Certified Respiratory Therapist Practice Exam 2025 – Complete Test Prep Guide

A bedside vital capacity of 400 mL in a 50-kg patient indicates significantly reduced lung capacity, which can lead to the conclusion that the patient cannot sustain prolonged spontaneous ventilation. Vital capacity is a measurement of the maximum amount of air a person can exhale after taking the deepest breath possible. For an adult, normal vital capacity values typically range between 3,000 mL to 5,000 mL, depending on various factors such as size, age, and sex.

Given that the patient's vital capacity is only 400 mL, this is far below the normal range and suggests compromised respiratory function. When a patient has such a low vital capacity, it severely limits their ability to effectively ventilate on their own over extended periods. Spontaneous ventilation relies on the capability of the respiratory muscles to move adequate volumes of air in and out of the lungs. A vital capacity as low as 400 mL would indicate insufficient muscle strength or lung volume to maintain adequate gas exchange during prolonged spontaneous ventilation.

Therefore, it is reasonable to conclude that this patient is likely unable to sustain prolonged spontaneous ventilation due to the inadequate lung capacity represented by the measured vital capacity.