Halothane, enflurane, and isoflurane can acutely reduce peak airway pressure and PaCO2 in ventilated patients and can supplement continued b-agonists and systemic corticosteroids. Halothane (0.5-2.0 per cent) is effective in most patients, but has a low therapeutic ratio in the acidotic hypovolemic patient. In addition, use of halothane for more than 24 h may be associated with bromide toxicity. Isoflurane in similar concentrations is safer than and probably as effective as halothane. Adequate gas-scavenging facilities must be available if anesthetic gases are to be used in an ICU environment.
Ketamine, an intravenous anesthetic with sedative, analgesic, anesthetic, and bronchodilating properties, has been used successfully for the emergency intubation of patients with severe asthma. The usual intubating dose is 1 to 2 mg/kg followed by an infusion of 10 to 40 pg/kg/min to maintain a bronchodilating and sedative effect. Ketamine increases catecholamine levels and directly relaxes bronchial smooth muscle.
Anecdotal uncontrolled experience has suggested that magnesium sulfate may offer useful bronchodilator properties but the mechanism is unknown. One possibility is that magnesium inhibits the calcium channels of airway smooth muscle, thus interfering in calcium-mediated smooth muscle contraction. It has been reported that intravenous administration of 1 g magnesium sulfate in the management of acute respiratory failure in a 72-year-old asthmatic produced sufficient improvement to avoid intubation. However, controlled studies have failed to confirm significant bronchodilator effect from the use of magnesium in acute asthma.
Heliox is a mixture of helium and oxygen which is generally available with He:O 2 mixtures of 80:20, 70:30, and 60:40. Helium is a low-density gas which reduces airway resistance and decreases respiratory work. A significant decrease in pulsus paradoxus and a significant increase in peak flow has been observed in patients with acute asthma breathing heliox for 15 min compared with controls. A significant reduction in both peak airway pressures and PaCO2 has been noted in intubated patients within 20 min of ventilation with heliox mixtures. If heliox is used during mechanical ventilatory support, gas blenders and flowmeters must be recalibrated to this low-density gas to obtain accurate measures of oxygen concentration or tidal volume.
Nitric oxide (NO) is an important short-acting endogenous vasodilator and bronchodilator. As it is extremely lipophilic, inhaled NO may not only diffuse directly from the alveoli into vascular smooth muscle causing vasorelaxation but also diffuse through the bronchial epithelial barrier to reach airway smooth muscle and produce airway relaxation. One study concluded that NO inhaled at 80 ppm exerted a weak bronchodilatory effect in asthmatics but not in patients with chronic obstructive pulmonary disease.
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If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.