Several clinical scenarios require specific alterations to the standard ACLS cardiac arrest algorithm. The American Heart Association’s 2025 guidelines identify more than two dozen special circumstances, including hypothermia, pregnancy, drowning, hyperkalemia, and various poisonings, where following the default algorithm without modification can reduce a patient’s chance of survival. The most commonly tested and clinically relevant alterations involve changes to drug timing, shock delivery, airway priorities, or the addition of targeted interventions not found in the standard protocol.
Hypothermia: Withhold Drugs Below 86°F
Severe hypothermia is one of the clearest examples of a modified ACLS approach. When a patient’s core temperature drops below 30°C (86°F), the heart becomes increasingly unresponsive to defibrillation and medications. The key alterations are:
- Withhold epinephrine and amiodarone entirely if the core temperature is below 30°C.
- Limit defibrillation attempts to three shocks. If ventricular fibrillation persists after three shocks and the temperature remains below 30°C, delay further attempts until the core temperature rises above that threshold.
- Double the epinephrine interval when the temperature is between 30°C and 35°C, giving it every 6 to 10 minutes instead of the standard 3 to 5 minutes.
The rationale is that a cold heart metabolizes drugs poorly and is electrically unstable. Rewarming is the definitive treatment, and resuscitation efforts should continue longer than usual because hypothermic patients have survived neurologically intact after prolonged arrests. The old advice to avoid moving hypothermic patients rapidly is no longer recommended. Getting the patient to a rewarming facility quickly is now considered more important than transport position.
Drowning: Prioritize Breathing Over Compressions
Standard BLS and ACLS sequences emphasize chest compressions first. Drowning flips that priority. Because the underlying problem is oxygen deprivation rather than a primary cardiac event, airway and breathing take precedence over compressions. Current guidelines recommend 5 initial rescue breaths before starting chest compressions, compared with the usual 2 breaths in standard BLS. Supplemental oxygen should be given as soon as it is available. The rest of the ACLS algorithm then proceeds normally, but recognizing that restoring oxygenation early is the single most important factor in neurological recovery.
Pregnancy: Uterine Displacement and Early Delivery
Cardiac arrest in a pregnant patient requires two simultaneous modifications. First, continuous manual left uterine displacement must be maintained throughout CPR. In late pregnancy, the uterus compresses the large blood vessels that return blood to the heart, making chest compressions ineffective unless the uterus is shifted to the left side. This is done by having someone physically push the uterus toward the patient’s left.
Second, if there is no return of spontaneous circulation within 5 minutes, a perimortem cesarean delivery should be performed. This is not primarily to save the fetus, though that may happen. Emptying the uterus relieves the compression on major blood vessels and dramatically improves the effectiveness of CPR for the mother. Teams should begin preparing for the procedure as soon as the arrest is recognized, since the 5-minute window is tight.
Opioid Overdose: Ventilation First, Not Naloxone
A common misconception is that naloxone (the opioid reversal agent) should be the first-line treatment during opioid-associated cardiac arrest. It is not. In a patient who is confirmed pulseless and receiving standard resuscitation with assisted ventilation, naloxone is unlikely to help and has a theoretical basis for harm during a period of low oxygen and high acid levels in the blood.
The appropriate alteration here is to prioritize aggressive ventilation and oxygenation alongside standard CPR. Naloxone is valuable for patients who still have a pulse but are breathing poorly or not at all. When it is uncertain whether a patient is truly pulseless, giving naloxone alongside CPR is reasonable. But for confirmed cardiac arrest, standard ACLS with an emphasis on high-quality ventilation is the correct approach.
Hyperkalemia: Calcium and Potassium-Lowering Agents
When dangerously high potassium levels cause cardiac arrest, the standard ACLS drugs alone will not fix the underlying electrical problem. The critical addition is intravenous calcium, which stabilizes the heart’s electrical activity within minutes. This is typically given as calcium gluconate or calcium chloride and should be administered early in the resuscitation. Beyond calcium, treatments that shift potassium out of the bloodstream are given simultaneously: insulin paired with glucose (to prevent dangerously low blood sugar) and sodium bicarbonate. In refractory cases, dialysis during CPR has been reported to achieve successful resuscitation. The rest of the ACLS algorithm continues, but without addressing the potassium, defibrillation and epinephrine are unlikely to restore a stable rhythm.
Post-Cardiac Surgery: Resternotomy Within 5 Minutes
Patients who arrest shortly after open-heart surgery follow a distinctly different protocol. The most important alteration is that emergency resternotomy, reopening the chest, should happen within 5 minutes if initial interventions fail. This allows for internal cardiac massage and internal defibrillation at much lower energy levels (20 joules compared to the standard 120 to 200 joules used externally).
Other key differences in this population:
- Avoid standard-dose epinephrine. The usual 1 mg dose risks causing severe high blood pressure on return of circulation, which can tear surgical connections on the heart and blood vessels and cause catastrophic bleeding.
- Use epicardial pacing wires. Most post-surgical patients have temporary pacing wires already in place. For bradycardia, asystole, or pulseless electrical activity, these wires should be activated immediately. A pacing pause should also be performed because an active pacemaker can mask underlying ventricular fibrillation.
- Attempt three rapid defibrillation shocks before moving to resternotomy if the rhythm is shockable.
Local Anesthetic Toxicity: Lipid Emulsion Therapy
When cardiac arrest results from an overdose of local anesthetics (a condition called local anesthetic systemic toxicity), the standard ACLS algorithm alone is insufficient. The specific alteration is the administration of 20% intravenous lipid emulsion, sometimes called “lipid rescue.” For patients over 70 kg, a rapid 100 mL bolus is given, followed by a 200 to 250 mL infusion over 15 to 20 minutes. For smaller patients, the bolus is 1.5 mL/kg of lean body weight, followed by an infusion at 0.25 mL/kg per minute. The bolus can be repeated and the infusion rate doubled if the heart remains unstable, up to a total dose of about 12 mL/kg. The lipid acts as a “sink,” pulling the fat-soluble anesthetic molecules away from heart tissue and restoring normal electrical function.
Anaphylaxis: Epinephrine Dosing Differs
Anaphylaxis-induced cardiac arrest requires epinephrine, which is already part of the standard ACLS algorithm, but the dosing strategy and route may differ from the typical 1 mg intravenous push given during standard arrest. Intramuscular epinephrine at a concentration of 1:1,000 is the first-line treatment for anaphylaxis before arrest occurs. If the patient progresses to cardiac arrest and intravenous access is available, an epinephrine infusion titrated to response is preferred over repeated bolus doses. The goal is aggressive volume resuscitation alongside epinephrine, since anaphylaxis causes massive fluid shifts out of the blood vessels. Recognizing anaphylaxis as the cause of arrest is critical because the addition of high-volume IV fluids and the avoidance of under-dosing epinephrine can make the difference between successful and failed resuscitation.
How to Think About These Alterations
The standard ACLS algorithm assumes a primary cardiac cause of arrest: a heart that has stopped due to an intrinsic rhythm problem. Every alteration listed above exists because something else is driving the arrest, whether it is temperature, oxygen debt, a toxin, a mechanical obstruction, or an electrolyte imbalance. The underlying principle is always the same: identify and treat the reversible cause while continuing high-quality CPR. Standard ACLS continues in the background, but the modification targets the specific reason the heart stopped, which the default algorithm was not designed to address.