Case Study: Asthma (2)
A 10-year-old female is diagnosed with acute appendicitis and is scheduled for an emergency laparoscopic appendectomy. She has a history of asthma for which she takes maintenance therapy with inhaled steroids, inhaled cromolyn, and a leukotriene antagonist. Three days ago she was treated in the emergency room for an acute asthma attack. She received inhaled albuterol and intravenous methylprednisolone. Some residual wheeze remains, and she states she is not back to her usual state of good health.
How will you approach the anesthetic management of this child?
Since this procedure is urgent, I don’t have much time to further optimize this child’s asthmatic condition prior to appendectomy. She should receive a nebulized albuterol treatment, either in the emergency room or upon arrival to the OR holding area, and one IV dose of methylprednisolone. I will also administer IV midazolam as a preoperative anxiolytic. Preoperative IV hydration is also important in this child – she has probably had limited oral intake recently, and I want to minimize thickening of her bronchial secretions.
Rapid sequence induction of general anesthesia is indicated in this patient owing to the nature of her abdominal process. It should be tailored so as to minimize the chances of bronchial reactivity following endotracheal intubation. Following an adequate interval of preoxygenation, I will administer (in approximate doses) fentanyl 2 mcg/kg, lidocaine 1.5 mg/kg, propofol 3 mg/kg, and rocuronium 1.2 mg/kg while an assistant holds cricoid pressure. This combination of medications should provide reliable intubating conditions within 60 seconds, and “gentle” ventilation (peak inspiratory pressures less than 15 cmH2O) performed if necessary to prevent hypoxemia. For maintenance of general anesthesia I can use any of the inhalational agents (except for desflurane because of its airway irritating properties), and continue administration of fentanyl as needed.
How will the presence of asthma change your ventilator settings?
Minute ventilation settings should be appropriate for this child’s age and weight. However, asthmatic patients with a significant degree of airway obstruction will require a longer than usual expiratory time, and a slower ventilatory rate to allow for complete alveolar emptying. In the worse-case scenario, asthmatic patients can develop air trapping, which can lead to tension pneumothorax. However, this rarely occurs in patients who do not exhibit severe airway obstruction at the time of institution of mechanical ventilation. I would choose a pressure ventilation mode over a volume ventilation mode to minimize abrupt increases in peak inspiratory pressures should bronchospasm occur. It is primarily a matter of personal preference and whether one desires to trigger a ventilator alarm if the peak inspiratory pressure is above a predefined setting, or if the delivered tidal volume is below a predefined limit.
During the procedure, you detect wheezing by auscultation, the capnograph changes to an up-sloping shape, and the delivered tidal volume decreases, all of which indicate the onset of bronchospasm. What will you do?
The likelihood of asthmatic-related bronchospasm in this patient is high, but I will initially rule out other obvious causes such as right main stem bronchial intubation (which often happens when a patient is placed in the Trendelenburg position), and excess secretions in the endotracheal tube. I will increase the concentration of the inhalational agent (within hemodynamic limits), and increase the inhaled oxygen concentration if necessary. If none of these rapidly reverse the wheeze, I will administer inhaled albuterol through the endotracheal tube. The most practical way of doing this intraoperatively is by using a metered-dose inhaler that is connected to the anesthesia breathing circuit between the inspiratory limb and patient Y-piece. This can be performed by inserting the bronchodilator canister into a 60-mL syringe barrel and using the plunger to actuate the medication (see figure), or by directly inserting the canister into the breathing circuit using a specialized adapter (see figure). Access into the circuit is attained through a removable cap, through which the spray is actuated just prior to a positive-pressure breath. In practice, however, a very low percentage of the bronchodilator actually reaches the lungs because it adheres to the circuit and endotracheal tube. The smaller the diameter of the endotracheal tube, the less actuated medication will actually reach the lungs. Therefore, multiple administrations of albuterol are delivered (usually between 10 and 20) until bronchospasm is relieved, or until the patient develops tachycardia from absorption of the adrenergic agonist.