A 22-year-old medical student presents with recurrent episodes of sinus tachycardia (HR 115 bpm) triggered by stress. Vital signs show BP 128/76, RR 16, temp 37°C, SpO2 98%. ECG demonstrates normal P-wave morphology with shortened PR intervals. Troponin is normal. He denies palpitations at rest. After starting metoprolol, tachycardic episodes resolve. Which phase of the sinoatrial nodal action potential is most directly affected by decreased sympathetic stimulation?
- A)Phase 1 early repolarization in atrial cells
- B)Absolute refractory period of skeletal muscle
- C)Phase 0 depolarization in ventricular myocytes
- D)Slope of phase 4 spontaneous depolarization in pacemaker cellsGABARITO
- E)Plateau phase of Purkinje fibers
Explicação
Sympathetic stimulation increases the slope of phase 4 spontaneous depolarization in pacemaker cells, making the SA node fire more rapidly. Beta blockers blunt this effect and thereby slow the sinus rate. Ver explicação completa e trilha adaptativa →