What are the differences in clinical use between terbutaline and nitroglycerin?

Pharmacology and Clinical Use: Terbutaline vs Nitroglycerin

Fundamental Pharmacologic Differences

Terbutaline and nitroglycerin have completely different mechanisms of action and clinical applications—terbutaline is a beta-2 adrenergic agonist used primarily for bronchodilation and uterine relaxation, while nitroglycerin is a nitrate vasodilator used for cardiac ischemia, heart failure, and hypertension. 1

Terbutaline Mechanism and Effects

• Beta-2 selective adrenergic agonist that causes smooth muscle relaxation in bronchi and uterus 2
• Produces bronchodilation with significant improvement in airflow rates in both major and smaller airways 2
• Causes uterine relaxation (tocolysis) by activating beta-2 receptors in myometrium 3, 4
• Cardiovascular effects include tachycardia and palpitations as common side effects due to some beta-1 receptor activity 3, 5
• Onset of action: 5 minutes when given subcutaneously or by aerosol 2

Nitroglycerin Mechanism and Effects

• Endothelium-independent vasodilator that dilates both venous capacitance vessels (reducing preload) and arterial vessels (reducing afterload) 1
• Reduces myocardial oxygen demand while enhancing oxygen delivery through coronary artery dilation 1
• Causes venodilation leading to decreased blood pressure, which can compromise perfusion if excessive 6
• Dilates epicardial coronary arteries and promotes collateral flow to ischemic regions 1
• Onset of action: 1-7 minutes sublingually; immediate with IV administration 1

Clinical Applications

Terbutaline Primary Uses

• Acute bronchospasm treatment: 0.25-0.50 mg subcutaneously or 0.375 mg by aerosol provides highly effective relief within 5 minutes 2
• Maintenance asthma therapy: Oral terbutaline 2.5 mg every 6-8 hours, adjusted based on clinical response 2
• Acute intrapartum fetal resuscitation: 250 mcg IV provides successful resuscitation in 71.9% of cases with nonreassuring fetal heart rate tracings 4
• External cephalic version: 0.25 mg subcutaneous terbutaline achieves 55% success rate for version at term 3

Nitroglycerin Primary Uses

• Unstable angina/NSTEMI: IV nitroglycerin 5-200 mcg/min for patients with ongoing ischemic symptoms not relieved by sublingual doses 1
• Acute heart failure with pulmonary edema: High-dose IV nitroglycerin (3 mg isosorbide dinitrate every 5 minutes) combined with low-dose furosemide reduces intubation rates (13% vs 40%) and myocardial infarction (17% vs 37%) compared to low-dose nitrates 1
• Acute coronary syndromes with hypertension: IV nitroglycerin for relieving ischemic pain and managing pulmonary congestion 6
• Uterine relaxation for retained placenta: Nitroglycerin serves as an alternative to terbutaline or halogenated anesthetics for uterine relaxation during placental removal 1

Head-to-Head Comparisons

For Uterine Relaxation (External Cephalic Version)

Terbutaline is superior to nitroglycerin for external cephalic version, achieving significantly higher success rates (55% vs 23%, P=0.01) 3. However, terbutaline causes more palpitations (17.2% vs 0%, P=0.02) and higher maternal heart rates 3.

For Acute Intrapartum Fetal Resuscitation

Both agents show equivalent efficacy for acute fetal resuscitation (terbutaline 71.9% vs nitroglycerin 64.2%, P=0.38) 4. However:

• Terbutaline provides superior tocolysis with lower contraction frequency (2.9 vs 4 per 10 minutes, P<0.002) and reduced tachysystole (1.8% vs 18.9%, P=0.003) 4
• Nitroglycerin causes significant maternal hypotension (mean arterial pressure drops from 81 to 76 mmHg, P=0.02), while terbutaline does not affect blood pressure 4

Critical Safety Considerations

Terbutaline Warnings

• Dose-dependent cardiovascular side effects including tachycardia and palpitations 2, 3
• Continuous nebulization produces similar plasma concentrations and cardiovascular effects as intermittent dosing, with peak heart rate increases of approximately 20 bpm 5
• No evidence of serious myocardial complications (CPK-MB elevation) even with prolonged nebulization 5

Nitroglycerin Warnings

• Contraindicated within 24 hours of sildenafil or 48 hours of tadalafil use 1
• Contraindicated with hypotension due to risk of further blood pressure reduction 1
• Rapid tolerance (tachyphylaxis) develops within 24 hours with continuous use, particularly with transdermal patches 6
• Risk of excessive blood pressure reduction compromising organ perfusion 6
• Venodilating effects can cause cardiovascular decompensation through decreased preload, especially when combined with anesthetic agents 1

Intraoperative Nitroglycerin Considerations

• Prophylactic intraoperative nitroglycerin has unclear benefit (Class IIb recommendation) for preventing myocardial ischemia in high-risk noncardiac surgery patients 1
• May lead to cardiovascular decompensation through preload reduction 1
• Should only be used when hemodynamic effects of other agents and intravascular volume status are considered 1

Route-Specific Pharmacokinetics

Terbutaline

• Inhalation route offers fastest onset, maximal response, and longest duration compared to subcutaneous or oral routes 2
• Subcutaneous and oral routes show similar trends but decreased magnitude of effect 2
• No definite dose-response dependency within tested aerosol and oral dosages 2

Nitroglycerin

• IV administration preferred over transdermal for acute situations due to more predictable absorption 1
• Sublingual/oral isosorbide dinitrate provides 8-12 times longer duration (3-4 hours) than sublingual nitroglycerin (15-30 minutes) 7
• Transdermal patches develop rapid tolerance and provide uneven absorption intraoperatively 1, 6