cardiac depolarizations and preventing tetanic contraction of cardiac muscle.

The absolute refractory period (ARP) 🛑 is the phase during which no new action potential can be initiated, no matter how strong the stimulus. It begins at the start of Phase 0 (rapid Na⁺ influx) and extends through Phases 1, 2, and early Phase 3. This period corresponds to the time when voltage-gated Na⁺ channels are either open or inactivated. Since the inactivated state cannot respond to depolarization, the heart is protected from premature contractions. This physiological safeguard ensures unidirectional conduction and helps prevent re-entry arrhythmias.

The relative refractory period (RRP) ⚠️ occurs during late Phase 3, when some Na⁺ channels begin to recover, but not all are fully reset. During this time, a stronger-than-normal stimulus can elicit another action potential — but the response will be weaker and slower, increasing the risk of abnormal conduction and ectopic activity. Clinically, this is the window where premature ventricular contractions (PVCs) or ventricular tachycardia may be triggered if stimuli occur inappropriately.

In Phase 4, the cell returns to its resting membrane potential, and all channels are reset. This marks the resting period, where a normal action potential can again be initiated by a normal threshold stimulus.

We also explain the differences in refractory periods between fast-response tissues (atria, ventricles, Purkinje fibers) and slow-response tissues (SA and AV nodes), and how antiarrhythmic drugs like class I (Na⁺ channel blockers) and class III (K⁺ channel blockers) alter refractory periods to suppress arrhythmias.

By the end of this session, you’ll clearly understand the timeline, channel states, and clinical relevance of refractory periods, preparing you to answer questions involving arrhythmia mechanisms, drug effects, pacemaker physiology, and ECG abnormalities on USMLE Step 1.

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