Question

Mr. C., age 68 with a history of COPD, presented in the emergency department 2 days ago febrile with a productive cough of large amounts of purulent sputum and in acute respiratory failure. He was diagnosed with community-acquired pneumonia and treated with antibiotics, hydration, aggressive pulmonary hygiene, and supplemental oxygen therapy. Although his oxygenation improved, he continued to be diaphoretic, using accessory muscles of respiration and complaining, I am exhausted, and I can’t get enough air. Arterial blood gas revealed: pH 7.31, PaCO2 59 mm Hg, PaO2 89 mm Hg, SaO2 91%, HCO329 mEq/L. The decision was made to intubate and place him on mechanical ventilation with the following settings: assist control (AC) mode, tidal volume (VT) 625 mL, respiratory rate 16 breaths/min, FiO2 70%, and 5 cm H2O of positive end-expiratory pressure (PEEP). He was transferred to the critical care unit.

Two days later, his presentation at 0500 is as follows:

• Ventilator settings: Mode AC, VT 625 mL, respiratory rate 12, FiO2 0.35, 5 cm H2O PEEP

• Total RR 16, peak inspiratory pressure (PIP) 22 mm Hg

• Moderate amounts sputum, rhonchi that clear with coughing, chest x-ray shows clearing pneumonia

• Negative inspiratory force -37 cm H2O, spontaneous tidal volume 5 mL/kg, vital capacity (VC) 10 mL/kg, minute ventilation 10 L/min, rapid shallow breathing index of 37

• Temperature 99.0° F oral, heart rate 86 beats/min, blood pressure 132/84 mm Hg

• No vasopressor or inotropic agents, urinary output is good, NPO since admission.

• Alert and oriented to person, place and situation, hypervigilant and restless, has not slept well since admission

The team decides to perform a ventilator weaning trial. At 0620 Mr. C. is placed on 10 cm H2O of pressure support with a FiO2 40%. Assessment at 0720 is as follows:

• Heart rate 108 beats/min

• Blood pressure 157/90 mm Hg

• Respiratory rate 36 breaths/min and labored

• SpO2 90%

He is diaphoretic and alternates between picking at his gown and falling asleep and needing to be aroused to stimulate breathing. He is placed back on the ventilator at the previous settings.
Late that morning, during rounds, Mr. C. is started on nutritional support via tube feeding and given trazodone at night for sleep. He is allowed a morning nap and has physical therapy that afternoon.

The following day he passes the pre-wean screening and is again placed on PS 5 above 5 cm H2O PEEP. Assessment findings 30 minutes into the weaning trial are as follows:

• Heart rate 84 beats/min

• Blood pressure 120/76 mm Hg

• Respiratory rate 18 breaths/min, unlabored

• SpO2 96%

Mr. C. is calm, cooperative, and oriented, so the weaning trial is continued for 90 minutes. Arterial blood gas results were pH 7.34, PaCO2 48 mm Hg, PaO274 mm Hg, HCO3 24 mEq/L, and SaO2 95%. The decision was made to extubate, and the patient was discharged from the critical care unit the following day.

Questions:

1.​What were the results of Mr. C.’s first arterial blood gas analysis? What factors contribute to these results?

2.​What other parameters should/could be assessed as part of the initial wean screen?

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3.​Why is pressure support the mode used for the weaning trial?

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4.​How long does a weaning the trial last?

5.​Is Mr. C. tolerating weaning, and how do you know?

Answer 1

1. The patient's ABG shows respiratory acidosis with partial compensation. He is a known case of COPD which has exacerbated due to the lung infection. This has led to inadequate ventilation and CO2 returntion leading to respiratory acidosis.

2. Screening for weaning should ensure adequate GCS, low FiO2 requirement (<0.5), resolving lung condition, presence of spontaneous breaths, hemodynamic stability.

3. Pressure support will provide additional support to patients own efforts at breathing. Eg: Suppose the patient is spontaneously breathing but is able to generate only enough pressure to obtain a tidal volume of 300 ml. The patient will compensate by increasing respiratory rate. But eventually he will suffer from respiratory fatigue and may need reinitiation of mechanical ventilation. But if the ventillao gives support on top of the patient's own attempts, the patient won't have respiratory fatigue and his efforts will improve over time and there is greater chance of successful weaning.

4. Typically weaning trial lasts from 30 mins to 2 hours.

5. He is tolerating weaning. The respiratory rate is adequate but not tachypneic. There is no sympathetic overactivity- normal heart rate and BP. And he is maintaining adequate saturation.