A 21-year-old man is brought to the emergency department after being found unresponsive in a public restroom. He has a history of injection drug use. Temperature is 36.1 C (97 F), blood pressure is 110/70 mm Hg, pulse is 62/min, and respiratory rate is 8/min. On physical examination, the patient is unconscious with pinpoint pupils. Endotracheal intubation is performed, and mechanical ventilation is begun. The ventilator settings are adjusted to provide a minute ventilation of 6.0 L/min. Calculated alveolar ventilation is 4.2 L/min. Which of the following most likely accounts for the difference between these 2 parameters?

Question

A 21-year-old man is brought to the emergency department after being found unresponsive in a public restroom.  He has a history of injection drug use.  Temperature is 36.1 C (97 F), blood pressure is 110/70 mm Hg, pulse is 62/min, and respiratory rate is 8/min.  On physical examination, the patient is unconscious with pinpoint pupils.  Endotracheal intubation is performed, and mechanical ventilation is begun.  The ventilator settings are adjusted to provide a minute ventilation of 6.0 L/min.  Calculated alveolar ventilation is 4.2 L/min.  Which of the following most likely accounts for the difference between these 2 parameters?

Quizplus · Accepted Answer

11ec160b_3cea_92f6_9249_bf968bc0e524_MD0001_00 Minute ventilation is the total volume of new air that enters the respiratory pathways per minute. It can be calculated by multiplying the average tidal volume by the number of breaths per minute (minute ventilation [L/min] = tidal volume [L] x breaths/min). Alveolar ventilation refers only to the volume of new air reaching the gas exchange areas (ie, alveoli and respiratory bronchioles) per minute. It can be calculated by subtracting dead space volume (which does not participate in gas exchange) from the tidal volume: Alveolar ventilation (L/min) = (tidal volume − dead space volume) × breaths/min The total volume of dead space in the lungs is known as physiologic dead space. It consists of both the anatomic dead space of the conducting airways (ie, nose, trachea, bronchi, bronchioles; normally ~150 mL) and alveolar dead space due to well-ventilated but poorly perfused alveoli. Because it is difficult to directly measure physiologic dead space, it is often estimated in mechanically ventilated patients by comparing arterial (a) and expiratory (E) pCO₂ levels: Physiologic dead space = tidal volume × ([P_aCO₂ − P_ECO₂]/P_aCO₂) (Choices B and C) Functional residual capacity is the volume of air remaining in the lungs after a normal exhalation. It is the sum of residual volume (the volume of air remaining in the lungs after maximal exhalation) and expiratory reserve volume (the volume difference between normal end-expiration and maximal end-expiration). (Choice D) Respiratory rate is the number of breaths taken per minute and is a factor in calculating both minute ventilation and alveolar ventilation. (Choice E) Tidal volume is the volume of new air that enters the lungs with each normal, unforced breath. It is a factor in calculating both minute ventilation and alveolar ventilation. Educational objective: Minute ventilation is equal to the product of tidal volume and respiratory rate and includes dead space ventilation. Alveolar ventilation is equal to the product of respiratory rate and the difference between tidal volume and dead space volume.

A 21-Year-Old Man Is Brought to the Emergency Department After