Excessively high heart rate ( 170) can ? - PGIMER Sangrur NO - 2023

PGIMER Sangrur NO - 2023

Applied Physiology

Hard

Appeared in: PGIMER Sangrur NO - 2023

Explanation

Cardiac output is the product of heart rate and stroke volume (CO = HR x SV).
An excessively high heart rate (>170 bpm) drastically reduces the diastolic filling time of the ventricles.
This inadequate filling time leads to a lower preload and a subsequent decrease in stroke volume.
The significant drop in stroke volume is greater than the increase in heart rate, resulting in a net decrease in overall cardiac output.

Option A: This is incorrect because at excessively high rates, the heart's pumping efficiency decreases. There isn't enough time for the ventricles to fill, so less blood is pumped with each beat, causing overall output to fall.
Option C: This is the opposite of what happens. Preload is the volume of blood in the ventricles at the end of diastole (filling). Because the filling time is so short, the ventricles don't fill completely, leading to a decreased preload.
Option D: Afterload is the resistance the heart must pump against. The primary effect of a very high heart rate is on filling (preload), not directly on the resistance in the arteries.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Relationship between heart rate and cardiac output as background academic context rather than a clinical decision trigger.
Nurses must recognize that patients with tachyarrhythmias (like SVT or VT) are at high risk for hemodynamic instability due to decreased cardiac output.
Key nursing assessments include monitoring for hypotension, dizziness, syncope, shortness of breath, and altered mental status, which are all signs of poor perfusion.
What if the heart rate was moderately high, around 130 bpm, in a patient with a fever? In this scenario, the increased heart rate is a compensatory mechanism, and it would likely increase cardiac output to meet the body's higher metabolic demands. The principle changes based on whether the rate is physiologic or pathologically excessive.

How to Approach the Question

First, identify the keywords in the question: 'Excessively high heart rate (>170)'. The word 'excessively' is a critical qualifier.
Recall the fundamental formula for cardiac output: Cardiac Output = Heart Rate × Stroke Volume.
Think about the two components. While the Heart Rate is very high, consider how this extreme rate affects the Stroke Volume.
Reason that a very fast rate leaves very little time for the heart's chambers (ventricles) to fill with blood between beats. This phase is called diastole.
Conclude that if the ventricles don't fill properly, the stroke volume (the amount of blood pumped per beat) must decrease.
Finally, weigh the two factors. The drop in stroke volume is more significant than the increase in rate, leading to an overall decrease in cardiac output.

Concept Tested & Keywords

Concept Tested: Relationship between heart rate and cardiac output
Stem keywords: Excessively high heart rate, >170
Lead-in keywords: can
Clinical cues: The term 'excessively' is key, indicating a pathological rather than physiological rate.

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