CHAPTER 2: THEORY:2.1 Definition of stethoscope:
is an acousticmedicaldevice forauscultation,or listening to the internal sounds of a human body. It is often used to listen to lung andheart sounds.It is also used to listen tointestinesand blood flow inarteriesandveins.In combinationwithsphygmomanometer,it is commonly used formeasurements of blood pressure.
2.2 Anatomy of stethoscope:
Fig 2.2 Anatomy of an Acoustic Stethoscope
The headset is the metal part of the stethoscope onto which the tubing is fitted. The headset ismade up of the two ear tubes, tension springs, and the ear tips. The wearer can adjust thetension to a comfortable level by pulling the ear tubes apart to loosen the headset or crossingthem over to tighten.
Ear tip :
Soft-sealing ear tips offer increased comfort, seal and durability, and feature a surfacetreatment that increases surface lubricity and reduces lint and dust adhesion.
Ear tube :
The ear tube is the part to which the ear tips are attached.
Tunable Diaphragm :
A traditional stethoscope consists of a bell and a diaphragm. The bell is used with light skincontact to hear low frequency sounds and the diaphragm is used with firm skin contact to hearhigh frequency sounds.
The stem connects the stethoscope tubing to the chest piece.
The tubing consists of two openings. The tubing on all Littmann stethoscopes is manufacturedfrom polyvinyl chloride (PVC). The tubing does not contain either natural rubber latex or drynatural rubber.
The chest piece is the part of the stethoscope that is placed on the location where the userwants to hear sound.
2.3 Electronic stethoscope:
5An electronic stethoscope overcomes the low sound levels by electronically amplifying body sounds.However, amplification of stethoscope contact artifacts, and component cutoffs (frequency responsethresholds of electronic stethoscope microphones, pre-amps, amps, and speakers) limit electronicallyamplified stethoscopes’ overall utility by amplifying mid-range sounds, while simultaneouslyattenuating high- and low- frequency range sounds. Currently, a number of companies offer electronicstethoscopes. Electronic stethoscopes require conversion of acoustic sound waves to electrical signalswhich can then be amplified and processed for optimal listening. Unlike acoustic stethoscopes, whichare all based on the same physics, transducers in electronic stethoscopes vary widely. The simplest andleast effective method of sound detection is achieved by placing a microphone in the chestpiece.Because the sounds are transmitted electronically, an electronic stethoscope can be awirelessdevice,can be a recording device, and can provide noise reduction, signal enhancement, and both visual andaudio output.
Fig 2.3 Electronic Stethoscope
2.4 The Heart:
6The heart is the pump station of the body and is responsible for circulating blood throughout the body.It is about the size of our clenched fist and sits in the chest cavity between two lungs. Its walls aremade up of muscle that can squeeze or pump blood out every time the organ “beats” or contracts.Fresh, oxygen-rich air is brought to the lungs through the trachea or windpipe every time we take abreath. The lungs are responsible for delivering oxygen to the blood, and the heart circulates the bloodto the lungs and different parts of the body.
2.5 Functioning of the Heart:
The heart is divided into four chambers made up of a right and a left unit, separa
ted from each other bya partition wall known as a septum.Each chamber is subdivided into an upper and a lower chamber. The upper chamber is known as anatrium while the lower chamber is referred to as a ventricle.The right atrium (RA) sits on top of the right ventricle (RV) on the right side of the heart while the leftatrium (LA) sits atop the left ventricle (LV) on the left.The right side of the heart is responsible for sending blood to the lungs, where the red blood cells pick up fresh oxygen. This oxygenated blood is then returned to the left side of the heart. From here theoxygenated blood is transported to the whole body supplying the fuel that the body cells need tofunction. The blood cells of the body extract or remove oxygen from the blood. The oxygen-poorblood is returned to the right atrium, where the journey begins. This round trip is known as thecirculation of blood.
Fig 2.4 functioning of the heart
The figure shown above is a section of the heart, as viewed from the front. It demonstrates the fourchambers. It is also observed that there is an opening between the right atrium (RA) and the rightventricle (RV). This is actually a valve known as the TRICUSPID valve. It has three flexible thin
7parts, known as leaflets, that open and shut. The figure below shows the mitral and tricuspid valves, asseen from above, in the open and shut position.
Fig 2.5 Heart Valves
When shut, the edge of the three leaflets touch each other to close the opening and prevent blood fromleaving the RV and going back into the RA. Thus, the tricuspid valve serves as a trapdoor valve thatallows blood to move only in one direction – from RA to RV. Similarly, the MITRAL valve allowsblood to flow only from the left atrium to the left ventricle. Unlike the tricuspid valve, the mitral valvehas only two leaflets.In the top diagram, we also notice thin thread like structures attached to the edges of the mitral andtricuspid valves. These chords or strings are known as chordae tendineae.They connects the edges of the tricuspid and mitral valves to muscle bands or papillary muscles. The papillary muscles shortenand lengthen during different phases of the cardiac cycle and keep the valve leaflets from floppingback into the atrium.The chords are designed to control the movement of the valve leaflets similar to ropes attached to thesail of a boat. Like ropes, they allow the sail to bulge outwards in the direction of a wind but preventthem from helplessly flapping in the breeze.When the three leaflets of the tricuspid bulge upwards during contraction or emptying of the ventricles,their edges touch each other and close off backward flow to the right atrium. This important featureallows blood to flow through the heart in only ONE direction, and prevents it from leaking backwardswhen the valve is shut. The two leaflets of the mitral valve functions in a similar manner and allowsflow of blood from the left atrium to the left ventricle, but closes and cuts off backward leakage intothe left atrium when the left ventricle contracts and starts to empty.
2.6 Electrical conduction system of theHeart:
The normal electrical conduction of theheartallowselectrical propagation to be transmitted from thesinoatrial nodethrough both atria and forward to theatrioventricular node. Normal/baseline physiologyallows further propagation from the AV node to thePurkinje Fibersand bundle branches. Both the SA andAV nodes stimulate themyocardium.It is the timeordered stimulation of the myocardium that allowsefficient contraction of the heart, thereby allowingselectivebloodperfusion throughout the body.
2.7 Conduction pathway:
Signals arising in the SA node (and propagating to theleft atrium) stimulates the atria to contract. In parallel,action potentials travel to the AV node via internodalpathways. After a delay, the stimulus is conductedthrough thebundle of Histo the bundle branches a
ndthen to thepurkinje fibersand the endocardium at theapex of the heart, then finally to the ventricularmyocardium.Microscopically, the wave of depolarization propagatesto adjacent cells viagap junctionslocated on theintercalated disk.The heart is a
. Ina functional syncytium, electrical impulses propagatefreely between cells in every direction, so that themyocardium functions as a single contractile unit. Thisproperty allows rapid, synchronous depolarization of the myocardium. While normally advantageous, thisproperty can be detrimental as it potentially allows thepropagation of incorrect electrical signals. These gap junctions can close to isolate damaged or dying tissue,as in amyocardial infarction.
2.8 Electrocardiogram (ECG or EKG):
An EKG is an important part of the initial evaluation of a patient who is suspected to have a heart relatedproblem. Small sticky electrodes are applied to the patient’s chest, arms and legs.
The electrical activity created by the patient’s heart is processed by the EKG machine and then printedon a special graph paper. This is then interpreted by the physician. The EKG can provide important
Electrical conduction system of the heart
Fig 2.6 Isolated conductionsystem of the heart Fig 2.7 Heart; conduction system