We all learn the Hs and Ts in medical school, but few people find them practical. It's a lot of letters and things to remember, and systematically working through them is inefficient and cognitively burdensome, especially during the chaos of running a code. The Hs and Ts are among the least useful of the mnemonics, comparable with the infamous AEIOU-TIPS or MUDPILES.
A much simpler mnemonic addresses the reversible causes of cardiac arrest, which instead of listing the numerous reversible causes, provides a checklist of concrete actions which, if made standard during a resuscitation, will cover each of the Hs and Ts: ABCD-Ultrasound.
A: Airway and Access
Hs and Ts equivalent: Hypovolemia/hemorrhage, hydrogen ions, hypoxia
Establishing an airway and providing proper ventilation (no more than one breath every six seconds, or 10 breaths a minute) is the mainstay of addressing acidosis (hydrogen ions) and hypoxia. Good evidence suggests that supraglottic devices are at least equivalent to endotracheal tubes. (JAMA. 2018;320:779; https://bit.ly/3uQL0Hy; Am J Emerg Med. 2018;36:2298.) Some evidence even holds that bag-mask ventilation is adequate. (JAMA. 2013;309:257; https://bit.ly/3dkjCM1.)
Establishing access serves as a reminder to run fluids wide open to address hypovolemia in undifferentiated cardiac arrest. Consider giving uncrossmatched blood if your physical exam suggests hemorrhage as a cause (trauma or GI bleed); hemorrhaging patients need blood, not crystalloid. (Emerg Med Pract. 2011;13:1.) Intraosseous access provides a faster, more successful, and safer alternative to crash central line placement if peripheral IVs are not easily obtained. (Resuscitation. 2012;83:40; Crit Care Med. 2015;43:1233; https://bit.ly/2RuMh8K.)
B: Blood Gas and Bicarb
Hs and Ts equivalent: Hydrogen ions, hyperkalemia/hypokalemia, toxins
I love point-of-care testing. Obtaining a venous blood gas with electrolytes provides high-yield information in a cardiac arrest. Give calcium and bicarb if the potassium is through the roof (hyperkalemia). You can consider sodium bicarbonate for profound acidosis (1 amp of bicarb will raise the pH by approximately 0.1), but recognize that the mainstay of addressing acidosis in cardiac arrest is adequate ventilation.
These values can also be used for prognostication and to guide terminating resuscitative efforts; one retrospective review of out-of-hospital cardiac arrest showed no patients who survived neurologically intact had a pH less than 6.8 or a potassium greater than 8.5 mEq/L during CPR. (CritCare. 2017;21:322; https://bit.ly/3uYCGpn.)
I included bicarb here as a reminder of it as a treatment for one potential toxic cause of cardiac arrest. Give it if the history indicates and according to your rhythm analysis (wide complex PEA). Giving it indiscriminately to address acidosis has not been proven to be beneficial. (J Clin Med Res. 2016;8:277; https://bit.ly/3gdzYrZ.)
C: Core Temperature
Hs and Ts equivalent: Hypothermia
Hypothermia should be evident based on the patient's presenting history, but it's not a bad idea to obtain a core temperature on patients with undifferentiated cardiac arrest. As the old adage goes, “You're not dead until you're warm and dead.”
Hs and Ts equivalent: Thrombosis (myocardial infarction)
This is the most straightforward because it is already built into our ACLS algorithm. Myocardial ischemia is the most common cause of ventricular fibrillation and tachycardia, so obtaining astute rhythm analysis and defibrillation and a prompt post-ROSC ECG are the main ways to address myocardial infarction (one of two thromboses).
Hs and Ts equivalent: Thrombosis (pulmonary embolism), tamponade, tension pneumothorax
The main components of sonography in cardiac arrest are echocardiography and bilateral lung ultrasounds to check for lung sliding. A subxiphoid view will reveal an effusion that is causing tamponade and evaluate for right ventricle dilation in the case of massive pulmonary embolism (thrombosis). You can also consider a deep vein thrombosis ultrasound if your physical exam reveals unilateral leg swelling, but this is not needed for every patient. Add on a lung ultrasound to check for sliding to rule out tension pneumothorax, and that covers the last three of our Ts.
Dr. Cunninghamis a second-year emergency medicine resident at Maricopa Medical Center in Phoenix. Follow him on Twitter@HappyDays_EM.