Which two mechanical events will occur as a result of the QRS wave on the EKG? (select two answers)

Question

BlackTom AI · Accepted Answer

The QRS complex represents ventricular depolarization, so it precedes and triggers the mechanical events of ventricular contraction.

1) Relevant concepts
- QRS = ventricular electrical depolarization.
- Electromechanical coupling: a short delay follows depolarization before contraction begins.
- Ventricular systole begins when ventricles contract and intraventricular pressure rises enough to close the AV valves (first heart sound, S1).
- Isovolumic (isovolumetric) contraction: the very early phase of systole when ventricles contract with all valves closed (AV valves closed, semilunar valves not yet open), so volume is constant while pressure rises.

2) Step-by-step reasoning
Step 1: QRS → ventricular myocytes depolarize.  
Step 2: After the electromechanical delay, the ventricles begin to contract. That contraction marks the start of ventricular systole.  
Step 3: As contraction begins, intraventricular pressure quickly rises, closing the AV valves. At that moment semilunar valves remain closed because pressure has not yet exceeded arterial pressures. This period with contracting ventricles and both sets of valves closed is the isovolumic contraction period. Thus both “start of ventricular systole” and “start of the period of isovolumic contraction” follow the QRS.

3) Calculations/intermediate steps
No numerical calculation is required; the temporal sequence is: QRS → electromechanical delay (~10–100 ms) → onset of ventricular contraction → AV valve closure (S1) → isovolumic contraction → semilunar valve opening → ejection.

4) Why the correct answers are correct
Both chosen events are the immediate mechanical consequences of ventricular depolarization: contraction begins (systole starts) and the initial contraction phase is isovolumic contraction.

5) Why the other options are incorrect
- Closure of the semilunar valves occurs at the end of systole (when ventricular pressure falls below arterial pressure), associated with ventricular repolarization/T wave, not the QRS.  
- End of isovolumic relaxation describes the transition from relaxation to filling (AV valves opening) later in diastole, not an event triggered by the QRS.

6) Conclusion
Therefore, the QRS wave leads to the start of ventricular systole and the start of the period of isovolumic contraction, which matches the provided correct answers.

Which two mechanical events will occur as a result of the QRS wave on the EKG? (select two answers)多项选择题

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