ACLS · Reversible causes · 8 cards
H’s and T’s of ACLS: spotting the reversible cause
The H’s and T’s of ACLS are the ten reversible causes of cardiac arrest, five starting with H and five with T. The 2025 AHA Guidelines for CPR and Emergency Cardiovascular Care ask you to hunt for them in every arrest, above all in PEA and asystole. Each card below hides one.
Find the cause
What is the most appropriate immediate treatment for a patient in cardiac arrest due to hyperkalemia?
Explanation
Keyed answerA. Administration of calcium chloride or calcium gluconate
Your pick
Among these options, calcium (chloride or gluconate) is the immediate drug aimed at the heart: it stabilizes the myocardial cell membrane against the effects of a high potassium while other treatments (insulin with glucose, bicarbonate in selected cases) shift potassium into cells. The 2025 AHA Guidelines note that calcium's effectiveness in hyperkalemic cardiac arrest is not well established, so it supports, and never replaces, high-quality CPR and treating the cause.
What is the appropriate use of sodium bicarbonate during cardiac arrest?
Explanation
Keyed answerB. For known pre-existing hyperkalemia, bicarbonate-responsive acidosis, or tricyclic antidepressant overdose
Your pick
Sodium bicarbonate during cardiac arrest should be considered for specific situations: known pre-existing hyperkalemia, known pre-existing bicarbonate-responsive acidosis, or tricyclic antidepressant overdose. It is not recommended for routine use in cardiac arrest as it can worsen intracellular acidosis, produce excess CO2 that diffuses into ischemic myocardial cells, and cause hypernatremia and hyperosmolarity.
What is the appropriate management of cardiac arrest due to suspected opioid overdose?
Explanation
Keyed answerC. Follow standard ACLS protocols while also administering naloxone
Your pick
For cardiac arrest due to suspected opioid overdose, providers should follow standard ACLS protocols while also administering naloxone. Naloxone will not treat the cardiac arrest itself but may prevent recurrence if ROSC is achieved. CPR and other ACLS interventions should not be delayed to administer naloxone.
What is an important consideration when selecting equipment for needle decompression of tension pneumothorax during ACLS?
Explanation
Keyed answerA. Select a catheter of adequate length (at least 5-8 cm) to reach the pleural space
Your pick
An important consideration when selecting equipment for needle decompression of tension pneumothorax is needle length. Standard IV catheters (usually 14-16 gauge) are often too short, especially in larger patients. Current recommendations suggest using longer catheters (at least 5-8 cm) to successfully reach the pleural space through the chest wall, at the 2nd intercostal space in the midclavicular line or the 4th–5th intercostal space just anterior to the midaxillary line (now preferred in adults).
Why is it essential to assess serum potassium in a patient with bradycardia showing no response to atropine?
Explanation
Keyed answerC. Correcting hyperkalemia may resolve bradycardia without the need for pacing
Your pick
Hyperkalemia is a reversible cause of bradycardia and must be corrected to restore normal conduction without unnecessary invasive interventions.
What is the appropriate dose of naloxone for reversal of opioid overdose in an adult with respiratory depression?
Explanation
Keyed answerB. 0.4-2 mg IV/IO/IM/IN, titrated to adequate respirations and level of consciousness
Your pick
The appropriate initial dose of naloxone for reversal of opioid overdose in an adult with respiratory depression is 0.4-2 mg IV/IO/IM/IN, titrated to adequate respirations and level of consciousness. The goal is to restore adequate respirations without precipitating withdrawal or pain.
What is the role of point-of-care ultrasound (POCUS) during cardiac arrest resuscitation?
Explanation
Keyed answerD. To identify potentially reversible causes of arrest and assess for cardiac activity during planned pauses
Your pick
Point-of-care ultrasound (POCUS) during cardiac arrest can help identify potentially reversible causes such as cardiac tamponade, pneumothorax, hypovolemia, or pulmonary embolism. It can also detect the presence or absence of cardiac activity, which may inform prognosis and decisions regarding continuation of resuscitation efforts. POCUS should be performed during planned pauses and should not interrupt CPR for more than 10 seconds.
What is the pathophysiological mechanism for the development of tachycardia in a patient with acute pulmonary embolism?
Explanation
Keyed answerD. Decreased cardiac output resulting in compensatory sympathetic nervous system activation
Your pick
In acute pulmonary embolism, obstruction of pulmonary vessels causes increased pulmonary vascular resistance, right ventricular strain, and decreased left ventricular filling. This leads to decreased cardiac output, triggering sympathetic nervous system activation and tachycardia as compensatory mechanisms to maintain tissue perfusion.
0%
Answer a few cards and come back to this one.
- Cardiac arrestnot yet
- Brady & tachynot yet
- Rhythms & drugsnot yet
Reread a low topic: Algorithms · Pretest
The thin line on each bar marks 84%, the written-exam pass mark AHA Training Centers usually quote; your instructor confirms the exact rule.
Pick an answer and the card turns over to the explanation. Keys: A to D or 1 to 4, arrows to move, F to turn the card.
5H · 5T
All ten H's and T's of ACLS in one table
The list has been the same ten causes for years, and the 2025 adult cardiac arrest algorithm keeps it unchanged. What the exam tests is not the list itself but whether you can find the cause hidden in a case: a dialysis history, a recent operation, a ventilator alarm. The middle column is what to look for.
| Cause | The clue in the question stem | What fixes it (the exam answer) |
|---|---|---|
| Hypovolemia | Bleeding, trauma, GI bleed; flat neck veins; a fast, narrow PEA | Volume: fluids or blood, and stop the bleeding |
| Hypoxia | Asthma, drowning, choking or a displaced tube before the arrest; the rate slows first | Oxygen and ventilation; confirm the airway with waveform capnography |
| Hydrogen ion (acidosis) | Diabetic ketoacidosis, kidney failure, sepsis, a long down time; low pH | Good ventilation and CPR, treat the cause; bicarbonate only for a specific indication, never routinely |
| Hypo-/hyperkalemia | Missed dialysis, crush injury; peaked T waves, wide QRS. Low: diuretics, vomiting | High: calcium and a potassium shift (insulin with glucose). Low: potassium (and magnesium) replacement |
| Hypothermia | Cold exposure, immersion, found outdoors; low core temperature | Standard ACLS while you actively rewarm; do not stop early because the patient is cold |
| Tension pneumothorax | Chest trauma, a new central line, rising ventilator pressure; one-sided absent breath sounds, distended neck veins | Needle decompression, then a chest tube |
| Tamponade, cardiac | Penetrating chest wound, recent heart surgery; distended neck veins, muffled heart sounds, effusion on ultrasound | Pericardiocentesis (or surgical drainage) |
| Toxins | Opioid use, pinpoint pupils; tricyclic overdose with a wide QRS; beta-blocker or calcium-channel-blocker overdose | Standard ACLS plus the specific antidote: naloxone for opioids (without delaying CPR), bicarbonate for tricyclics; a toxicologist for the rest |
| Thrombosis, pulmonary | Recent surgery, a swollen leg, sudden breathlessness then collapse; large right ventricle on ultrasound | Fibrinolytics or embolectomy when PE is the likely cause |
| Thrombosis, coronary | Chest pain before the collapse, ST elevation, VF that keeps coming back | After return of circulation: emergency coronary angiography and reperfusion |
PEA
Treatment for pulseless electrical activity
Treatment for pulseless electrical activity is short to state: high-quality CPR, epinephrine 1 mg IV/IO as soon as you can and then every 3–5 minutes, a rhythm check every 2 minutes, and no shock. That holds the patient while you look for the reason. PEA means the heart’s electrical system still works, so something mechanical or metabolic is stopping the pump, which is exactly what the H’s and T’s list.
Our drawing, after the 2025 adult cardiac arrest algorithm
- No pulse, organized rhythm on the monitor: PEA
- CPR 2 min, IV/IO access, epinephrine 1 mg as soon as possible
- Search the H’s and T’s during every cycle: history, monitor, ultrasound if it does not pause compressions
- Rhythm shockable at the 2-minute check?
Yes
- Shock, then the VF/pVT side of the algorithm
No
- CPR 2 min, epinephrine every 3–5 min, treat the cause you found
- Back to the rhythm check
Asystole runs the same loop. The full arrest algorithm, including the shockable side, is on the ACLS algorithms page.
Cases
Find the cause: four short cases
The deck above has 8 cards on hyperkalemia, toxins, tension pneumothorax, pulmonary embolism and ultrasound in arrest. These four short cases cover the causes the deck leaves out. Read the stem, name the cause before you read the bold word, then the fix.
01Collapse three days after heart surgery
Distended neck veins, quiet heart sounds, an organized rhythm without a pulse. The clue is the recent surgery plus the veins: tamponade. The fix is pericardiocentesis; epinephrine alone will not refill a squeezed heart.
02Pulled from cold water
Core temperature far below normal, VF on the monitor. Hypothermia: run standard ACLS, shocks and epinephrine included, while active rewarming starts, and do not call the code early because the skin is cold.
03Asthma attack, then arrest
Hours of wheezing, then slowing heart rate and PEA. Hypoxia: oxygen, an airway you have confirmed with capnography, and ventilation without over-breathing.
04Crushing chest pain, then VF that keeps returning
The story points at coronary thrombosis. Defibrillate and run the algorithm; once circulation returns, persistent ST elevation or recurrent VF sends the patient for emergency coronary angiography.
Every Megacode case can hide one of these. Practice saying the cause out loud as team leader in the Megacode practice cases.
Memory
How to memorize the H's and T's
Most people can recite eight of the ten and lose the last two under pressure. Grouping helps more than a rhyme:
A memory card for the 5 H's and 5 T's
- H: what the blood carries
- Volume (hypovolemia), oxygen (hypoxia), acid (hydrogen ion), salt (potassium)
- H: the body itself
- Temperature (hypothermia)
- T: something squeezes the heart
- Tension pneumothorax, tamponade
- T: something blocks a vessel
- Pulmonary and coronary thrombosis
- T: something was swallowed or injected
- Toxins
Run the list in that order in your head at every 2-minute rhythm check, and write the H’s and T’s on the inside of your hand for the first few practice cases. By the third case the order sticks.
Traps
Exam traps around reversible causes
Reversible causes sit inside the cardiac arrest topic, the biggest share of the written exam in our estimate. Mixed cards on it are in the ACLS practice questions, and the ACLS study guide shows where the topic fits in a week of study.
FAQ
Questions people ask
What are the 5 H's and 5 T's of ACLS?
The five H’s are hypovolemia, hypoxia, hydrogen ion (acidosis), hypo- or hyperkalemia and hypothermia. The five T’s are tension pneumothorax, cardiac tamponade, toxins, pulmonary thrombosis and coronary thrombosis.
Are the H's and T's only for PEA?
No. They apply to every cardiac arrest, including VF that keeps returning after shocks. PEA and asystole are where the exam asks about them most, because finding the cause is often the only way out.
What are the three shockable rhythms?
ACLS names two shockable arrest rhythms: ventricular fibrillation and pulseless ventricular tachycardia. Lists of three usually add polymorphic VT (torsades), which, without a pulse, is defibrillated like VF.
How is PEA treated in ACLS?
CPR, epinephrine 1 mg IV/IO as soon as possible and every 3–5 minutes after that, rhythm checks every 2 minutes, no shock, and a search for the reversible cause.
What changed for the H's and T's in 2025?
The list did not change. Under the 2025 AHA Guidelines for CPR and Emergency Cardiovascular Care, ultrasound to find a cause may be considered by experienced providers if it does not interrupt CPR, and routine calcium or bicarbonate is still not part of the arrest algorithm.
One more cycle, then rest
Nobody walks into the ACLS exam knowing every dose cold, and you do not need to. Ten cards a day, a look at the topic you miss most, then the next ten.