What is the role of subcutaneous (SC) terbutaline in the management of severe asthma?

Subcutaneous Terbutaline in Severe Asthma

Subcutaneous terbutaline is an effective second-line bronchodilator for acute severe asthma when patients fail to respond adequately to nebulized beta-agonists, are unable to cooperate with inhaled therapy, or when nebulizers are unavailable. 1

When to Use Subcutaneous Terbutaline

Administer subcutaneous terbutaline in the following specific scenarios:

• When signs of acute severe asthma persist 15-30 minutes after initial nebulized therapy (inability to complete sentences, pulse >110 bpm, respiratory rate >25/min, PEF <50% predicted) 1
• When patients cannot cooperate with nebulized therapy due to severe distress 2
• When nebulizer equipment is unavailable in pre-hospital or resource-limited settings 1
• In intubated patients not responding adequately to inhaled beta-agonist therapy 2

Dosing and Administration

The standard dose is 0.5 mg (0.5 mL) subcutaneously, which can be repeated if needed. 1, 3

• Peak plasma concentration occurs at median 0.5 hours (range 0.08-1.0 hours) after subcutaneous injection 4
• Bronchodilation begins within 3 minutes, with peak effect at 15 minutes 3
• Terminal half-life is approximately 5.7 hours 4
• About 90% is excreted in urine at 96 hours, with 60% as unchanged drug 4

Efficacy Compared to Alternatives

Subcutaneous terbutaline (0.5 mg) produces equivalent bronchodilation to subcutaneous adrenaline (0.5 mg) in acute severe asthma. 3

• At 5 minutes post-injection: terbutaline increased PEF by 23% and FEV1 by 37% 3
• At 15 minutes: terbutaline increased PEF by 40% and FEV1 by 58% 3
• No significant difference in heart rate, blood pressure, or pulsus paradoxus compared to adrenaline 3
• Both drugs produced rapid symptom relief within 3 minutes without serious adverse effects 3

However, when comparing 1.0 mg terbutaline to 0.5 mg epinephrine, terbutaline caused significantly greater tachycardia (25% increase above baseline) despite equivalent bronchodilation. 5 This suggests the optimal subcutaneous dose is 0.5 mg, not higher.

Integration into Treatment Algorithm

Follow this stepwise approach:

1. First-line therapy: Oxygen 40-60% + nebulized salbutamol 5 mg or terbutaline 10 mg + prednisolone 30-60 mg orally or hydrocortisone 200 mg IV 1

2. Reassess at 15-30 minutes: Measure PEF, respiratory rate, pulse, ability to speak 1

3. If acute severe features persist: Give subcutaneous terbutaline 0.5 mg OR repeat nebulized therapy + add ipratropium 0.5 mg 1

4. If life-threatening features present: Consider aminophylline 250 mg IV over 20 minutes or IV terbutaline 250 mcg over 10 minutes 1

Long-Term Use in Brittle Asthma

For highly selected patients with brittle asthma experiencing frequent life-threatening exacerbations despite maximal therapy, continuous subcutaneous terbutaline infusion may reduce hospital admissions. 6, 7

• In 17 patients with brittle asthma, mean lowest daily PEF rose from 142 L/min to 297 L/min 6
• Long-term treatment (mean 86.7 months) reduced mean hospital admissions from 6.7 to 3.3 per year (p=0.045) 7
• Both continuous infusion and 6-hourly divided dose regimens were equally effective 6
• 59% of patients reduced oral corticosteroid requirements 7

However, this approach should only be used in specialized severe asthma centers due to significant risks. 7

Critical Safety Considerations and Pitfalls

Cardiovascular effects are the primary concern:

• Terbutaline can cause clinically significant tachycardia, blood pressure changes, and ECG abnormalities (T-wave flattening, QTc prolongation, ST depression) 4
• Use with extreme caution in patients with coronary insufficiency, cardiac arrhythmias, or hypertension 4
• The 1.0 mg dose causes excessive tachycardia (25% increase) compared to 0.5 mg 5

For long-term subcutaneous use:

• Common side effects include painful subcutaneous nodules (one patient required withdrawal) 6
• Serious complications occur especially with central venous access devices 7
• Treatment duration ranged 2-40 months in successful cases 6

Avoid these common errors:

• Do not delay subcutaneous terbutaline in patients unable to cooperate with nebulizers—this wastes critical time 2
• Do not use doses exceeding 0.5 mg subcutaneously due to disproportionate cardiac effects 5
• Do not use long-term subcutaneous infusions outside specialized centers with appropriate monitoring capabilities 7
• Do not forget to administer systemic corticosteroids concurrently, as beta-agonists alone are inadequate 4

Recognize deterioration: If patients require more frequent terbutaline doses than usual, this signals asthma destabilization requiring treatment escalation, particularly addition of anti-inflammatory therapy. 4