The H’s and T’s are a well-known memory device throughout the house of medicine. The mnemonic has been used for decades and is designed to help clinicians sort through the reversible causes of cardiac arrest. But this device is deeply flawed, not only in its construction and usability as a memory aid, but in prioritizing the approach to cardiac arrest. 

The H’s and T’s

No commonly agreed upon number of H’s and T’s exists. In 2000, the American Heart Association formally published five H’s and five T’s in their journal, Circulation.¹ Since then, clinicians and educators have taken this device and adapted it significantly, often changing the number of diagnoses as well as what they stand for. The H’s and T’s suffer from a profound lack of consistency, limiting its use as a teaching device and as a bedside tool.

As a principle, memory aids should be easy to recall and offload cognitive burden. Mnemonics are rooted in the educational theory of chunking, a concept that encourages learners to memorize one device rather than multiple individual pieces of information.² While the H’s and T’s promote chunking, their use of only two letters to prompt recall of multiple different diagnoses is ineffective.

Instead of the H’s and T’s, we believe a structured, action-based approach to address cardiac arrest is a more effective bedside memory tool to consider the reversible causes of cardiac arrest. This is a novel perspective that is supported in the Anesthesia literature, where a similar discussion around reframing the causes of perioperative cardiac arrest in a more usable way has been suggested.³ We recommend the following framework:

1. Assign necessary roles
2. Perform bedside ultrasound for pericardial effusion and pneumothorax
3. Obtain a point-of-care blood gas
4. Consider systemic lytic therapy and/or extracorporeal membrane oxygenation (ECMO) (see Table 1)
   

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The 2025 American Heart Association Advanced Cardiovascular Life Support (ACLS) guidelines recommend that using point-of-care ultrasound may be considered to look for reversible causes. The text also states that although arterial blood gas may help clinicians individualize resuscitations, the limited evidence has yielded “uncertain benefits.”⁴

Action-Based Approach

1) Assign Necessary Roles

Numerous pieces have addressed the importance of optimal team dynamics for cardiac arrest.⁵ We recognize that personnel availability varies widely depending on the clinical setting. Some may have residents, fellows, attendings, pharmacists, nurses, technicians, and equipment acquisition personnel at the bedside, whereas some cardiac arrests may be run with ad hoc teams and minimal personnel.

Rather than suggest a rigid framework with specific personnel assignments, we suggest this mental framework of necessary roles when managing a patient in cardiac arrest:

• Airway management;
• Compressions;
• Defibrillator and monitor setup; and
• Vascular access and medications.

By assigning these roles, clinicians can cognitively offload multiple aspects of high-quality CPR and ACLS protocols in favor of focusing on interventions for reversible causes.

2) Ultrasound

We advocate for the use of point-of-care (POC) ultrasound in all cardiac arrest patients for evaluation of pneumothorax and pericardial effusion. Although there have been other proposed uses for ultrasound during cardiac arrest (transesophageal echocardiography for hand placement, evaluation of cardiac activity for prognostication, etc.), this approach is designed to address reversible causes.

3) Tamponade

Pericardial effusion may result from malignancy, uremia, inflammation, or infection, or it may be idiopathic. Because of the wide variety of risk factors and the often-limited history that accompanies patients in cardiac arrest, an effusion is most sensibly diagnosed with a POC cardiac ultrasound.

Although patients in pulseless electrical activity (PEA) who have organized cardiac activity may show sonographic evidence of cardiac tamponade, patients in ventricular arrhythmias or asystole are unable to exhibit this physiology. Therefore, any circumferential pericardial effusion in a pulseless patient should be considered for pericardiocentesis as some literature suggests higher rates of return of spontaneous circulation (ROSC).⁶

4) Pneumothorax

Pneumothoraces can be either the primary etiology of cardiac arrest or can result during resuscitation from ventilation and chest compressions. Classically, clinicians are taught to look for the presence of lung sliding as a sonographic finding to rule out pneumothorax. However, this can be impractical during active chest compressions and may need to be performed during pulse check. 

The presence of B-lines should also be considered a rule-out test for pneumothorax. B-lines are an artifact created by rapid reflections of sound waves through fluid-filled lungs, which definitionally requires uninterrupted connection between the visceral and parietal pleura, thus meaning no air is present in the pleural space. B-lines can often be seen during chest compressions, likely from lung consolidation, pulmonary edema, and/or alveolar hemorrhage from compressions.

• Check for B-lines during compressions.
• If unable to find B-lines, consider looking for lung sliding during pulse check, prior to proceeding to chest decompression.

5) Blood Gas

The information available from a POC blood gas can elucidate several reversible causes of cardiac arrest in a concise manner that may prompt lifesaving interventions without having to recall them individually. Metabolic derangements such as hyper- or hypokalemia, hypoglycemia, and hypocalcemia can all be readily assessed to guide medication administration and improve the outcome of a resuscitation. Some point-of-care devices also measure hemoglobin which may hint at an occult hemorrhagic etiology.

If vascular access via peripheral intravenous (IV) access is readily obtained, we argue that a venous blood gas should be sent at the time of access, as the clinical information from a venous sample has been shown to be comparable to arterial samples. We do not endorse spending undue time in a resuscitation attempting to obtain peripheral IV access if attempts have not been successful. If intraosseous (IO) access is the available route of medication administration, a rapid arterial stick can be performed to obtain similar clinical information. 

Current evidence and guidelines do not recommend routine administration of calcium and sodium bicarbonate in cardiac arrest.⁷ For patients with a history of end-stage renal disease (ESRD) and no POC blood gas capabilities, calcium is certainly reasonable. But for patients with a history of ESRD and where POC blood gas is available promptly, it is reasonable to check given the likely harm of calcium in an inappropriate setting.

6) Lytics, ECMO

When acute coronary syndrome or pulmonary embolism in particular is suspected as the etiology of arrest, the use of lytic agents should be considered. In routinely doing so, one inherently considers these two reversible causes of cardiac arrest as part of their differential diagnosis without having to recall if they are missing a letter in the H’s and T’s mnemonic. 

Whether in a highly resourced setting or not, it is valuable to consider the possibility of invoking more advanced measures beyond that of advanced cardiovascular life support in every case of cardiac arrest. Learners will practice in a wide variety of clinical settings after their training, so instilling a foundational paradigm that includes advanced capabilities is still necessary for all trainees. Practically, in hospitals with extracorporeal cardiopulmonary resuscitation (ECPR) programs (utilizing extracorporeal membrane oxygenation [ECMO] for cardiac arrest patients), early activation of these resources is paramount. With the growing availability of these resources, we propose including it in the algorithm as applicable to individual practice settings.⁸

Conclusion

Clinicians are taught countless memory devices throughout their education, though few provide action-based frameworks that can be invoked in high-stress scenarios. Standardized approaches that prompt action rather than requiring active recall of facts can streamline learning, reduce extraneous cognitive load, and allow clinicians to direct their attention to pertinent parts of patient care. This four-step action-based approach to cardiac arrest is a teaching tool that can be deployed at the bedside in real time to impact both learning and patient care.

Dr. Grossman is education faculty at Harlem Hospital Center and an assistant professor of clinical emergency medicine at Columbia University Irving Medical Center. He focuses on innovating new approaches to clinical education and demystifying complex concepts for learners of all levels. 

Dr. Rosenthal is a critical care fellow at New York Presbyterian Columbia University Medical Center. She completed her emergency medicine residency training at Mount Sinai Hospital and Elmhurst Hospital Center. 

Dr. DeVivo is an assistant professor of emergency medicine and holds appointments in both the Department of Emergency Medicine and the Institute for Critical Care Medicine at the Icahn School of Medicine at Mount Sinai Hospital. Over the past five years, he has designed, developed, and now leads a novel approach to combined emergency medicine and critical care medicine training. He is the program director of this combined training program and continues to both teach and attend in the emergency department, as well as the Surgical and Transplant ICUs at Mount Sinai Hospital. 

References

1. Wyckoff M, Singletary EM, Soar J, et al. International Consensus on Cardiopulmonary Resuscitation and Emergency Cardiovascular Care Science With Treatment Recommendations: Summary From the Basic Life Support; Advanced Life Support; Neonatal Life Support; Education, Implementation, and Teams; First Aid Task Forces; and the COVID-19 Working Group, Circulation. 2021; 145(9): (e645-e721).
2. Lewis JB Jr., Mulligan R, and Kraus N. (2018). The Pharos. Winter. The importance of medical mnemonics in medicine. pp. 30-42. 
3. Meng L, Rasmussen M, Abcejo AS, et al. Causes of perioperative cardiac arrest: mnemonic, classification, monitoring, and actions. Anesth Analg. 2024; 138 (6): 1215-1232.
4. Wigginton JG, Agarwal S, Bartos JA, et al. Part 9: Adult advanced life support: 2025 American Heart Association guidelines for cardiopulmonary resuscitation and emergency cardiovascular care. Circulation. 2025;152 (16) Suppl. 2.
5. Meaney PA, Bobrow BJ, Mancini ME, et al. Cardiopulmonary resuscitation quality: Improving cardiac resuscitation outcomes both inside and outside the hospital: a consensus statement from the American Heart Association. Circulation. 2013;128(4): 417-435.
6. Ávila-Reyes D, Acevedo-Cardona AO, Gómez-González JF, et al. Point-of-care ultrasound in cardiorespiratory arrest (POCUS-CA): Narrative review article. The Ultrasound Journal, 2021; 13(46).
7. Link MS, Berkow LC, Kudenchuk PJ, et al. Part 7: Adult Advanced Cardiovascular Life Support: 2015 American Heart Association Guidelines Update for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation. 2015; 132(18, Suppl. 2), S444–S464.
8. Vallentin MF, Granfeldt A, Meilandt C, et al., Effect of Intravenous or Intraosseous Calcium vs Saline on Return of Spontaneous Circulation in Adults With Out-of-Hospital Cardiac Arrest: A Randomized Clinical Trial. JAMA. 2021;326(22):2268–2276.