Ventilation for Acute Hypoxic Respiratory Failure

Significant reduction in intubation rate.^11–16NPPV = noninvasive positive-pressure ventilation. Reprinted with permission from Intensive Care Med.¹⁷

NIV and Acute Hypoxic Respiratory Failure (AHRF): Post extubation Respiratory Failure

Given that the use of NIV avoids intubation and decreases the number of nosocomial infections, length of ventilation, hospital LOS, and overall costs, should every patient receive NIV? What about using NIV for post-extubation respiratory failure?

Estaban and his collegues⁸ conducted a large, multicenter, multinational, randomized controlled trial, to determine the effects of NIV on post extubation respiratory failure. Respiratory failure was defined as having 2 or more of the following: pH of <7.35 plus a PaCO₂ >45 mm Hg, a respiratory rate >25 breaths/min for at least 2 hours, hypoxemia with Sao₂ <90% or PaO₂ <80 mm Hg on FIO₂ >50%, clinical signs of respiratory muscle fatigue, or increased respiratory effort. The criteria for reintubation were no decrease in respiratory acidosis, declining mental status, refractory hypoxemia, hypotension refractory to treatment with fluids and vasopressors, the inability to clear secretions, and clinical signs of fatigue.

Patients who had been mechanically ventilated for greater than 48 and had been electively extubated after passing a 2 hour spontaneous breathing trial exhibiting the preset criteria of respiratory failure and meeting criteria for re-intubation were included. Patients were randomized to receive either medical therapy as directed by their treating physician or medical therapy as directed by their treating physician with the addition of NIV. Crossover from standard therapy to NIV was allowed but was considered failure of standard therapy. Patients in the NIV group were managed with a standard ICU ventilator with an oronasla mask interface. The initial settings were a pressure support set to deliver a tidal volume of greater than 5 mL/kg and a respiratory rate to assure 25 breaths per min total. Subsequent settings were at the discretion of the treating physician.

Of the 908 patients enrolled, 244 developed respiratory failure within 48 hours, 23 required emergent reintubation. Therefore, 221 were randomized. There were no significant differences in severity of illness at baseline, time of extubation, or time of randomization. The results showed no difference in the re-intubation rates for the NIV group vs the medical therapy group (48% vs 51%), The median time to reintubation in the NIV group was 12 hours, compared with 2.5 hours for the standard therapy group. However, mortality for the NIV group was 25%, compared with 14% for the medical therapy group. This trial was terminated early by the data safety monitoring board because of the significant mortality increase in the reintubated NIV patients. The reason for the increased mortality was unclear. It may have been related to the delay in inevitable reintubation. The patients may have needed to be intubated earlier, had a time delay that created excessive exhaustion for them, or had aspirations that were caused by the unprotected airway.

Nava and collegues⁹ also sought to determine the use of NIV in post extubation respiratory failure. They conducted a multicenter, multinational, randomized controlled trial comparing NIV used immediately post extubation to standard medical care. Endpoints included need for reintubation, tie spent in intensive care and in hospital, as well as intensive care unit and hospital mortality.

There were 97 patients who had been mechanically ventilated for greater than 48 hours who were considered at risk of developing post-extubation respiratory failure (ie, they had congestive heart failure, previous respiratory failure, comorbid conditions, weak cough, and an increase in Pco₂) enrolled. After a successful weaning trial, the patients were randomized to receive NIV for >8 hours a day in the first 48 hours or standard medical therapy. Both groups had similar at baseline characteristics. Compared with the standard medical therapy group, the NIV group had a 12% lower mortality rate, a 16% lower reintubation rate, and a decreased LOS. However, this study also showed reintubation to be associated with an increase in ICU mortality (60%).

NIV Failure Predictors

If respiratory failure increases the risk of mortality, then it is important to know which patients are going to experience respiratory failure. The question is whether or not that can be determined.

Rana et al¹⁰ performed an observational cohort outcome study to assess patients with acute lung injury (ALI) initially treated with NIV and to identify specific risk factors for NIV failure. All patients meeting ALI criteria and who were treated with NIV between March and October 2004 at ICUs in 2 tertiary-care institutions were included. Data on demographics, APACHE III scores, degree of hypoxemia, and ALI risk factors were recorded. Univariate and multivariate regression analyses were performed to identify patients at risk for NIV failure. Fifty four NIV patients were enrolled. Of the fifty four, nineteen had a secondary diagnosis of shock. NIV failed in 38 of the 54 patients. NIV failed in all nineteen of the patients with shock. Shock was the most significant predictor followed by the APACHE III score (Table 1). Other failure predictors were the Sequential Organ Failure Assessment score, the ratio of PaO₂ to FIO₂, and base excess. It is important to note that in this study the mortality rate for patients who failed NIV was greater than the APACHE III score had predicted (68% vs. 39%) but for those successfully supported with NIV, the mortality rate was zero even though the APACHE III predicted mortality approached 21%.

Table 1. Failure predictors

APACHE III = Acute Physiology and Chronic Health Evaluation III; NPPV = noninvasive positive-pressure ventilation. Reprinted with permission from Intensive Care Med.¹⁸

NIV Failure

The high mortality rate associated with failed NIV shows just how crucial assessment of NIV efficacy is. It is also essential to make the assessment quickly. There are many studies suggesting that clinicians need to accept defeat and accept it fast because the time to failure is often rapid. Studies show failure in as little as two hours but most often failure is within the first 48 hours. Clinicians need to rapidly determine when NIV fails and when it fails quickly.

Non-invasive ventilation is most likely failing your patient if there is evidence of inability to clear secretions, worsening mental status, inability to accept any interface, hemodynamic instability, worsening oxygenation, progressive hypercapnia (pH < 7.20, or persistent tachynea/tachycardia. Clear cut contraindications are cardiac or respiratory arrest, non-respiratory organ failure, facial surgery or trauma, upper airway obstructions, an inability to protect the airway and/or high risk for aspiration, and an inability to clear secretions. Know that are there are times when NIV simply does not make sense. For example, patients with facial trauma and/or deformities are poor NIV candidates because fitting them for a mask is nearly impossible.

Conclusion

NIV is not new. In fact it has been around for over two hundred years. What is different now, however, is that disease processes have evolved. Older and sicker patients are being admitted to the ICU. Clinicians’ understanding of disease processes has evolved as well. More importantly, delivery devices have evolved. Advances in technology have provided clinicians with many options for the delivery of NIV.

So, when do you use NIV? Data do support a trial of NIV for AHRF. But remember, when it works, it works fast, and when it fails, it fails fast. More importantly, when it fails, it is associated with a higher risk of mortality. Although the data do not clearly define those at risk for NIV failure, there are some indications, such as a higher APACHE III score and the presence of shock. No matter what the data suggest, however, they should always be used with sound clinical judgment and experience because it is very difficult to determine which in patients ventilation is going to fail.

References

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Self-Assessment

What can clinicians not do using a freestanding, noninvasive ventilator?

(a) Limit the fraction of inspired oxygen (FIO₂)

(b) Titrate positive end-expiratory pressure

(c) Manipulate inspiratory time and mean airway pressure

(d) Directly control tidal volume
What is the most significant failure predictor for noninvasive ventilation (NIV)?

(a) Shock

(b) Acute Physiology and Chronic Health Evaluation III (APACHE III) score

(c) Sequential Organ Failure Assessment score

(d) Ratio of PaO₂ to FIO₂