Understanding ECG and The Cardiac Cycle

Cardiac Cycle and ECG


The cardiac cycle refers to a complete heartbeat from its generation to the beginning of the next beat, and so includes the diastole (relaxation), the systole (contraction) and the intervening pause. The frequency of the cardiac cycle is described by the heart rate, which is typically expressed as beats per minute.

Each beat of the heart involves five major stages. The first two stages, often considered together as the ventricular filling stage, involve the movement of blood from the atria into the ventricles. The next three stages involve the movement of blood from the ventricles to the pulmonary artery (in the case of the right ventricle) and the aorta (in the case of the left ventricle).

Important Changes related to Cardiac Cycle

  1. Mechanical changes (systole and diastole)
  2. Electrical changes (ECG)
  3. Volume changes (note that the volume of the ventricles concerns us more, we don’t care about changes in the atrial volume)
  4. Sound Changes: there are four heart sounds; we are concerned with only two of them i.e. lub and dubb.
  5. Pressure changes

Points to Consider

  1. During one cardiac cycle we have one atrial systole, one atrial diastole. And we also have one ventricular systole, one ventricular diastole, and so on.
  2. Remember that volume between the right and left ventricle is the same. But what differs is the pressure between them (left ventricle > right ventricle).
  3. For teaching purposes, we consider the heart rate to be 75 beats/min, and the time for one cardiac cycle is 0.8 seconds. Just to make things easier to understand and visualize. But practically, it could be anything else from 60-100 beats/min in normal situations.

Mechanical Changes during Cardiac Cycle

Atrial Changes and AV Delay

The atrial systole takes 0.1 seconds. And the atrial diastole takes 0.7 seconds (the total is 0.8 seconds = the cardiac cycle). We have AV delay; so that the atria and ventricles do not contract at the same time.

Therefore, atrial systole is followed by ventricular systole. In other words, once the atria finish their contraction, the ventricular contraction starts. The AV delay takes 0.1 seconds.

Ventricular Systole and Diastole

The ventricle systole takes 0.3 seconds. And the ventricular diastole takes 0.5 seconds (the total is 0.8 seconds = the cardiac cycle). Do not be confused to think that the ventricle diastole takes 0.4 seconds. This time (0.4 seconds) is the time where atrial and ventricular diastole overlaps (both ventricles and atria are in diastole). While during the other 0.1 second, the atria are during systole (of the next cycle), and the ventricles are during diastole.

Mechanical Changes are Consequence of Electrical Changes

So, the atria and ventricles work together, we consider this as mechanical changes that occur in heart. Remember that mechanical changes are a consequence of electrical changes. This means that for mechanical changes to occur it should be preceded by electrical changes.

Electrical Changes in the Cardiac Cycle and ECG

Before atrial systole occurs we should record the P Wave in the ECG. And before ventricular systole occurs we should record the QRS Complex in the ECG. And before ventricular diastole occurs we should record the T Wave in the ECG.

How many action potentials we have to record? The answer should be two; one for the atrium and one for ventricle. The first or atrial action potential corresponds to the P wave on the ECG (atrial depolarization). Whereas the second or ventricular action potential includes QRS complex and the T wave together.

Now, by looking to the two action potentials; you should recognize that the atrial repolarization and the ventricular depolarization occur at the same time. Therefore, the atrial repolarization is not seen (masked).

Volume and Sound Changes

As we mentioned earlier, we do not really care about the atrial volume; the ventricular volume is the important one. The blood volume of the right ventricle is equal to that of the left ventricle; the ventricles differ only in pressure (pressure in the left > right).

Before atrial contraction takes place, ventricular volume is 100 ml, for both the right and left ventricles.

Even before contraction of the atria, the AV valve is open because the atrial pressure is more than that of the ventricle (they open passively due to the pressure). This occurs at the beginning of ventricular diastole.

Atrial Pressure

The pressure in the atrium is almost zero. Accordingly, the ventricular pressure has to be less than zero (negative). You may wonder how can we have negative pressure? Our reference – when we say the pressure is zero – is the atmospheric pressure (760 mmHg).

So when the pressure is zero in the atrium, it means that the pressure is equal to the atmospheric pressure The pressure in the ventricle is negative (between –1 and –2). This means that it less than the atmospheric pressure. Here (–2) would mean (760 – 2 = 758 mm Hg)

When we say someone’s blood pressure is 120/80; it means that it is greater than atmospheric pressure by this value. Which is equivalent to (120+760)/(80+760) = 880/840.

So, to make things easier we consider zero is the pressure of the atrium. If you don’t like dealing with negative numbers, you can consider the ventricular pressure to be 0; so the atrial pressure is +1 or +2.

During this period, the AV valve opens and blood goes from the atrium to the ventricle straight away. This is followed by atrial contraction, pushing a certain extra amount (25 mL) of blood to the ventricles faster. Now, the blood volume becomes 125 mL at the end of diastole of the ventricles. This is what we call the end diastolic volume (EDV).

But even if the atria don’t contract (as in atrial fibrillation); blood would also flow to the ventricles because the AV valves are open. But, it will move with fewer amounts and less speed in this case (the volume in the ventricle would increase by 10 mL only).

Ventricular Systole

The ventricle is guarded by two valves: the AV valve and the Semilunar valve. The left semilunar valve separates the aorta from left ventricle (aortic valve), whereas the right semilunar valve separates the pulmonary trunk from right ventricle (pulmonary valve).

The aortic pressure during diastole is 80 mmHg, and the pulmonary pressure during diastole is 8 mmHg. When the ventricle starts to contract, its pressure is going to increase above zero (that of the atrium). Therefore, the blood tries to move from the ventricles back to the atria: The AV Valve closes.

Isovolumetric Contraction

Thus, the blood tries to move from the ventricles to the atria; the AV valves will shut. This creates a sound. And this sound is called LUBB (the first heart sound because of closure of AV valve). This corresponds to the QRS complex on the ECG. During this stage, the four valves of the heart (two AV & two semilunar) are closed. And the volume in the ventricle does not change. Therefore; we call this phase Isovolumic Contraction.

Rapid Ejection, Slow Ejection and Ejection Fraction

Isovolumic Contraction is a short phase in which ventricular volume is constant and the four valves are closed. By the end of this phase the pressure increases very rapidly, and when it reaches higher than 80 mmHg in left ventricle causing the semilunar valve to open. This leads to the first rapid ejection (about 70%), then the slow ejection (about 30%).

Until the end of systole, the volume of blood that stays in the ventricle called the end systolic volume (ESV). It is equal to 55 mL. The fraction of the EDV that is ejected called ejection fraction usually equal to about 60%.

Stroke Volume and Cardiac Output

The amount of blood ejected in each beat (cardiac cycle) from either right or left ventricle is called the stroke volume. And here it equals (125 – 55 = 70 mL/beat).

Stroke volume = end diastolic volume (EDV) – end systolic volume (ESV).

If we want the stroke volume in one minute we multiply it by the heart rate (in this case its 75 beats/min); this is what we call the cardiac output.

Cardiac Output (mL/minute) = Stroke Volume × Heart Rate

Isovolumetric Relaxation

During systole the pressure in the left ventricle is very high, its value is higher than the aortic pressure which makes the blood flow from the ventricle to the aorta, and then to the systemic circulation.

When blood flows, its pressure will become less and less until it becomes less than 80 mmHg in case of left ventricle (in right ventricle it gets less than 8 mmHg). So the blood tries to move from the aorta back to the left ventricle; closing the left semilunar or aortic valve.

The AV valves are still closed, and this is the diastolic phase where the 4 valves are closed; which is called Isovolumic Relaxation. Usually Isovolumic Relaxation takes longer time than Isovolumic Contraction.

The closure of semilunar valve will cause the second heart sound DUP. During ventricular systole, the AV valves are closed. Therefore, there is a filling of blood inside the atria. Deoxygenated blood in the right atrium & oxygenated blood in the left atrium.


Diastasis is slow filling of ventricle before atria contract. The pressure in the Isovolumic Relaxation phase keeps decreasing until it becomes lower than the atrial pressure; causing the AV valve to open.

Rapid filling occurs, then slow filling. Before the atrial systole, the blood volume in the ventricle rises up to 100 mL (passively by pressure gradient). When the atria contract, another 25 mL are added. Total = 125 mL = end diastolic volume (EDV).

So we have, rapid filling in ventricular diastole and rapid ejection in ventricular systole. To sum things up, closure of AV valve gives us the first heart sound: LUBB (occurs at the beginning of the ventricular systole). Closure of Semilunar valve gives us the second heart sound: DUP (occurs slightly after the end of ventricular systole), or at beginning of its diastole.

Third and Fourth Heart Sound

There are sounds due to the movement of blood around a closed valve, it sounds like murmur. We may also hear other sounds. The third heart sound is heard when there is rapid filling of blood from atria into the ventricles (opening of the AV valve). Also atrial contraction gives us the fourth heart sound. But usually, we hear only hear the two previously mentioned heart sounds only.

Pressure Changes

We will talk about the change of pressure in the left ventricle.

  1. Pressure in the left ventricle is different from that of the right ventricle.
  2. If the pressure in left ventricle before atrial systole (during diastole) is zero; the pressure in atria will be +1 or +2.
  3. When the atria contact the pressure will increase, and it may reach up to +5 mmHg.
  4. This is followed by ventricle contraction, the AV valve will close (1st heart sound) LUBB. The Isovolumetric Contraction phase starts (both AV valve and aortic semilunar valves are closed).
  5. Now there will be sharp increase in pressure until it reaches the aortic pressure (80 mmHg); once it exceeds it, the aortic valve will open, and it keeps rising to be higher than the aortic pressure.
  6. Why the ventricular pressure must be higher than the aortic pressure? There must be a pressure gradient that prevent the flowing of blood from the ventricles to the aorta.

When the aorta pushes blood to the closing semilunar valve, the blood is compressed and press on the wall of the aorta and the aortic pressure slightly rises, and makes a peak called Dicrotic Notch or Incisura.

The waves of Atria

First rise in pressure in atria occurs during atrial systole which causes a wave called atrial pressure wave or A wave. After atrial systole, the ventricle systole begins. And when the ventricle contracts, blood tries to regurgitate towards the atria; causing the AV valve to close (because of high pressure in ventricle). This also increases the pressure inside the atrium (because of the force of blood on its wall). This causes another waves called C wave.

After that the pressure inside the atria start to develop due to venous filling (the AV valve is closed during this phase) until it reaches a maximum just before the ventricular diastole, this is called V wave. This occurs just before the AV valve opens due to the drop in ventricular pressure, and the rapid filling occurs.