Now, That's What I Call High Yield: Cardiology Vol. 1

Cardiology is one of the foundational topics of Step 1 and of medicine as a whole. It’s a common source of anxiety for residents and Step 1 test-takers alike. It’s difficult to pick a few subtopics within cardiology to deem “high-yield” because all of the topics within it may appear on the exam! Plus, having a high grade of understanding will help you understand other pathways and systems throughout the body—it’s an excellent blend of fluid dynamics, pressures, math, and a place to build fundamental test-taking skills.

Given the expansive landscape that is cardiology, this post will be split into two parts, but will still not cover everything there’s to know about the topic.

High-Yield Cardiology Topics for Step 1

Fetal Circulation

The circulation between mother and fetus is a complicated but conquerable process. It begins with oxygenated blood from the mother via the umbilical vein to the ductus venosus in the fetus which delivers this blood to the inferior vena cava and thus bypasses the hepatic circulation.

The majority of this blood enters the left atrium via the foramen ovale.

Deoxygenated blood from the superior vena cava passes through the right atrium to the right ventricle followed by the pulmonary artery but is then diverted to the descending aorta via the ductus arteriosus.

But why does this bypass work? It’s due to the high resistance in the fetal pulmonary circuit.

At the time of an infant’s first breath, multiple changes take place. The high pulmonary artery resistance in the fetus decreases in an infant which leads to increased left atrial pressure versus right atrial pressure which closes the foramen ovale. The ductus arteriosus is finally closed by increase in O2 and decrease in prostaglandins.

Many of these fetal anatomic structures have adult derivatives which are useful for you to connect in your mind. It’s also worth thinking through how the changes in resistance and pressure take place from a physics standpoint and how one leads to the other. Finally, it’s often a good idea to think of opposites.

For example, in the case of the ductus arteriosus, decreased prostaglandins and increased O2 close this structure, but how may you be able to keep it open as a physician? By giving prostaglandins perhaps? Or what if you want to close a ductus arteriosus that’s stuck open? What medicine class acts against prostaglandins? That would be NSAIDs!

Cardiac Output

Cardiac output is just as it sounds, it’s the amount of blood leaving the heart to perfuse all parts of the body. There are two ways to think about cardiac output, the mathematical and the logical way. When does blood leave the heart? During systole.

If you want more blood to leave, have more systoles, AKA increase the heart rate. What if every stroke only had (arbitrarily) 1 ml versus every stroke having 10 ml? More blood would leave the heart in the same number of beats in the latter.

Therefore, cardiac output = Stroke volume x heart rate.

There are many many variables which affect each of these equation parts. If you pour water into a pipe with diameter of 1 cm versus one with diameter 10 cm, how long would it take for the same volume to pass through? This is an example of afterload, or resistance to flow after the heart. Decreasing the afterload increases the stroke volume because it’s easier to get more blood out of the heart. Preload is simply the amount of blood that’s available to be pumped out of the heart. Increase the preload and your stroke volume increases.

Finally, if you swing a hammer at a carnival strongman game with 50% of your strength, you can expect to not ring the bell versus if you use 100% of it. This is analogous to contractility. If the heart pumps “harder” due to anxiety, more of the preload will be pumped out of the heart and thus increase the stroke volume.

Murmurs

When auscultating the heart, there are four important areas to place your stethoscope, the pulmonic, aortic, mitral, and tricuspid area. When it comes to identifying murmurs, however, identifying WHEN they occur in the cardiac cycle often help identify the murmur to a greater degree. There are several ways to categorize these murmurs in your head, so this will be just one method.

Understanding when blood is flowing through a valve normally will help identify the pathology. In general, murmurs from a stenosis will occur during the same cycle that blood is normally flowing through the valve, but the blood is struggling to make its way through. Murmurs from regurgitation happen from blood going backwards through a valve and, therefore, the sound from the murmur occurs during the phase of the cardiac cycle when blood normally does not move through the valve.

Once you are able to identify when in the cardiac cycle the murmur is occurring, it’s a simple matter to identify which area you are auscultating to identify the valve in question. There’s a bit of nuance when it comes to the mitral valve. If it’s holosystolic, then mitral regurgitation is most likely. If there’s a midsystolic click followed by a murmur, the patient has mitral valve prolapse.

Finally, discerning between ventricular septal defect and tricuspid regurgitation can be challenging. Regurgitation will be described as “high pitched” while ventricular septal defect will be described as harsh sounding. Patent ductus arteriosus is easy to remember because it’s the only murmur that’s continuous through both phases of the cardiac cycle

As stated before this is only one of many possible methods for remembering the murmurs and you can be sure that at least one or two questions will be on your test.

EKGs

Reading EKGs is a famously difficult task. It would take an entire textbook to thoroughly cover the topic. The most important piece of advice in regard to interpreting EKGs on an exam is to always use a consistent and systematic approach every single time you see an EKG and to not deviate from your method.

A common method to use when approaching an EKG is as follows:

Rate – Identify the beats per minute
Rhythm – Identify the rhythm as either Regular, regularly-irregular, or irregularly-irregular
Intervals – PR, QT, QRS
Axis – This is done using the “Thumb Rule” where you imagine your LEFT thumb is LEad I and your RIGHT thumb is AVF. If the QRS is down in each respective lead, then the corresponding thumb points down. For example, if the QRS is up in both, both of your thumbs are up and that is great because that is a normal axis. If the QRS in Lead I is down and the QRS is up in AVF, only your RIGHT thumb is up and that is a right axis.
Morphology – Are there any abnormalities to the “shape” of the wave?

Common Arrhythmias

Some common arrhythmias that you should be able to (almost) see on sight are Afib, Aflutter, Vfib, Vtach, and Torsades. Knowing these does not mean you can skip your systematic approach, however!

Atrial fibrillation

Famously has a rhythm described as “Irregularly-irregular.” Atrial fibrillation is hallmarked by a lack of P-waves and ventricular depolarization (QRS’s) occurring at seemingly random intervals. Don’t confuse this with ventricular fibrillation, a wavy jumble of nothing. If it’s VFib, shock it!

Atrial flutter

See a sawtooth pattern? A-flutter. AV block comes in 4 flavors. First degree is nothing more than a long PR interval. Second degree is either a PR that goes longer, longer, longer dropped QRS (type I) or random dropping of QRS (type II). Type I is no big deal, type II necessitates a pacemaker. Third degree, or complete heart block involves a complete dissociation between atrial and ventricular depolarization. The atria pound away at a set rate, as do the ventricles, they are just not talking to each other and coordinating their efforts.

Other arrhythmias that are important to be familiar with are Wolff-Parkinson-White, and atrioventricular block (Type 1, 2A, 2B, and 3).

Baroreceptors

Regulatory receptors in the carotid body that help regular blood pressure. Their goal is to maintain homeostasis via the glossopharyngeal nerve. Slower firing leads to an INCREASE in blood pressure and faster firing DECREASES blood pressure.

If you check the carotid pulse of a patient on both sides, you will artificially increase pressure, decreasing the rate of fire and therefore decrease the blood pressure (and possibly cause syncope).