Hemodynamic Effects of 0.5 mg IM Adrenaline on Heart Rate
Intramuscular adrenaline 0.5 mg typically increases heart rate by approximately 2-10 beats per minute in adults, though the primary therapeutic effect is on blood pressure rather than heart rate, with mean arterial pressure rising by 13-30 mmHg. 1, 2
Expected Heart Rate Response
The heart rate response to IM adrenaline is modest and variable:
In critical care transport patients receiving push-dose epinephrine (10-20 µg IV), the median heart rate increase was only 2.0 beats per minute (range -1.0 to 9.3 bpm). 1 While this was IV administration, it demonstrates that epinephrine's chronotropic effect is less pronounced than its pressor effect.
Pediatric data shows more variability, with children ≤2 years old experiencing the greatest percentage increase in heart rate following low-dose epinephrine boluses. 3 However, adult responses are typically more blunted.
The primary hemodynamic effect is on blood pressure, not heart rate—mean arterial pressure increases by 13-30 mmHg following epinephrine administration. 1, 2
Pharmacokinetic Considerations
IM injection in the vastus lateralis achieves peak plasma concentrations in 8±2 minutes, compared to 34±14 minutes with subcutaneous deltoid injection. 4, 5 This rapid absorption explains why effects manifest within minutes of administration.
Clinical Context Matters
The heart rate response depends heavily on the underlying clinical scenario:
In anaphylaxis (the primary indication for 0.5 mg IM adrenaline), patients may be bradycardic or tachycardic at baseline, and epinephrine works to normalize cardiovascular function rather than simply increase heart rate. 6
In hypotensive states, the baroreceptor reflex may be impaired, leading to variable chronotropic responses. 1
Patients on beta-blockers may have blunted heart rate responses to epinephrine, though the alpha-adrenergic effects on blood pressure remain intact. 6
Dose-Response Relationship
Higher doses of epinephrine produce greater hemodynamic effects—doses ≥0.2 µg/kg resulted in significantly greater blood pressure increases (30 mmHg vs 10 mmHg) compared to lower doses. 2 However, the standard 0.5 mg IM dose (approximately 0.007 mg/kg in a 70 kg adult) is designed for safety and efficacy in anaphylaxis, not maximal chronotropic effect.
Critical Safety Points
The maximum single IM dose is capped at 0.5 mg for adults to minimize cardiac adverse effects, as higher doses increase arrhythmia risk without improving outcomes. 4
There is no absolute contraindication to epinephrine in anaphylaxis, even in elderly patients or those with cardiac disease—the risk of death from untreated anaphylaxis far exceeds epinephrine-related risks. 4, 5
Severe hypertension (SBP >220 mmHg) is rare following appropriate IM dosing—no cases occurred in a series of 96 prehospital administrations. 2
Common Pitfalls
Do not expect dramatic heart rate increases—epinephrine's primary benefit in hypotension is vasoconstriction and increased cardiac contractility, not chronotropy. 1, 2
Avoid confusing IM dosing (1:1000 concentration, 0.5 mg maximum) with IV dosing (1:10,000 concentration)—using 1:1000 IV can cause fatal overdose. 7
The effect is short-lived (minutes), necessitating repeat dosing every 5-15 minutes if symptoms persist. 4, 5