Page 682 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 55: Status Asthmaticus 501

demonstrating benefit to heliox as a driving gas, but there are also data lung injury. Patients who are intubated before they arrest generally do
153
to the contrary. The likely reason for reported lack of benefit is the well. In a recently published retrospective observational study in a single
154
failure to ensure a heliox delivery system that prevents room air entrain- ICU, the authors reported their findings in 280 episodes of status asth-
ment, which reduces inspired helium concentration. 153 maticus in 227 patients over a 30-year span. Mortality rate was 0.35%
8
despite a high percentage of patients requiring mechanical ventilation.
Antibiotics: Because viruses trigger most infectious exacerbations of In another report that described the outcomes of 78 inner-city patients
asthma (and bacterial pneumonia is rare), there is no clear role for anti- with status asthmaticus admitted to an ICU, there were three deaths. 170
biotics in treating acute asthma. Antibiotics are frequently prescribed for Intubation is indicated for impending respiratory failure and cardio-
an increase in sputum volume and purulence. However, purulence may pulmonary arrest. Changes in posture, mental status, speech, accessory
reflect an abundance of eosinophils, not polymorphonuclear leukocytes. muscle use, and RR can indicate progressive ventilatory failure that does
The importance of Mycoplasma pneumoniae and Chlamydia pneu- not need blood-gas or PEFR confirmation. In the final analysis, the deci-
moniae in acute asthma is unknown. Lieberman and colleagues used sion to intubate rests on a clinician’s estimate of the patient’s ability to
paired serology to demonstrate evidence for mycoplasma infection in maintain spontaneous respirations.
18% of patients hospitalized for acute asthma. The Expert Panel from Oral intubation is preferred because it allows for placement of an
155
the NIH does not recommend the use of antibiotics in asthma exacerba- adequately sized endotracheal tube (eg, 8.0 mm inside diameter [ID]
tion in the absence of other clinical indications such as pneumonia. for adult women, 8.0-8.5 mm ID for adult men) to facilitate removal of
62
Graham and associates selected 2 out of 128 studies adequate for review mucus and decrease airflow resistance. Nasal intubation is acceptable
and concluded that the role of antibiotics is difficult to assess. 156 in an awake patient anticipated to be difficult to position and intubate
■ NONINVASIVE POSITIVE PRESSURE VENTILATION (fiberoptic guidance may facilitate intubation in this setting), but is
Noninvasive positive pressure ventilation (NIV) by face mask is an complicated by the need for a smaller endotracheal tube, the possibility
of nasal polyps and increased risk of sinusitis.
option for patients with hypercapnic respiratory failure who do not
require intubation. Continuous positive airway pressure (CPAP) Postintubation Hypotension: Hypotension has been reported in 25% to
helps overcome the adverse effects of PEEPi and decreases the inspi- 35% of patients following intubation. It stems from loss of vascular
171
ratory work of breathing. Bronchial dilation also occurs during tone due to the direct effects of sedation and loss of sympathetic activ-
157
CPAP. Advantages of NIV over intubation include decreased need for ity, hypovolemia, and DHI (especially when inadequate time is not
158
sedation and paralysis, decreased incidence of nosocomial pneumonia, allowed for exhalation). The presence of DHI is signaled by diminished
decreased incidence of otitis and sinusitis, and improved patient breath sounds, hypotension, tachycardia, and high airway pressures, and
comfort. Disadvantages include increased risk of aspiration when importantly these findings should lead to a trial of apnea or hypopnea
159
there is gastric insufflation, skin necrosis, and diminished control of (2-3 breaths/min) in a well-oxygenated patient. This maneuver is both
ventilatory status compared with invasive ventilation. diagnostic and therapeutic as 30 to 60 seconds of exhalation drops
Data regarding the efficacy of NIV in acute asthma are limited. In intrathoracic pressure allowing for greater filling of the right atrium
one study of 21 acute asthmatics with a mean PEFR of 144 L/min, and ultimately improved hemodynamics and lower airway pressures.
160
nasal CPAP of 5 or 7.5 cm H O decreased RR and dyspnea compared Improved cardiopulmonary parameters after such a trial, however, does
2
to placebo. In another study, Meduri and colleagues reported their not exclude pneumothorax, which has been reported to be as high as
observational experience with NIV during 17 episodes of acute severe 6% in intubated asthmatic patients. 171,172 Careful inspection of the chest
asthma. The average duration of treatment was 16 hours and NIV x-ray is mandatory because the lungs may not collapse completely in the
161
generally improved dyspnea, HR, RR, and blood gases. Two NIV-treated setting of DHI and widespread mucus plugging. When tension pneu-
, and there were no mothorax is considered, chest tubes generally should not be placed until
patients required intubation for worsening Pa CO 2
NIV complications. Soroksky and colleagues reported their results of a trial of apnea or hypoventilation has failed or there is radiographic
a randomized, placebo controlled trial of conventional asthma treat- evidence of pneumothorax.
ment plus 3 hours of NIV (n = 15) versus conventional treatment plus
sham NIV (n = 15) in ED patients aged 18 to 50 years of age with an Initial Ventilator Settings and Dynamic Hyperinflation: Expiratory time,
tidal volume, and severity of airway obstruction determine the level
FEV <60% of predicted and an asthma attack duration less than 7
1
days. The protocol sets the initial expiratory pressure at 3 cm H O of DHI (Fig. 55-1). Minute ventilation and inspiratory flow determine
162
To avoid dangerous levels of DHI, initial minute
expiratory time.
173,174
2
and the initial inspiratory pressure at 8 cm H O. Expiratory pres-
2
sure was increased by 1 cm H O every 15 minutes to a maximum of ventilation should not exceed 115 mL/kg/min or approximately 8 L/min
in a 70-kg patient. This goal is achieved using an RR between 12 and
175
2
5 cm H O and the inspiratory pressure was increased by 2 cm H O
2
2
every 15 minutes to a maximum pressure of 15 cm H O or until RR 14/min and a tidal volume between 6 and 8 mL/kg (ideal body weight).
The use of low tidal volumes avoids excessive peak lung inflation, which
2
was less than 25/min, whichever came first. The mean increase FEV was
1
53.5 ± 23.4 with NIV compared with 28.5 ± 22.6 in the control arm can occur even with low minute ventilation.
Shortening the inspiratory time by use of a high inspiratory flow rate
(p = 0.0006). There was also a significant decrease in hospitalization
rates with NIV (17.6% vs 62.5%). Two meta-analyses and guidelines (eg, 60 LPM using a constant flow pattern) further prolongs expiratory
time. High inspiratory flows increase peak airway pressure by elevating
provide provisional further support for NIV in acute asthma. 163-166
airway resistive pressure, but peak airway pressure per se does not corre-
late with morbidity or mortality. High inspiratory flow and high airway
MANAGEMENT OF THE INTUBATED ASTHMATIC pressures may redistribute ventilation to low-resistance lung units, risk-
■ INTUBATION ing barotrauma, but these concerns are based largely on mathematical
and mechanical lung models.
Another concern in spontaneously
176,177
Approximately 10% of patients admitted with a primary diagnosis of breathing patients is that high inspiratory flow rates in the assist-control
asthma are admitted to an intensive care unit; approximately 2% are mode can increase RR and thereby decrease expiratory time. 178
intubated. While this percentage may be small, and there has been a There is no consensus as to which ventilator mode should be used in
recent decline in the number of patients requiring ICU stay in some asthmatics. In paralyzed patients, synchronized intermittent mandatory
centers, these patients generally incur greater costs, stay in hospital ventilation (SIMV) and assist-controlled ventilation (AC) are equivalent.
longer, and are at increased risk of morbidity and mortality. 167-169 In patients triggering the ventilator, SIMV may be preferred by some
The goals of intubation and mechanical ventilation are to maintain intensivists because of the unproven concern that minute ventilation will
oxygenation, prevent respiratory arrest, and minimize ventilator-induced be higher during AC, since each triggered breath receives a guaranteed
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