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Show full transcript for ACLS Secondary Survey H's and T's video

In this lesson, we'll be going over the most common causes of cardiac arrest, which we touched on briefly at the end of the last lesson, as presented as what's commonly referred to as the H's and T's.

This lesson will include a little information on common treatments for specific H's and T's, and at the end of the lesson, we'll provide you with a Word about diagnosing and treating underlying causes.

Common Causes of Cardiac Arrest – the H's

  1. Hypovolemia – can often be corrected with fluid replacement.
  2. Hypoxia – can be corrected with appropriate oxygenation and tissue perfusion.
  3. Hydrogen ion or acidosis (respiratory or metabolic) – if respiratory, you can correct it with oxygen and respirations, and if metabolic, you might need sodium bicarbonate to correct it.
  4. Hypokalemia – when dealing with hypokalemia, you may need to administer potassium.
  5. Hyperkalemia – when dealing with hyperkalemia, you need to administer calcium chloride.
  6. Hypothermia

Pro Tip #1: It's important to remember that with hypokalemia, you may see flat T-waves on the ECG, as well as something called U-waves. If you do see these, administer potassium magnesium per the protocols.

Common Causes of Cardiac Arrest – the T's

  1. Tension pneumothorax – can often be relieved with needle decompression and later with a chest tube.
  2. Cardiac tamponade – this would require surgical intervention to correct.
  3. Toxins
  4. Pulmonary thrombosis – this would require a corrective procedure or thrombolytic therapy.
  5. Coronary thrombosis – the same as above is applicable, but additionally, treatment may also include percutaneous coronary intervention, commonly known as PCI.

Pro Tip #2: Percutaneous Coronary Intervention, or PCI, (formerly known as angioplasty with stent) is a non-surgical procedure that uses a catheter to place a small structure called a stent to open up blood vessels in the heart that have been narrowed by plaque buildup, a condition known as atherosclerosis.

Warning: It's important to note that the most common causes of pulseless electrical activity (PEA) are hypoxia and hypovolemia, and both are potentially reversible. Which is why it's vital to look for evidence of these problems when assessing your patients.

A Word About Diagnosing and Treating Underlying Causes

Patients in cardiac arrest, such as VFib, pulseless V-tach, asystole, and PEA, require rapid assessment and management, as their cardiac arrest may be caused by an underlying and potentially reversible issue or condition.

If you can quickly identify a specific condition that has caused or contributed to PEA and correct it, you may achieve ROSC. The identification of the underlying cause is extremely important in cases of PEA and asystole. When you're searching for the underlying cause, consider the following:

  • Consider frequent causes of PEA by recalling the H's and T's
  • Analyze the ECG for clues to the underlying cause
  • Recognize hypovolemia
  • Recognize drug overdose and/or poisoning


Hypovolemia is a common cause of PEA and initially produces the classic physiologic response of a rapid, narrow-complex tachycardia. And it typically produces increased diastolic and decreased systolic pressures.

As the loss of blood volume continues, blood pressure will drop and will eventually become undetectable. However, the narrow QRS complexes and rapid rate will continue.

You should consider hypovolemia as a cause of hypotension, which can deteriorate to PEA. Providing quick treatment can often reverse this pulseless state by rapidly correcting the hypovolemia. Common nontraumatic causes of hypovolemia can include occult internal hemorrhage and severe dehydration.

Cardiac and Pulmonary Conditions

Acute coronary syndromes involving a large amount of heart muscle can present as PEA. That is, occlusion of the left main or proximal left anterior descending coronary artery can present with cardiogenic shock rapidly progressing to cardiac arrest and PEA.

However, in patients with cardiac arrest and without known pulmonary embolism, routine fibrinolytic treatment provided during CPR shows no benefit and is therefore not recommended.

Massive or saddle pulmonary embolism obstructs flow to the pulmonary vasculature and causes acute right heart failure. In patients with cardiac arrest due to presumed or known pulmonary embolism, you should consider administering fibrinolytics.

Pericardial tamponade may be a reversible condition. In the peri-arrest period, volume infusion in this condition may help while definitive therapy is initiated. Tension pneumothorax can often be effectively treated once recognized.

Drug Overdoses or Toxic Exposures

Certain drug overdoses and toxic exposures may lead to peripheral vascular dilatation and/or myocardial dysfunction with resultant hypotension. Your approach to poisoned patients should be aggressive, as the toxic effects can progress rapidly and may be of limited duration.

In these situations, myocardial dysfunction and arrhythmias may be reversible. Numerous case reports confirm the success of many specific limited interventions with one thing in common: they buy time.

Treatments that can provide this level of support include:

  • Prolonged basic CPR in special resuscitation situations
  • Extracorporeal CPR
  • Intra-aortic balloon pumping
  • Renal dialysis
  • Intravenous lipid emulsion
  • Specific drug antidotes, such as digoxin immune Fab, glucagon, and bicarbonate
  • Transcutaneous pacing
  • Correction of severe electrolyte disturbances, such as potassium, magnesium, calcium, and acidosis
  • Specific adjunctive agents

It's important to note that if the patient shows signs of ROSC, post-cardiac arrest care should be initiated.