Staten Island Corner: The Intubated Asthmatic

Welcome back to this month’s edition of Staten Island Corner. I decided to review the literature on the proper initial management of intubated asthmatic patients. Although asthma is a very common disease and something that we deal with on a daily basis in the Emergency Department, it has the potential of being a very dangerous and potentially fatal disease. The overall mortality rate from asthma exacerbations is less than 0.1%, but the mortality rate of intubated asthmatics is about 8%. There are more than 5,000 deaths reported annually in the US and 100,000 deaths estimated worldwide from asthma. About 1/3 of patients worldwide admitted to the ICU with acute asthma require mechanical ventilation. There are two types of asthma patients who require intubation: those who are exhausted but still have preserved lung function and those who are exhausted who have extreme airway obstruction and hyperinflation that will make mechanical ventilation difficult. Over 50% of patients who have a life threatening asthma episode may not have histories suggesting this severity and any patient with asthma can present with a life threatening attack. The majority (80-85%) of asthma related deaths occur in patients with severe and poorly controlled asthma who deteriorate gradually over days or weeks. The other 10-15% of deaths occur suddenly and unexpectedly without previous long term deterioration. Severe asthma exacerbation leads to hypercapnic respiratory failure and a mixed acidosis. Risk factors for death from asthma include: past history of sudden and severe exacerbations, prior intubation for asthma, prior admission to ICU for asthma, two or more hospitalizations in previous year for asthma, three or more ED asthma visits in the previous year, hospitalization or ED visit for asthma in previous month, use of more than 2 short acting beta agonist inhalers in the previous month, current systemic steroid use or recent withdrawal from steroids, difficulty perceiving airflow obstruction or its severity, comorbidities (COPD or cardiovascular), serious psychiatric disease or psychosocial problems, low socioeconomic status and urban residence, illicit drug use, sensitivity to alternaria.

What asthmatics should be intubated? Patients who present with apnea or coma should obviously be intubated immediately. Patients with increasing hypercapnia, exhaustion, depressed mental status, hemodynamic instability, and refractory hypoxemia should be considered strongly for intubation. Intubation should be done semi-electively (before respiratory arrest) in a controlled setting. There is no evidence for a specific number on ABG that is an indication for intubation. All efforts should be made to avoid intubation in asthma patients because of the serious complications that can occur. Once the decision to intubate the patient has been made, RSI is recommended and ketamine is recommended as the induction agent. The emergency physician’s job has just begun once the tube is secured. This is not the patient that the ED doctor can just call respiratory and have the patient connected to the ventilator and admit the patient to the ICU. This requires continuous reassessment of clinical status and adjustment to the ventilator. Deterioration soon after intubation (hypotension, hypoxemia, hypoventilation, or hemodynamic instability) can be caused by sedative drugs used for intubation, pre-existing hypovolemia, misplacement of tube, worsening of a pre-existing unrecognized pneumothorax which has been made worse by positive pressure ventilation, or worsening of hyperinflation by excessive hand bagging.

Deep sedation with doses of opiods or other medications sufficient to suppress respiratory drive and occasionally the use of paralytics (20-60 min) is recommended during the initial stages of mechanical ventilation to gain control. Propofol is generally recommended because of its rapid on/off, bronchodilation, and potent respiratory suppression qualities. It can be titrated to anesthetic-depth sedation and therefore may allow you to avoid the use of paralytics if the blood pressure can tolerate that high a dose of propofol. Ketamine is also favored because of its bronchodilating properties. Bronchodilation appears within minutes of IV administration and lasts 20-30 min after. Inadequate sedation is a common reason for spontaneous breathing in an intubated asthmatic that can worsen hyperinflation and increase the risk of barotrauma, hypotension, and cardiovascular collapse. Dynamic hyperinflation can cause hemodynamic compromise in the same way that a tension pneumothorax would (increased intrathoracic pressure causing decreased preload). Hyperinflation occurs because of obstruction of the expiratory airflow leads to trapping of air. Hyperinflation flattens the diaphragm and therefore reduces its generation of force because of the stretch on the muscle fiber. It also increases the dead space which increases the minute ventilation required to maintain adequate ventilation. If the patient is not adequately sedated and is breathing at a high respiratory rate, the ventilator will augment these breaths giving a larger tidal volume which will increase hyperinflation, dead space, and CO2, worsening the overall picture. A viscous cycle can ensue if the inspiratory cycle is allowed to begin before the preceeding exhalation has finished. The patient may become agitated and fight the vent (dyssynchrony). If unable to achieve adequate ventilation with sedation alone, paralytics may be required. Paralytics allows patient-ventilatory synchrony, promotes more effective ventilation, lowers the risk for barotrauma, reduces oxygen consumption, reduces CO2 production, and reduces lactate accumulation.

The ventilator settings play a critical role in the intubated asthmatic patient. As initial settings, it is recommended to limit the tidal volume to 6-10 ml/kg, RR 10-15, PEEP of zero, I:E ratio >1:3, and FiO2 of 100% (after initial stages, the FiO2 should be titrated to keep the O2sat above 88%.) Each mode of ventilation has advantages and disadvantages. Pressure-control has the advantage of always limiting the amount of hyperinflation because the lungs will not be inflated to a pressure above the set pressure. Therefore, even if the amount of lung resistance increases, the ventilator will never deliver pressure above the set pressure. The disadvantage is however that if the airway resistance is high, it will be difficult to deliver an effective tidal volume. This is the safest mode of ventilation because it limits the amount of barotrauma, but does so at the expense of decreased CO2 clearance, decreasing pH, and less effective bronchodilator delivery. Volume-control on the other hand provides better ventilation and delivery of bronchodilator therapy, but does so at the risk of hyperinflation.

Permissive hypercapnia or controlled hypoventilation is the ventilator strategy that is recommended in intubated asthmatics. The goal is to decrease dynamic hyperinflation, provide adequate oxygenation and ventilation, and minimize barotrauma and hypotension. Hypercapnia is usually well tolerated as long as PaCO2 is kept below 90. High breathing rates and high tidal volumes worsen hyperinflation because there is inadequate time for exahalation. The most critical determinant of hyperinflation is the expiration time which can be maximized by shortening the inspiratory time by increasing the flow rate. There is a plateau in expiratory flow time so after a certain point there is no benefit to further expiratory time (after about 4 seconds).

It is very important to realize that mechanical ventilation is only a stabilizing measure in respiratory failure from an asthma exacerbation and it does not treat the underlying asthma. These patients require all of the asthma medications including bronchodilators, steroids, epi, magnesium, etc. During mechanical ventilation, bronchodilators can be administered by wet nebulization or MDI with a holding chamber or spacer. It is also critical that these patients be monitored closely for deterioration while on the ventilator. Hypotension during mechanical ventilation may require you to disconnect the vent and allow for exhalation of trapped air and reduction of hyperinflation. In a patient with sudden or persistent hypotension, you must assess for the possibility of a tension pneumothorax. Hyperinflation can develop at any time in these patients and must constantly be on the provider’s radar. The American College of Chest Physicians conclude that plateau pressure is the best predictor of hyperinflation in intubated asthmatic patients and this pressure should be kept below 35.

Take Home points:

1) Asthma can be DEADLY

2) Only intubate an asthmatic as a last resort, BUT don’t wait too long

3) Use Ketamine as inducing agent.

4) Use A LOT of sedation, preferably propofol which will suppress the respiratory drive

5) Use paralytics if necessary to gain control

6) Pressure support mode is safest; Tidal volume 6-10 ml/kg, RR 10-15, PEEP of zero, I:E ratio >1:3, FiO2 to keep sat above 88%, and keep plateau pressure less than 35

7) Beware of hyperinflation and pneumothorax

8) Don’t forget to continue to give continuous asthma medications (intubation doesn’t cure asthma)

Hodder R, Lougheed MD, FitzGerald M, Rowe BH, Kaplan AG, McIvor RA. Management of acute asthma in adults in the emergency department: assisted ventilation. CMAJ. 2010 Feb 23;182(3):265-72.

Medoff BD. Invasive and noninvasive ventilation in patients with asthma. Respir Care. 2008 Jun;53(6):740-48.

Papiris S, Kotanidou A, Malagari K, Roussos C. Clinical review: severe asthma. Crit Care. 2002 Feb;6(1):30-44.