Item 2 of 2
During evaluation, the patient becomes unresponsive and develops cardiac arrest with pulseless electrical activity. Cardiopulmonary resuscitation and intubation are performed, and the patient is placed on a mechanical ventilator. Once stabilized, he is admitted to the intensive care unit, and appropriate treatment is administered. Several days later, a spontaneous-breathing trial is performed to determine readiness to wean off the ventilator. After the mechanical ventilator settings are adjusted, the following changes are noted as reflected on the graph below. Dead space ventilation is shaded in black.
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Which of the following breathing patterns most likely contributed to this patient's change from the left bar to the right on the graph above?
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Physiologic (total) dead space includes the anatomic dead space (the permanent volume of dead space within the conducting airways) plus the alveolar dead space (the volume of air in the respiratory zone that does not participate in gas exchange). Increases in physiologic dead space occur in many lung diseases, including pulmonary embolism, emphysema, and acute respiratory distress syndrome.
Dead space volume remains relatively constant on a minute-by-minute basis. However, lower tidal volumes increase the proportion of each breath composed of dead space. If minute ventilation (tidal volume x respiratory rate) is unchanged, the consequence of the low-tidal-volumes is an increase in dead space ventilation (inefficient breathing).
Patients undergoing mechanical ventilation weaning typically have weakened respiratory muscles and therefore tend to breathe at low tidal volumes to minimize the work of breathing. This hypoventilation triggers an increase in respiratory drive that increases respiratory rate to maintain minute ventilation. When patients are being weaned off the ventilator, the ratio of their respiratory rate/tidal volume, known as the rapid shallow breathing index (RSBI), is measured; a low RSBI indicates relatively high tidal volumes, relatively efficient breathing, and a lower likelihood of recurrent respiratory failure once ventilatory support is discontinued.
(Choice A) Increases in both respiratory rate and tidal volume result in increased minute ventilation. However, the graph shows that minute ventilation remains the same after ventilator adjustment.
(Choice C) A decrease in respiratory rate and an increase in tidal volume would keep minute ventilation constant; however, the total volume of dead-space air breathed each minute would decrease rather than increase.
(Choice D) Decreases in both respiratory rate and tidal volume would result in decreased, rather than constant, minute ventilation.
Educational objective:
Patients being weaned from mechanical ventilation typically breathe at low tidal volumes, with a compensatory increase in respiratory rate to maintain minute ventilation. Because at low tidal volumes a higher proportion of each breath is composed of dead space, this type of breathing leads to an increase in wasted ventilation (inefficient breathing).