Question 2 of 30

A 72-year-old, 70-kg man presents in respiratory distress. He is barrel-chested and thin, with marked accessory muscle use and retractions. He complains, through pursed lips, that he cannot catch his breath and his inhaler does not help. Admission blood gases: pH 7.28, Pao2 45 mm Hg, Paco2 70 mm Hg. Despite therapy his mental status and respiratory rate deteriorate, and you intubate him. Chest x-ray confirms proper tube placement and no other acute problem. After 5 minutes of manual ventilation, the most appropriate initial ventilator settings are:

Pressure-cycled ventilator, Fio2 60%, assist-control rate 18, tidal volume 600 mL.

Volume-cycled ventilator, Fio2 70%, intermittent mandatory ventilation (IMV) rate 4, tidal volume 600 mL.

Volume-cycled ventilator, Fio2 70%, PEEP 5 cm H2O, assist-control rate 20, tidal volume 600 mL.

Volume-cycled ventilator, Fio2 80%, assist-control rate 10, tidal volume 450 mL.

Volume-cycled ventilator, Fio2 100%, IMV rate 20, tidal volume 450 mL.

Although you should make an effort to avoid intubating the patient with decompensated COPD, the deteriorating mental status and respiratory rate indicates worsening hypoxia, increasing hypercarbia, and fatigue. This patient has a chronic respiratory acidosis with a compensatory metabolic alkalosis, since a Paco2 of 65 would otherwise result in a pH of 7.16 (for every 10 mm Hg change in CO2 from 40 mm Hg, pH changes by 0.08). If the patient is hyperventilated to a “normal” Paco2 he will become alkalotic (pH 7.52). Alkalosis shifts the oxygen dissociation curve to the left, decreases O2 delivery, and may result in hypotension, arrhythmias, and seizures. A rule of thumb is that total minute ventilation for an adult is normally 10 L/min, and alveolar ventilation approximately 5 L/min. Dead-space ventilation for the average intubated patient is 50% of the total minute ventilation, or approximately 5 L/min (physiologic dead space + ventilator tubing). In general, doubling the minute ventilation halves the Paco2. Observations of the adverse effects of barotrauma and volutrauma have led to recommendations of lower tidal volumes than in years past (tidal volume = 5.0–10 mL/kg). An initial tidal volume of 5.0–8.0 mL/kg is indicated in the presence of obstructive airway disease and acute respiratory distress syndrome (ARDS). The goal is to adjust the TV so that plateau pressures are less than 35 cm H2O. Assist control for the fatigued patient is probably preferable to IMV in the emergency department, although some patients may hyperventilate and require sedation. The initial Fio2 should be high, but prolonged 100% O2 may promote atelectasis, so a lower concentration should be used after resuscitation and adjusted according to blood gas results. Pressure-cycled ventilators and PEEP is not often indicated in the patient with decompensated COPD.

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