Systolic blood pressure slightly falls during normal inspiration due to?

WCL Staff Nurse - 2019

Applied Physiology

Hard

Appeared in: WCL Staff Nurse - 2019

Explanation

During inspiration, the fall in intrathoracic pressure increases venous return to the right side of the heart.
This leads to increased blood volume in the pulmonary circulation (increased capacitance), which transiently reduces the amount of blood returning to the left ventricle.
Reduced left ventricular filling (preload) leads to a decreased stroke volume and a slight fall in systolic blood pressure.
This phenomenon is a normal physiological response to the mechanics of breathing.

Option A: Pulmonary compliance relates to the stretchability of lung tissue. While it's essential for breathing, it is not the direct hemodynamic cause of the blood pressure drop during inspiration.
Option B: This is the opposite of what happens. The pooling of blood in the expanded pulmonary circulation during inspiration leads to a transient decrease, not an increase, in left ventricular filling.
Option D: An increase in systemic vascular resistance (SVR) would cause blood pressure to rise, not fall. SVR might increase as a reflex response to a significant drop in blood pressure, but it is not the initial cause of the fall during inspiration.

Clinical Relevance

Nursing practice connection: Knowing Physiology of blood pressure changes during respiration helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Understanding this normal physiology is crucial for identifying its exaggeration, known as pulsus paradoxus (a drop >10 mmHg), which is a critical sign of conditions like cardiac tamponade, constrictive pericarditis, and severe obstructive lung disease.
A nurse measuring blood pressure should be aware that minor fluctuations with respiration are normal. However, if a large drop is suspected, they should measure it carefully by inflating the cuff above systolic pressure and slowly deflating it, noting the pressure at which Korotkoff sounds are first heard (only during expiration) and then the pressure at which they are heard throughout the respiratory cycle. The difference is the pulsus paradoxus.
What if? If a patient on a ventilator with positive pressure ventilation is observed, the opposite effect occurs. The positive pressure during the inspiratory phase impedes venous return to the right heart, which can decrease cardiac output and blood pressure. This is a key consideration in managing hemodynamically unstable patients on mechanical ventilation.

How to Approach the Question

First, identify the core physiological process in the question: the effect of normal inspiration on systolic blood pressure.
Recall or deduce the mechanical and hemodynamic changes that occur during inspiration. Think about pressures in the chest cavity and how they affect blood flow.
Trace the path of blood: inspiration causes negative thoracic pressure, which pulls blood into the right heart. This 'extra' blood has to go through the lungs before reaching the left heart.
Consider the consequence of this pathway: the lungs' vessels expand to hold this blood, causing a slight delay or reduction in blood flow to the left ventricle.
Evaluate each option based on this physiological sequence. The correct option will accurately describe a step in this chain of events.
Eliminate options that describe the opposite effect (e.g., increased LV filling) or are unrelated or compensatory mechanisms (e.g., increased SVR).

Concept Tested & Keywords

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E6 Medicine Davidson Principles Practice 24e p. 467-469