• SOM 4th Yr Curriculum

• CLERKSHIP INFORMATION
• Introduction / Goals and Objectives
• Rotations
  • HMC
  • UWMC
  • VAMC
• Seminars
• Faculty Only
• PULMONARY EDUCATION
• Pulmonary Med Library
• Arterial Blood Gases
• Cardiopulmonary Exercise Testing
• Mechanical Ventilation
• Pulmonary Function Testing
• Pleural Effusion
• Radiology

Mechanical Ventilation

Case 4 Answers

63 year-old woman was intubated four days ago for respiratory failure secondary to sepsis from a presumed pneumonia. She is on appropriate antibiotics, is now off pressors, and her WBC count has declined to the normal range. During your pre-rounding, you note that her F_IO₂ is down to 0.4 and she is on a PEEP of 5 cm H₂O. On these settings, the ABG shows pH 7.36, pCO₂ 46, PO₂ 75, HCO₃- 26. She has a weak cough and continues with copious secretions, requiring suctioning every 30 to 60 minutes.

At what point do you start considering whether your patient is ready to come off the ventilator?

There are several conditions that should be satisfied before you can consider separating your patient from the ventilator. First, the underlying process that put them on the ventilator should be better or improving. Second, the patient should be able to maintain adequate oxygenation on minimal support (eg. P_aO₂ > 80 mm Hg on an F_IO₂ of 0.5 and PEEP < 8.0 cm H₂O). Finally, the patient should be able to maintain an adequate acid-base balance without requiring high levels of minute ventilation (>12 liters/minute) in order to do so. If higher levels of minute ventilation are required, the ventilatory demands on the patient may be high and they are at risk for tiring out once off they are taken off the ventilator.

How do you determine if the patient is capable of being separated from the ventilator?

In the past, clinicians used to look at several different variables, referred to as “weaning parameters” in an effort to assess readiness for separation from the ventilator. For example, if a patient’s vital capacity was > 10 ml/kg, then the patient was likely to tolerate being off the ventilator. None of the different parameters that were used had perfect predictive ability and this approach has since been supplanted by a different strategy utilizing a trial of spontaneous breathing. If patients meet certain criteria on the respiratory therapist’s morning rounds, they are placed on CPAP, t-piece or a low level of pressure support and are monitored for a period of 30-120 minutes while they breathe on their own. An arterial blood gas is usually drawn at the end of this period. A successful trial is one in which the patient looks comfortable, maintains a good respiratory rate (< 25 breaths/minute), takes in sufficient tidal volumes (> 5 ml/kg) and maintains stable vital signs including heart rate, blood pressure and S_aO₂. The arterial blood gas should also show relatively stable oxygenation and no evidence of increasing P_aCO₂. Patients can fail the trial for a variety of different reasons including vital sign instability, worsening oxygenation, hypercarbia or other signs of insufficient ventilatory capacity. Patients who pass their trial of spontaneous breathing are deemed ready to be separated from the ventilator.

Suppose your patient demonstrates that she can be separated from the ventilator. Should she be extubated?

When a patient passes a spontaneous breathing trial, they are ready to be separated from the ventilator. In other words, they no longer need the ventilatory or oxygen support of the machine at their bedside. It is important to remember, however, that the decision to separate a patient from the ventilator is distinct from the decision to remove the endotracheal tube. Some patients can be separated from the ventilator but still require an endotracheal tube. In order to qualify for extubation, patients should be free of upper airway problems, should be able to protect against aspiration of gastric or oral contents and should be able to cough and clear secretions without a need for frequent suctioning. In this case, the patient has a weak cough and copious secretions, problems that would lead you to predict that she might decompensate if extubated. As a result, the endotracheal tube should remain in place until these issues resolve.

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