BRIDGEPORT, CONN—Spontaneous breathing trials (SBTs) are not always accurate in predicting which patients are ready to be extubated. A recent study suggests that the patient’s cough strength, as measured by peak expiratory flow (PEF), may help determine whether an endotracheal tube can be safely removed.[1] The same measurement can also help assess the patient’s risk of morbidity and mortality.

“The best way to determine whether a patient who is hemodynamically stable and adequately oxygenated still needs the ventilator is simply to turn off the ventilator and allow the patient to perform an SBT,” noted senior investigator Constantine A. Manthous, MD, an Associate Clinical Professor of Medicine at Yale School of Medicine. However, he added, “there have not been good predictors of whether patients still require the endotracheal tube after they’ve passed their SBT—that’s what we’ve been working on.”

This study’s results indicate that in patients who have passed an SBT, “cough peak flows greater than 60 L/min measured just before extubation are predictive of successful extubation,” said Dr. Manthous, who is also Director of Medical Intensive Care at Bridgeport Hospital. He added that the test is simple to perform. “As the patient is completing the SBT,” he explained, “you disconnect [the patient] from the ventilator, connect the endotracheal tube to a peak flow meter—with a filter in between—and ask the patient to cough maximally.”

PEF PREDICTS SUCCESS

The Bridgeport study included 95 ICU patients who were receiving mechanical ventilation through an endotracheal tube. A two- to three-minute SBT was performed once a patient’s ratio of arterial oxygen tension to fraction of inspired oxygen (Pao2/Fio2) exceeded 120. If the SBT was successful, the patient was asked to cough into a peak flow meter connected in-line via a filter to the endotracheal tube; the best of three coughs was recorded as the patient’s cough PEF. Cough PEFs ranged from 30 to 180 L/min; in four cases, PEFs could not be measured due to patients’ inability to comprehend instructions. A total of 115 separate extubation attempts were evaluated in the study.

Within 72 hours of 13 extubation attempts in 11 patients, reintubation was required because hypoxemia, stridor due to airway edema, acute hypercapnia, cardiac arrest, or failure to maintain a clear airway developed. If a patient did not require reintubation within 72 hours after endotracheal tube removal, the extubation was deemed successful.

Mean cough PEFs preceding unsuccessful extubations were significantly lower than those measured before successful extubations (64.2 vs 81.9 L/min). Receiver-operator analysis identified the optimum cut-off point for predicting extubation failure: A cough PEF of 60 L/min or less had a sensitivity of 69% and a specificity of 74%.

Overall, patients having PEFs of 60 L/min or less had a fivefold higher risk of failure than did the patients with higher PEFs. Furthermore, these patients had longer median hospital stays (18 vs 12 days), during which they were 19 times as likely to die as were the patients with higher PEFs.

WAITING TO EXTUBATE

When considering extubation for a patient who passes an SBT but has a cough PEF below 60 L/min, “proceed with caution,” Dr. Manthous recommended. He added, “If the patient has no secretions—so that a strong cough is not vital to preventing the patient from drowning—and has intact mental status, it would not be unreasonable to do a careful trial of extubation.” But, he stressed, “a weak cough appears to increase the risk of extubation failure.”

Ideally, the decision to attempt extubation would be based on a combination of identified risk factors. Said Dr. Manthous, “We are just completing a follow-up study that awaits peer review … in which cough flows greater than 60 L/min, the amount of secretions, and ability to complete four simple tasks—opening the eyes, following with the eyes, grasping with the hand, and sticking out the tongue—were predictive of extubation outcomes.”

—Mimi Zucker, PhD

Reference
1. Smina M, Salam A, Khamiees M, et al. Cough peak flows and extubation outcomes. Chest. 2003;124:262-268.

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