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Dr. Shapiro is Assistant Professor of Clinical Medicine, Columbia University, and Director of Medical Intensive Care Unit, St. Luke’s-Roosevelt Hospital.
Correspondence to: Janet M. Shapiro, MD, Division of Pulmonary-Critical Care Medicine, St. Luke’s Hospital MU 316, 1111 Amsterdam Ave, New York, NY 10025
Intensive care management of status asthmaticus has advanced in recent years. Knowledge of the inflammatory basis of severe asthma and the pathophysiologic consequences of airway obstruction1 has translated into improvements in the medical and ventilatory approaches.
Still, status asthmaticus carries a significant risk of morbidity and mortality in the ICU. In this issue of CHEST (see page 1616), Afessa and colleagues present an observational study of 132 medical ICU admissions for status asthmaticus in an urban hospital. There was a notable predominance of African-American patients (67%) and women (79%). Invasive mechanical ventilation was required in 48 patients (36%), and noninvasive ventilation was initiated in 27 patients (20%), 5 of whom subsequently required intubation.
Eleven patients died, giving a mortality rate of 8.3%. Features distinguishing nonsurvivors were lower initial arterial pH and higher PaCO2, higher APACHE (acute physiology and chronic health evaluation) II score, development of sepsis and organ failures, and need for mechanical ventilation. The occurrence of pre-ICU cardiopulmonary arrest can explain many of the findings. Three of four patients suffering pre-ICU cardiopulmonary arrest and anoxic encephalopathy died. The mean APACHE score of 25 among nonsurvivors is much higher than that among survivors in this series and in the literature.2 Pre-ICU cardiopulmonary arrest or tension pneumothorax had occurred in five of the patients with the highest APACHE II scores. The authors note a higher mortality rate than predicted based on APACHE II; however, the group includes patients in whom circulatory arrest may be considered the principal ICU diagnosis.
The mortality rate of 21% among intubated patients is striking. In contrast to the high mortality of such patients in prior years,3 several series over the past 2 decades report lower mortality rates of 0 to 6%,4 5 6 although mortality was 22% in one study.7 Of 75 patients intubated for status asthmaticus over a 5-year period at my hospital in New York City, 2 patients (2.6%) died; both had suffered prehospital cardiorespiratory arrest with resultant anoxic encephalopathy.8 Most ICU deaths thus appear to be consequences of pre-ICU cardiopulmonary arrest.5 6 8
The frequency of mechanical ventilation varies in the literature from 30%4 to 100%6 7 and is likely subject to differences in disease severity in particular locations,9 10 ICU admission criteria, and clinicians’ decisions to initiate mechanical ventilation. The ventilator management is guided by the principle of preventing dynamic hyperinflation. Based on studies1 of trapped gas volume, targeting a minute ventilation of 115 mL/kg and plateau pressure < 25 cm H2O will usually prevent severe dynamic hyperinflation and unnecessary extreme hypercapnia. A low tidal volume (8 mL/kg) and a slow respiratory rate (10 to 14/min) are set to provide a prolonged expiratory time, thereby minimizing air trapping and its consequences of hemodynamic compromise and barotrauma. Measurements of peak airway pressures and auto-positive end-expiratory pressure have not been found to correlate with complications. Permissive hypercapnia (with levels of PaCO2 up to 90 mm Hg) is generally tolerated when adequate oxygenation is achieved; the main contraindication is intracranial disease.11 Permissive hypercapnia is therefore avoided in patients who have already suffered an anoxic brain injury and cerebral edema following cardiorespiratory arrest.
Hypotension is a common complication after initiation of mechanical ventilation, reported by Williams et al5 in 20 to 41% of cases. Barotrauma was found to complicate status asthmaticus in 14 to 27% of their patients.5 In the current study, three of the four cases of pneumothorax developed during bag ventilation after intubation. The authors raise an excellent point that the approach to manual bag ventilation in the field is a potential area for education and improvement.
Noninvasive ventilation was attempted in 20% of patients in the study by Afessa et al. Although there are no trials comparing noninvasive and invasive ventilation, noninvasive ventilation may have a role in selected patients with status asthmaticus.12 Most patients with severe asthma, including many presenting with hypercapnia,13 improve rapidly with conventional therapy and do not require intubation, so it may be difficult to prove the benefit of noninvasive support. The fact that there were two deaths among the five patients initially treated with noninvasive ventilation who required intubation raises the possibility that delaying invasive ventilatory support led to refractory deterioration.
Data concerning the six patients in the current study who developed neurologic dysfunction are not presented. Major neurologic syndromes may complicate critical care of status asthmaticus. Myopathy has been attributed to concomitant neuromuscular blockade and corticosteroid administration.2 14 15 In one series, myopathy was detected in 10% of 64 episodes of status asthmaticus, occurring in 30% of those who received neuromuscular blockade.2 Consequences of myopathy include increased duration of mechanical ventilation and hospital stay, as well as the need for rehabilitation.2 14 Neuromuscular blocking agents are therefore avoided in patients with status asthmaticus. Critical illness polyneuropathy may complicate the course of patients who acquire sepsis while in the ICU.16 Anoxic encephalopathy is a devastating consequence of cardiopulmonary arrest; of our six patients who had suffered pre-ICU cardiopulmonary arrest, two patients died and two patients remained in a coma and were discharged to nursing homes.8
Although the median duration of mechanical ventilation was 3 days in this series, as in the literature,6 many patients required assisted ventilation for a longer period, placing them at risk of ICU complications. Sepsis was reported in 13% of patients and was the immediate cause of death in three patients. GI bleeding and probable venous thromboembolism were proximate causes of death in one patient each in the current study.
ICU survival should be expected for those patients who have not suffered cardiorespiratory arrest. However, these patients remain at risk for future asthma death. One study17 of 121 survivors of status asthmaticus demonstrated a 15% mortality within the next 6 years; cigarette smokers were at especially high risk of death.
Afessa and colleagues provide a complete description of the ICU course in their patients with status asthmaticus. However, important questions remain. We lack information on the patients’ asthma histories and access to appropriate outpatient medical care. The authors describe many patients who were admitted to the ICU multiple times during the 3-year study.
Black and Hispanic individuals predominate among patients with severe asthma and fatal asthma in urban areas.10 18 Cigarette smoking and illicit drug use19 present additional exacerbating factors. A recent study20 reported that high-risk asthmatic women were hospitalized twice as often as men. Minority women account for > 70% of ICU patients with status asthmaticus in the current study and in most series in the literature.4 7 14 In addition to biological differences, contributing factors may include poverty, environmental concerns in the home, and gender differences in the quality of care.21 One study reported poor ability to utilize a metered-dose inhaler among women22 and raises the additional possibility of inadequate instruction offered to women.21
Patients may underestimate the degree of worsening airflow obstruction. Reduced perceptions of dyspnea and hypoxia were found among 11 patients following respiratory failure due to status asthmaticus.23 Psychosocial factors, including denial of illness24 and illicit drug use,19 can impede action in a deteriorating situation. Survivors of respiratory failure who are noncompliant with medical visits may be at particular risk of death. In a prospective study25 in which close outpatient follow-up was offered to 12 such patients, 2 of the 5 patients who declined the protocol died within the 18-month period.
Afessa and colleagues describe the high mortality and morbidity among patients admitted to an ICU for status asthmaticus. This examination reinforces the challenges to the pulmonary-critical care community to improve the entire spectrum of care—from the outpatient setting to the ICU—and thus enhance outcome in status asthmaticus.
References
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