Electrocardiography is the „English language of medicine” – everybody must understand it. That is why we chose ECG as the main topic of our new challenge, the Arrhythmia Challenge celebrating World Heart Day on 29th September. To read ECG effectively, you have to understand the physiology behind it and should know the features of the normal rhythm in order to recognise the differences in pathologic cases. Refresh your memories about electrophysiology and the cardiac conducting system with us!
Dig down deep in your memories for all your knowledge about physiology because you will need it now! To understand the definition of sinus rhythm, first, you have to understand the mechanism of the electrical conduction in the heart. So let’s go through it & start with the basics!
One of the first concepts you learn in med school and yet, you will hear about it many more times. The heart has a pacemaker that creates electrical pulses. These electrical currents spread through the heart and cause the change of the membrane potential due to the movement of charge across the membrane of myocytes. This process is called depolarization.
The pair of the one above. After depolarization the cells return to the resting membrane potential - the movement of charge reverses across the membrane of the myocytes, this is the second wave of electrical activity in one cardiac cycle. The cell reaches electroneutrality again. The process is called repolarization.
The mechanical consequence of the electrophysiological changes will result in a series of cardiac cycles after each other, resulting in a well-functioning heart. One cardiac cycle consists of two phases: systole and diastole. See their relation with the electrical changes!
The depolarization of myocytes manifests in muscle contraction. We speak about systole when the ventricular muscle contracts and pumps the blood into the aorta and the truncus pulmonalis.
Following contraction, cardiac muscle returns to a resting state, it repolarizes. This results in dilatation. The dilation of the ventricular muscle is called diastole. It coincides with the atrial contraction so that the blood can continue its way from the atria to the ventricles.
Cardiac conducting system
The cardiac conducting system is made of special cardiomyocytes which can generate electrical impulses and propagate them to the working muscles, causing contraction. Let’s see the bigger stations of this system!
Sinoatrial (SA) node
This is the so-called natural pacemaker of the heart! The cells in the SA node have a special ion channel called the “funny” sodium channel that enables them to create electrical pulses. The atrial myocardium transmits the electrical current to the non-conducting fibrous rings of the heart while causing a contraction in the late diastole. Here (in a healthy heart), the current has only one way to reach the ventricles: through the atrioventricular node.
Atrioventricular (AV) node
It is the second big station of the conducting system. It has similar features to the SA node except that here, the transmission is much slower, whereby it functions as an electrical gateway. This means a filtering function that saves the ventricles from higher rates and helps to preserve the cardiac function. It also has intrinsic pacing function that is mostly overwritten by the SA node.
The system includes the conducting branches in the interventricular septum. The His bundle starts from the AV node and quickly divides into two bundle branches in the interventricular septum. The left and right bundle branches of His are often called the Tawara branches. They lead to the Purkinje fibers located in the ventricular walls just below the endocardium. The Purkinje fibers transmit the electrical current to the cardiomyocytes, so the ventricles contract.
So what is sinus rhythm?
Now that we know the essential concepts, we return to our original topic - the normal sinus rhythm. What are the main characteristics of sinus rhythm?
It must be regular, meaning that the distance between the peaks of two R waves next to each other doesn’t differ in one electrocardiogram
In adults, the rate of normal sinus rhythm also has to be between 60 and 100 beats per minute (bpm)
It must originate from the SA node
It must respect the route of the normal conduction pathway: SA node → Atria → AV node → His-Purkinje → Ventricles
The atrial contraction must precede the ventricular contraction
The elapsed time between the beginning of an atrial contraction and the start of systole must be between 0,12 and 0,20 seconds
Finally, the systole is shorter than 0,12 second in normal settings.
After learning the basics, test your knowledge in practice by solving cases in InSimu Patient app! You can download it for iOS or Android.
Andrew R. Houghton, David Gray: Making Sense of the ECG – A hands-on guide