med 610 clinical respiratory diseases & critcare med

Mechanical Ventilation

Case 6 Answers

A 65 year-old woman is intubated emergently for a severe COPD exacerbation. She underwent a rapid sequence intubation using succinylcholine for paralysis and etomidate for sedation. Shortly after intubation, she becomes hypotensive with her blood pressure dropping from 145/85 prior to intubation to 95/60 post-intubation. On exam, she has a very prolonged expiratory phase and diffuse wheezing.

What is the differential diagnosis for this patient’s hypotension?

This patient decompensated immediately following intubation, a situation with a short differential diagnosis. Potential diagnoses include: esophageal intubation (the endotracheal tube can become dislodged after an initial correct placement and slide into the esophagus); mainstem bronchus intubation; problems with the ventilator circuit, tension pneumothorax, blood pressure effects of sedative medications; severe auto-PEEP (air-trapping and hyperinflation in patients with airflow obstruction leading to increased intrathoracic pressure, decreased venous return and impaired cardiac output); decreased venous return due to positive pressure ventilation and/or volume depleted state in a patient with a weak, pre-load dependent right heart (eg. severe pulmonary hypertension).

What can you do to sort through this differential and identify the etiology of the problem?

A thorough physical exam may reveal the source of the problem. Unilateral absence of breath sounds and elevated neck veins are suggestive of tension pneumothorax while unilateral absence of breath sounds, normal neck veins and tracheal deviation toward the quiet side of the chest suggest mainstem intubation. Worsening oxygen saturation and decreased/absent breath sounds bilaterally suggests esophageal intubation. A chest x-ray may also reveal the presence of a pneumothorax or mainstem intubation while a review of the medication record may point to a role for the sedative medications. To look for the presence of auto-PEEP, if the person is not making spontaneous respiratory efforts you can perform an expiratory pause of the ventilator and examine the difference between the total PEEP and the set-PEEP. On ventilators with graphical displays, you can also examine the flow versus time curves. In normal patients, expiratory flow returns to zero prior to the next breath. The patient is able to exhale fully and there is no longer expiratory airflow by the time the next breath is due. In patients with auto-PEEP, expiratory flow does not return to the zero line before the next breath is delivered. As a result of the airflow obstruction, expiratory airflow is slowed and continues up until the next breath is delivered. Observing this expiratory airflow pattern on the graphic display confirms the presence of auto-PEEP but does not tell you the magnitude of the problem.

How should you manage the most likely source of the problem?

Given that the patient was intubated for a severe COPD exacerbation and likely has a lot of airflow obstruction, the most likely explanation for her hypotension is auto-PEEP. This is a problem commonly seen in patients with severe asthma or COPD in which they trap air on exhalation and become progressively hyperinflated. This hyperinflation leads to increased intrathoracic pressure, which decreases venous return, impairs cardiac output and, as a result, leads to decreased blood pressure. Patients can even go into cardiac arrest (usually a pulseless electrical activity arrest) from this phenomenon. Several strategies are used to prevent or manage this problem. First, the minute ventilation can be decreased by lowering the respiratory rate or tidal volume. If you put less air into the lungs each minute, the patient has to exhale less air and, therefore, there is less potential for air-trapping. Second, you can provide more time for the patient to exhale. This is done by increasing the flow rate on inhalation and thereby decreasing the ratio of inspiratory time to expiratory time (I:E ratio). Finally, you can use bronchodilators and steroids to facilitate bronchodilation, decrease airway inflammation and promote exhalation. If a patient becomes bradycardic or pulseless, you should disconnect them from the ventilator and let their chest deflate as the trapped air escapes.

A key aspect of managing auto-PEEP is anticipating situations in which it might occur. Patients intubated during severe asthma and COPD exacerbations are prime candidates for this problem and you should always be on the alert for the problem in these situations.

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