Simple Anatomy

The heart is a simple musclular organ responsible for providing nutrition to other parts of the body as well as carrying waste away and out the system. To achieve this it must be able to pump blood to various organs in the body. The pumping action of the heart forces blood through the artieraries and caperleries where nutrition transfer and waste filter occurs. It is then sent back to the heart through the veins and ventricles.

The heart is made up of a series of chambers seperaed by valves as in the figure below. The 4 main chamberes are the (right atrium, right ventricles, left ventricles, left atrium as depicted in the figure below.

Anatomy of the heart.

The blood flows through the heart is as follows:

  • blood returns via the superior vena cava → right atrium collects blood until the tricuspid valve opens → right ventricle, then contracts pumping blood through the lungs → which is then passed to the left atrium → then the mitral valve opens which pumps it to other parts of the body

The sequence of blood flow in the heart is controlled by the contraction of the atriums and later the ventriculars muslces (myocardium).

Conduction system of the heart

The heart is made up of millions of individual cardiac muscle cells, and they must work sequentially to enable the proper functioning of the heart. The contractiton of the heart is trigered by depolarization. Specialized cardiac cells distributed around the heart have the ability of slowly leak ions across the cell memberane. This eventually hits a threshold, causing ** spontaneous depolarization**. Once a pacemaker cell depolarises it triggers the depolarization of adjacent cell leading to the depolarization of all cardiac cells.

The main pacemaker cells are located near the superior vena cava on top of the right atrium. It is called the sino-atrial node, it is the most important as it has the fastest depolarization rate, with an average rate of times per minute.

Other backup pace maker cells are scattered throughout the heart, specifically in the atrioventricular node, in the intraventricular septum and lower in the ventricular myocrdium. These have a much slower depolarization rate compared to the sino-atrial node, and only take over when the sinoatrial node stops working.

The pacemaker impulse travels through the heart as follows:

Anatomy of the heart.

The pulse starts at sinoatrial node → passes through the atrial muscle to the intreventicular node where it is delayed (on average) this allows for blood to flow from the atria into the ventricles before the ventricular muscles contract → after the delay it travels through the bundles of His (which splits the pulse) one to the left ventrical and one to the right ventrical → the Purkinje fibers take the pulses to the apex of the ventricles → and then muscle contraction begins (muscle contraction starts from the bottom in a squeezing way enabling a highly efficient pump)

Some notes:

  • interventricular septum: the wall seperating the left and right ventricles.
  • bundles of His: carries the impulse through the interventricular septum, where it is split.
  • Purkinje: responsible for conducting the pulse quickly throughout both ventricles.

Once the contraction occurs the heart must relax to allow for the accumulation of blood in the right atrium.

Some terminology:

  • Systole: contraction of the heart that expels blood (period $T_{systole}$)
  • Diastole: relaxation of the muscles that fills the heart with blood (period $T_{Diastole}$)

Diastole must be long enough to allow the heart to fill up with blood, hence it is as important as systole.

The start of a systole can be felt upon the mitral and tricuspid valves upon contraction of the ventricles. The second heart sound is caused by the aortic and pulmonary valves.

What is ECG?

The activation of the heart depends on the electrical polarization of the pacemaker muscle cells in a specific and repeatable sequence.

Electrodes are used on the surface of the skin to detect the small changes in the electrical activation of the heart.

The ECG provides information on the cardiac rhythm and:

  • It enables the detection of problems within the internal circuitry of the heart.
  • One can estimate the muscle mass of the heart given the electrical activation of the heart.
  • Certain diseases cause electrical changes in the heart cells. (such as diseases caused by coronary artery occulsions or narrowing of the heart called schemic heart disease)
  • Electrolyte and metabolic abnormalities (eg. hyperkalaemia)
  • Problems elsewhere in the body that effect the blood biochemistry can also effect the ECG.

Components of an ECG

ECG signals record electrical activiation at different chambers of the heart comprising of various segments.

  • P wave: caused by depolarization of both atria.
  • Q wave: represents first part of ventricular depolarisation
  • R wave: represents the intermediate part of ventricular depolarisation.
  • S wave: final part of the ventricular depolarisation.
  • PR interval: the delay caused by the conduction from atria and ventricles. (this interval allows for blood to flow from atria to ventricles before contraction). This is measured from the beginning of the P wave to the beginning of the QRS complex.
  • QRS complex: is caused by depolarisation of both ventricles. The shape depends on where the electrical activation or depolarization occurs. Note that QRS complex usually obscures the atrial repolarisation
    • In a 12 lead ECG, the different leads look at different anatomical areas of the heart.
      • Leads two, three, and AVG show changes in the interior part of the heart.
      • Leads C1 - C2 show changes in the interventricular septum.
      • Leads C6-1 and AVL show changes that occur in the apex of the left ventricle.
  • T wave: caused by ventricular repolarisation.
  • QT Interval: mesaured from the begginning of the Q wave to the end of the T wave. It measures the time taken form ventricular depolarisaton and repolarisation.

Note: In a 12 lead ECG there are only 10 leads, and the 12 refers to the number of different electrical views the 10 leads can provide you with. In other words 12 lead ECG refers to 12 different pictures of the electrical activity of the heart. Read more on here

Each ECG lead and its QRS complex has a typical characteristic pattern. So to recognize the ECG one has to recognize the usual patterns in the corresponding lead.

There are normal variants of the ECG QRS complex, if there is significant change one can infer that there has been some biological changes in the heart due to some disease or aging.

Procedure for optimal ECG contact

The following is the common procedure medical practictioners take for measuring ECG.

  • Shave and wash the surface of the skin near heart for optimal electrode contacts.
  • place leads on LA, RA, LF, and RF (as ground reference).

Problems in ECG Recordings

Some notes on electrical inference of ECG and problems thats occur when performing ECG recordings.

The isoelectric line of the ECG is at 0mV reference. This baseline represents when no current is flowing in the heart. In a healthy heart the baseline should lie on this isoelectric line.

ECG devices commonly use alternating current (AC) and hence can cause power-line noise in the range of $50Hz-60Hz$

Make sure you attach the wires to the electrodes in order from C1 to C6:

  • C1: red - 4th intercostal space at right sternal edge
  • C2: yellow - 4th intercostal space at left sternal edge
  • C3: green - in between C2 and C4
  • C4: brown - 5th intercostal space in the mid-clavicular line
  • C5: black - left anterior axillary line and at the same horizontal level as C4
  • C6: blue/purple - left mid-axillary line and at the same horizontal level as C4 and C5.

Color codings following the international Electrotechnical Commision system. The color coding may varry by country.

Figure depicting difference ECG lead placement on body.

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The Normal ECG Trace

When looking at ECG it should be studied in a sysematic way.

  • The sampling rate
  • Rhythm of the recorded pulses
  • Looking at the cardiac axis (in 12 lead ECGs), which tells us about the direction of the electrical activation of the heart. More on cardiac axis here
  • Then the component parts of the ECG.

In an ECG recording, the size of the deflections is indicative of the size of muscle causing the depolarisation. This can be used to find abnormalities in the size of either chambers in the heart. One can also look at the time axis to determine if intervals allow for appropiate functioning of the heart. For example, we can measure the width of the QRS complex to determine the time taken for depolarisation (important as the bundle of His may not always function properly)

Note: normal sizes of deflections are determined by doctors by looking at many ECG recordings of different healthy people in the same age group

Normal characteristics of ECG components

Each component of ECG has a characteristic which it follows when there aren’t abnormalities in the heart.

  • P wave: this should be upright in all leads except aVR, in which it should be inverted. It could have negative component in V_1 lead (known as biphasic), and this is normal as long as it is more negative than positive.
  • PR interval: this interval should be around 110 msec. It is normally of fixed duration (note: it can shorten with sympathetic activation and increased heart rate, it can also shorten with parasympathetic activity).
  • QRS complex: the total duration of the QRS complex should be less than 120 msec, a broad QRS complex may indicate ** interventricular conductiton abnormality**
  • QT interval: There is no single definition for a normal QT interval, as it has to be corrected by the heart rate. QT interval corrected by heart rate is denoted as QTc and generally for women it has a duration of 460 ms or longer and 450ms or longer for men. [2]. Prolongation of QT is related to increased risk of death. Below is some causes of QT prolongation [3]:
    • genetic abnormalities effecting potasium/sodium levels in cardiac cells.
    • use of drugs
    • electrolyte abnormalities
    • cardiac diseases, diabetes, alchoholic liver diseases..[4][5][6]
  • T wave: no set normal ranges.
  • The tallest R wave should not be greater than $2.5 mV$ in height
  • The deepest S wave should not be greater than $2.5 mV$ in depth.

Cardiac axis

  • The normal cardiac axis for a healthy heart is between -30° - 90°, if the axis is beyond -30°, called left axis devition; may indicate the following 1:
    • Left anterior hemiblock
    • Wolff-Parkinson-White syndrome
    • Inferior myocardial infarction
    • Ventricular tachycardia.
  • Obesity can cause leftward shift as well, but not beyond -30°. For axis greater than 90° it is called right axis deviation (bundle branch block).

A simple method of working out the axis of a patient, to determine whether they have left or right deviations is to look at the I, II leads and look at the QRS deflections. Then determine whether it is predominantly upwards or downwards.

  • Normal axis: both positive in I, II.
  • Left axis: positive in I, negative in II.
  • Right axis: negative in I and positive in II.
  • Exterme right axis or northwest axis deviation: negative in both I and II
Sinus rhythm

The sinus rhythm refers to the normal rhythm of the heart as it orginatets from the cardiac pacemaker in the sinoatrial node.

  • average HR is 60 - 100 bpm
  • rhythm is regular
  • P wave followed by QRS
  • P wave is positive in II and negative in aVR
  • PR interval is consistent, between consecutive beats

When breathing in and out the rate of P waves changes and causes heart ratet to change as depicted in the figure below. This is commonly refered to as sinus arrhythmia.

Sinus bradycardia refers to the sinus rhythm with a heart rate of less than 60 BPM. On the other hand sinus tachycardia is refered to as heart rae more than 100 BPM.


Below is a list of several abnormalities that can be seen on an ECG recording.

  • Ventricular hypertrophy: By making ECG measurements over time and taking point of amplitude heights of QRS complexes, it is possible to determine if the ventricles are getting bigger. Important in assessing the severity of aortic or mitral valve disease and success of treatments.
  • Supraventricular tachycardia: from time to time your heart beats very fast for reasosn other than exercising/fever/sress. This can be caused by faulty electrical conections in the heart.
  • First-degree atrioventicular block:a disease of electrical conduction system where PR interval is lengthened beyond 0.2 seconds.