صدق الله العظيم الاسراء اية 58

By Dr. Abdel Aziz M. Hussein Assist Prof. Medical Physiology

Heart is the pump Pulmonary circuits Systemic circuits

The heart is a hollow muscular organ Site: Left side of the thoracic cavity partly behind the sternum. Size and weight: It is in the size of a man's fist It weighs about 320 gm in males. In female it weighs about 250gm. Chambers: 4 chambers = 2 atria + 2 ventricles Atria Ventricles

Its wall consists of 3 layers; Endocardium Epicardium Myocardium

1. Nodal fibers 2. Conducting fibers 3. Contractile fibers Atria Ventricle

Myocardium is composed of 3 types of ms fibers; a) Nodal ms fibers e.g. Sinoatrial (SA) node and Atrioventricular node. SAN is the normal pacemaker of heart from which the cardiac impulse initiated. b) Conducting ms fibers include AV bundle, bundle branches and Purkinje fibers. These fibers conduct the cardiac impulse from atria to ventricles.

c) Contractile ms fibers They consist of atria and ventricles. The primary function of cardiac muscle cell is to contract to pump blood into arteries Electrical changes within the cardiac muscle cell initiate this contraction.

There are 2 types of membrane potentials on the cardiac muscle cell 1.One during rest called resting membrane potential (RMP) 2.And the second during action called action potential or cardiac impulses

RMP is the potential difference across cell membrane during rest i.e. inside the membrane is negative and outside is positive Its value differs according to type of muscle fiber e.g. in SAN it is -65 mv, and in ventricle ms is -90 mv It is measured by galvanometer with 2 microelectrodes one is inserted inside the cell and the other is placed outside the muscle cell

Ionic basis of RMP 1.K + diffusion to outside (main cause) 2.Na + diffusion to inside (little effect) 3.Electrogenic Na + -K + pump

Action potentials occur when the membrane potential suddenly depolarizes and then repolarizes back to its resting state. So cardiac potential consists of 2 phases; 1.Depolarization phase (inside cell become +ve and outside become –ve) mainly due to Na and Ca influx (inflow inside cell) 2.Repolarization phase (inside cell become -ve and outside become +ve) mainly due to K efflux (outflow outside the cell)

RMP Depolarization Repolarization

Depolarization phase Repolarization phase

The excitation wave (depolarization followed by repolarization) is initiated by the pace maker of the heart (SAN). Then it spreads through the atrial fibers in a rapid rate (1m/sec) to converge on the AVN. The conduction in the AVN is very slow (0.05 m/sec). The depolarization wave then travels rapidly down the AV bundle, bundle branches (1m/sec) and the purkinje fibers (4m/sec) to all parts of the ventricles.

The body is a good conductor of electricity because tissue fluids have a high concentration of ions that move (creating a current) in response to potential differences. The potential difference of about 120 mV between a depolarized and a resting area of heart muscle generates in the area of the heart an electric field that projects and conducted to the body surface.

These potential differences can be detected by electrodes put on the skin surface and amplified and recorded as a tracing on paper or on a monitor

Def. ECG is a record of the electrical activities of the cardiac muscle

Apparatus: The apparatus used for recording the electrical activities of the heart is called electrocardiograph or ECG ECG apparatus consists of; 1.Recording galvanometer 2.Electrodes

Galvanometer Electrodes

◊ Def., It is the particular arrangement of 2 electrodes of the ECG galvanometer.

◊ Types : Bipolar Limb leadsUnipolar leads

They measure the potential difference between 2 limbs at different levels of potential. 1.Lead I (RA (-) and LA (+)) 2.Lead II (RA (-) and LF (+)) 3.Lead III (LA (-) and LF (+))

Lead I Lead II Lead III

Einthoven`s triangle is an equilateral triangle, the sides of which represent the 3 bipolar leads & the heart lies in its centre. Einthoven`s law the sum of voltage in lead I and Lead III = voltage in lead II

Lead I Lead II Lead III

ECG Central t. Central t. + V leads + VL + VR -

They measure the potential at one point. The potential at one pole (-ve) of ECG is made zero by connecting it to central terminal that receives electordes from (RA, LA, and LF) Positive pole (exploring electrode may be placed at; 1.Limbs → unipolar limb (VR, VL and VF) 2.Chest → unipolar chest (V1- V6)

They measure the potential at limbs (RA, LA and LF) The voltage of potentials recorded by these leads is weak, so it must be augmented to form aVR, aVL, and aVL This done by disconnection of electrode connected to central terminal to limb to be augmented

Central term. Central term. VRaVR ECG

V1 V2 V3 MCL AAL MAL V5 V6 V4

6 unipolar chest leads from V1 to V6. 1.V1 (Rt 4 th intercostal space) 2.V2 (Lt 4 th intercostal space) 3.V3 (midway between V2 and V4) 4.V4 (Lt 5 th intercostal space midclavicular line 5.V5 (Lt 5 th intercostal space anterior axillary line ) 6. V6 (Lt 5 th intercostal space midaxillary line )

I II III aVRaVL aVF V1 V2 V3 V4 V5 V6

ECG paper is composed of a number of 1 and 5 mm squares It calibrated so that, Each small square (horizontal)= 0.04 sec. Each small square (vertical )= 0.1 mv

Consists of; 1.Waves (P,Q,R,S,T,U) 2.Segment (S-T segment) 3.Intervals (P-R interval, Q-T interval)

P Q R S T U

1- P wave: represents atrial depolarization. 2- QRS complex: represents ventricular depolarization. 3- T wave: represents ventricular repolarization 4- U wave: represents repolarization of papillary muscles

B) S-T segment: B) S-T segment: normally is isoelectric C) P-R interval: C) P-R interval: It represents the time of conduction of impulse from SAN to ventricles Normally 0.12 to 0.2 sec

It gives valuable information about; 1) The anatomical orientation of the heart 2) The relative size of the heart chambers. 3) A variety of disturbance of rhythm and conduction. 4) The extent, location and progress of ischemic damage of the myocardium. 5) The effects of altered electrolyte concentrations and drugs

T wave is inverted Deep Q S-T segment is not isoelectric i.e. elevated or depressed

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