Management of acute asthma in ICU

Dr.Zeeshan Waheed

INTRODUCTION

 Intensive therapy with inhaled bronchodilators and systemic corticosteroids is usually sufficient to reduce airflow obstruction and ameliorate symptoms in patients with acute asthma.

However, some patients develop respiratory failure and require supportive care with mechanical ventilation (approximately 4 percent of all patients hospitalized for acute asthma) [1].

Although life-saving, mechanical ventilation and its associated interventions (eg, sedatives, paralytics) can also cause morbidity and mortality

Krishnan V, Diette GB, Rand CS, et al. Mortality in patients hospitalized for asthma exacerbations in the United States. Am J Respir Crit Care Med 2006; 174:633.

Suitable candidates for trial of noninvasive ventilation in acute Asthma*

Clinical observations indicating probable need for elective intubation and mechanical ventilation in acute asthma*.

Indications for transferring a patient with acute asthma to a closely monitored setting or intensive care unit*.

Potential reasons for death from acute asthma.

Principles of initial mechanical ventilation for acute, potentially fatal asthma.

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Ventilator settings 

 The following are reasonable initial ventilator settings

•Respiratory rate 10 to 14 breaths/min•Tidal volume less than 8 mL/kg•Minute ventilation less than 115 mL/kg•Inspiratory flow of 80 to 100 L/min•Extrinsic positive end-expiratory pressure (extrinsic PEEP) less than 80 percent of the intrinsic PEEP

ADVERSE EFFECTS

1.Sequelae of dynamic hyperinflation • Cardiovascular collapse • Barotrauma • Increased work of breathing

2.VAP/Nosocomial Infection

Dynamic hyperinflation in status asthmaticus

• Patients with airway obstruction due to obstructive lung disorders, such as status asthmaticus, expiration time (te) is increased and the lungs take longer to reach FRC.

• In patients with obstructive lung disorders, the lung volume at end-inspiration (VEI) can progressively rise when ventilatory straregies increase inspiratory time (ti) (eg, generous tidal volumes (VT), low inspiratory flow rates) or shorten te (eg, higher respiratory rates), a phenomenon known as dynamic hyperinflation.

• The volume of trapped air (Vtrapped) above FRC rises at end expiration during dyanamic hyperinflation.

• Dynamic hyperinflation creates intrinsic PEEP and elevates the plateau pressure (Pplat), which can lead to cardiovascular collapse and barotrauma, as well as increase the work of breathing.

• Adjustments of the ventilator settings should try to minimize the risk of these events, preferably maintaining an inspiratory Pplat less than 30 cm H2O and an intrinsic PEEP less than 10 to 15 cm H2O.

• The following adjustments may help achieve these goals by decreasing air trapping :

Increasing the inspiratory flow will shorten the inspiratory time, increase the expiratory time, and allow the patient more time to exhale

Decreasing the tidal volume causes less lung inflation and gives the patient a smaller volume to exhale before the next breath

Decreasing the respiratory rate increases the expiratory time and allows the patient more time to exhale

Brenner B, Corbridge T, Kazzi A. Intubation and mechanical ventilation of the asthmatic patient in respiratory failure. J Allergy Clin Immunol 2009; 124:S19.

Additional considerations

Intrinsic PEEP — The amount of intrinsic PEEP can be difficult to measure accurately. As an example, widespread airway closure in patients with severe asthma may impede measurement of end-expiratory alveolar pressure, resulting in a falsely low intrinsic PEEP measurement . As a consequence, marked hyperinflation may be unrecognized.

Asynchrony — Even after sedation, some patients are unable to breathe in synchrony with the ventilator. Paralytic agents may be necessary in this setting. Adequate sedation must be maintained if paralysis is used [8]. Neuromuscular blockade in patients receiving high-dose corticosteroids increases the risk of post-paralytic myopathy.

Upper airway obstruction — Patients with status asthmaticus typically have increased airway pressures. Upper airway obstruction should be considered if a patient presumed to have status asthmaticus does not have elevated airway pressures. Rarely, upper airway obstruction (eg, vocal cord dysfunction, laryngeal malignancy) can mimic status asthmaticus.

ADJUNCTIVE THERAPY

General anesthesia — Induction of general anesthesia, either by intravenous infusion (eg, ketamine) or inhalation (eg, isoflurane), can reduce bronchospasm. This approach should not be undertaken lightly, due to the risk of profound hypotension . An anesthesiologist is required for the duration of the anesthesia and bronchoconstriction generally recurs when anesthesia is withdrawn. The duration of general anesthesia typically ranges from 2 to 12 hours.

Heliox — Heliox is a blend of helium and oxygen that has a lower density than air. Heliox can reduce resistance to airflow, enhance delivery of nebulized bronchodilators, and improve oxygenation compared to standard nitrogen-oxygen mixtures. However, it can also cause ventilator malfunction, including inaccurate measurement of tidal volume and oxygen concentration.

Extracorporeal life support — Oxygenation and carbon dioxide removal through an artificial membrane may be beneficial for patients with status asthmaticus complicated by refractory respiratory acidosis, although clinical outcome studies are lacking

Key messages for the management of assisted ventilation for acute, potentially fatal asthma*.

PROGNOSIS• Patients with status asthmaticus who require mechanical

ventilation have increased in-hospital mortality compared to patients who do not require mechanical ventilation (7 versus 0.2 percent) .

• Patients who survive to hospital discharge remain at high risk of death; this excess risk is only beginning to be recognized and may be worse than some malignancies. As an example, one study assessed the outcome of 145 survivors of status asthmaticus who required mechanical ventilation for a duration of six years [2].

• The one-, three-, and six-year mortality rates were 10, 14, and 23 percent, respectively. Most of the deaths were due to recurrent asthma.

• Marquette CH, Saulnier F, Leroy O, et al. Long-term prognosis of near-fatal asthma. A 6-year follow-up study of 145 asthmatic patients who underwent mechanical ventilation for a near-fatal attack of asthma. Am Rev Respir Dis 1992; 146:76.