Ventilator Management of the Intubated Patient With Asthma

Question

What initial ventilator settings should be used after intubation of a patient with a severe asthma exacerbation?

Response from Michael E. Winters, MD Assistant Professor of Emergency Medicine and Medicine, University of Maryland School of Medicine; Medical Director, Adult Emergency Services, University of Maryland Medical Center, Baltimore, Maryland

Managing the mechanical ventilator can be the most challenging aspect of caring for a critically ill patient with asthma. An improper setting can rapidly worsen lung hyperinflation, induce lung injury, and cause cardiovascular collapse, thus increasing patient morbidity and mortality. I will discuss key concepts in the mechanical ventilation of patients with a severe asthma exacerbation.

Ventilator Management in Asthma

When managing the ventilator in a patient with asthma, it is critical to remember that the primary problem is expiratory flow limitation. This limitation results from both anatomic and dynamic obstruction of the airways. As a consequence, these patients require prolonged expiratory times to reach static lung volumes. Unfortunately, patients with severe exacerbations often initiate inspiration before expiration ceases. The result is a progressive increase in lung volume, called intrinsic positive end-expiratory pressure (PEEP), auto-PEEP, or hyperinflation.

Minimizing hyperinflation and avoiding excessive airway pressures are key goals in ventilating the patient with asthma. These goals are best accomplished by hypoventilating the patient; that is, selecting a low respiratory rate and tidal volume in an effort to give the patient sufficient time for exhalation.

A consequence of hypoventilation is hypercapnia. In the absence of increased intracranial pressure or severe myocardial ischemia, hypercapnia is generally well-tolerated by intubated patients with asthma. In fact, patients can tolerate arterial CO₂ levels as high as 90 mm Hg without adverse effects, provided the arterial pH remains above 7.15. A common pitfall is selecting a high respiratory rate or tidal volume to normalize the arterial partial pressure of CO₂ (P_aCO₂). These settings will unequivocally worsen hyperinflation and rapidly lead to cardiovascular collapse. Routine arterial blood gas analysis is not necessary in the management of most patients with asthma.

Initial Ventilator Settings

Suggested initial ventilator settings for intubated patients with asthma are provided in the Table.

Table. Suggested Initial Ventilator Settings for the Intubated Patient With Asthma

Assist control mode Tidal volume: 7-8 mL/kg (using ideal body weight) Respiratory rate: 10-12 breaths/minute FiO2: 100% PEEP: 0 cm H2O

FiO₂ = fraction of inspired oxygen; PEEP = positive end-expiratory pressure

Controlled ventilation is the preferred way to ventilate patients with asthma initially, because they require deep sedation to tolerate the necessary ventilator settings. Currently, no consensus exists on the mode of mechanical ventilation for patients with asthma. Many clinicians prefer volume-controlled ventilation over pressure-controlled ventilation because pressure-controlled ventilation can produce variable tidal volumes, especially in patients who have significant resistance to airflow and hyperinflation.

The initial respiratory rate and tidal volume should be set to maintain a minute ventilation of less than 10 L/minute. A respiratory rate of 10-12 breaths per minute, and a tidal volume of 7-8 mL/kg of ideal body weight are common initial settings. Immediately following intubation, the oxygen concentration (F_iO₂) should be set at 100%. Maintaining adequate arterial oxygen levels is usually not a problem in patients with asthma. Therefore, the F_iO₂ can usually be reduced rapidly, with the goal of maintaining arterial oxygen saturations above 88%.

The use of external PEEP in patients with asthma continues to be debated. In general, no benefit from external PEEP is derived in the heavily sedated patient with asthma who is on controlled ventilation. However, as the patient is weaned from mechanical ventilatory support, low levels of external PEEP may be useful.

Monitoring for Hyperinflation

The ventilated patient with asthma must be monitored for persistent or worsening hyperinflation. Hyperinflation can be measured by the volume at end inspiration (Vei), auto-PEEP, and plateau pressure (P_plat). Vei is a measure of expired gas from end inspiration to functional residual volume during a 40- to 60-second apnea period. Because Vei measurement can be difficult to obtain and requires skilled staff and a paralyzed patient, it is not routinely used to assess hyperinflation.

Auto-PEEP represents the lowest average alveolar pressure during the respiratory cycle. It is measured using an end-expiratory hold, with values >15 cm H₂O indicative of hyperinflation. Auto-PEEP can also be detected by examining the flow tracings. Expiratory flow that continues at the onset of inspiration indicates that breath stacking (hyperinflation) is occurring. Auto-PEEP measurements can significantly underestimate the degree of hyperinflation when communication between the alveoli and proximal airways is poor.

The recommended method to monitor patients for hyperinflation and injurious airway pressures is P_plat, the average end-inspiratory alveolar pressure. P_plat is measured using an end-inspiratory pause. Values > 30 cm H₂O indicate hyperinflation and excessive airway pressures. Of note, peak airway pressure (P_peak) measurements do not correlate with patient outcomes and thereforeare not useful for assessing hyperinflation.

Adjusting the Ventilator

If the P_plat value exceeds 30 cm H₂O, the ventilator must be adjusted to extend exhalation time. Adjustments include alterations in respiratory rate, tidal volume, and inspiratory flow rate. Lowering the respiratory rate has the greatest effect on improving hyperinflation and should be the first adjustment made in this scenario. If P_plat remains above 30 cm H₂Odespite lowering the respiratory rate, tidal volume can be reduced by 1 mL/kg. Below 6 mL/kg, further reductions in tidal volume are limited as a result of progressive increases in dead space fraction.

If P_plat exceeds 30 cm H₂Odespite the lower respiratory rate and tidal volume, the inspiratory flow rate can be increased. On most ventilators, the inspiratory flow rate is set at 60 L/minute. For the patient with asthma who has elevated P_plat, the inspiratory flow rate should be increased to 80 to 90 L/minute. Increasing the inspiratory flow rate will increase P_peak. Elevated P_peaks levels do not correlate with patient outcomes.

Medscape Emergency Medicine © 2010  WebMD, LLC

Cite this: Michael E. Winters. Ventilator Management of the Intubated Patient With Asthma - Medscape - Dec 13, 2010.