Heart lung machine

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 Function

     The heart-lung machine (HLM) is a device used to provide blood circulation and oxygenation while the heart is stopped. It is a means of keeping a patient alive while his/her heart is stopped or even removed from the body. Usually called the heart-lung machine, the device also is referred to as cardiopulmonary bypass, indicating its function as a means to substitute for the normal functions of the heart (cardio) and lungs (pulmonary).

Need for heart lung machine

    During open heart surgery for installation of a valve prosthesis or cortrection of congential mal formation, the heart cannot maintain the circulation. It is necessary to provide extra corporeal circulation with the special machine called heart lung machine. The heart lung machine replaces the function of heart and lungs there by providing the rest of the body with a continuous supply of oxygenated blood.

Mechanical functions of the heart

        * The unoxygenated blood returned to right side of the heart at a pressure of 0-5 mmHg.

         * From right side of the hot the blood test pump into lungs through pulmonary arteries.

     It is the capability of the heart to give flow of blood consistently. It pushes blood out into the body and through the lungs. It should work the entire day of life to keep up with the wellbeing of the tissues all through the body.

    The heart can breakdown now and again, and expect a medical procedure to address the issue. Specialists looked for a way to stop the heart so they could address deserts yet keep the patient alive by circling blood by another means. For a long time, no such means could be found. Some heart medical procedure was done while the organ actually siphoned, making sensitive medical procedure for all intents and purposes inconceivable. 

   Specialists then found that they could stop the heart by bringing down the patient's internal heat level, a condition called hypothermia, and by flooding the heart with a cool arrangement. In its condition of fake hibernation the body required less blood dissemination, however, best case scenario, that gave specialists a couple of brief minutes to complete the medical procedure. They were as yet restricted with respect to the strategies they could do due to the extreme time requirements.

KEY TERMS

Chamber (plural: atria) — One of two upper offices of the heart. They are getting units that hold blood to be siphoned into the lower chambers, the ventricles.

Isolated organs — Organs eliminated from a creature's body for study. Along these lines, their capability not set in stone without impact by different organs.

Oxygenation — Providing oxygen to blood to be coursed all through the body.

           Through the body and is needing oxygenation. One more association is made by shunting into the aorta, the principal course driving from the heart to the body, or the femoral vein, an enormous conduit in the upper leg. 

         Blood is eliminated from the vena cavae, passed into the heart-lung machine where it is cooled to bring down the patient's internal heat level, which diminishes the tissues' requirement for blood. The blood gets oxygen which powers out the carbon dioxide and it is sifted to eliminate any garbage that ought not be in the course like little clusters. The handled blood then returns into the patient in the aorta or femoral supply route.

           During medical procedure the professional observing the heart-lung machine cautiously watches the temperature of the blood, the tension at which it is being siphoned, its oxygen content, and different estimations. At the point when the specialist approaches the finish of the technique the professional will build the temperature of the intensity exchanger in the machine to permit the blood to warm. This will reestablish the ordinary body intensity to the patient before he is removed the machine.


 Components of HLM

Cardioplegia

Cannula

Blood Reservoir

Heparin Pumps

Roller pumps/Centrifugal pump

Oxygenators

Heat exchangers

 

1. Cardioplegia
   
     Heart-lung machine circuit consists of a separate circuit for infusing a solution into the heart itself to produce cardioplegia to stop the heart from beating, and to provide myocardial protection (to prevent the death of heart tissue).


2. Cannula
    
    A venous cannula removes oxygen-deprived blood from a patient's body. An arterial cannula is sewn into a patient's body and is used to infuse oxygen-rich blood.

    A cardioplegia cannula is sewn into the heart to deliver a cardioplegia solution to cause the heart to stop beating.


3. Blood reservoir

    The blood reservoir serves as a receiving chamber for venous return, facilitates a venous bubble trap, provides a convenient place to add drugs, fluids, or blood, and adds storage capacity for the perfusion system.

    Reservoirs may be rigid (hard) plastic or soft, collapsible plastic bags.


4. Heparin Pump

    Once using a heart-lung machine during open-heart surgery, A powerful anticoagulant, Heparin should be given to the patient in order to reduce the blood's ability to clot, reducing the risk of clots forming in the heart-lung machine.


5. Pumping system 

   i) Roller pumps

     Roller pumps compress a segment of the blood-filled tubing. Blood is then pushed ahead of the moving roller.

    Roller pumps consist of tubing, which is compressed by two rollers 180° apart. Forward flow is generated by roller compression and the flow rate depends upon the diameter of the tubing and the rate of rotation.

    Excessive compression of the tubing in a roller pump increases the risk of hemolysis.

    ii) Centrifugal pump

    Centrifugal pumps have replaced roller pumps at many institutions; mainly because they cannot over-pressurize the heart-lung machine system.

    It consists of smooth plastic cones, which when rotated rapidly, propel blood by centrifugal force. An arterial flow meter is required to determine forward blood flow, which varies with the speed of rotation and the afterload of the arterial line.


6. Oxygenators

    Oxygenators not only supply vital oxygen for the blood but also transport carbon dioxide, anesthetics and other gases into and out of circulation.


There are three types of oxygenator

            1. Membrane oxygenators

            2. Bubble oxygenator
 
           3. Film oxygenator


  1. Membrane oxygenators

      Membrane oxygenators imitate the natural lung by a thin membrane of either microporous polypropylene or silicone rubber between the gas and blood phases.




    2. Bubble oxygenator

     The bubble oxygenator's venous blood drains directly into an Oxygenator chamber into which oxygen is infused through a diffusion plate. The diffusion plate produces thousands of small oxygen bubbles within the blood. Gas exchange occurs across a thin film at the blood-gas interface around each bubble. Carbon dioxide diffuses into the bubble and oxygen diffuses outward into the blood.

7. Heat Exchanger

     Control body temperature by heating or cooling blood passing through the perfusion circuit.

     In the heat exchanger, blood flows through spiraling coils made of stainless steel. The inner walls of the coils are coated with polymers to limit blood-surface interactions. The circulating water is chilled to nearly o °C in an ice bath and heated by an electric resistance coil to an absolute maximum of 42 °C.




Schematic setup of heart lung machine

Stopping of Heart-lung machine

     Once the operation is completed, The surgeon gradually occludes the arterial pressure and pulse, and central venous pressure is monitored and adjusted. When cardiac performance is satisfactory and stable, protamine is given to reverse cardioplegia and blood return from the surgical field is discontinued.

Complications
  • Hemolysis
  • Capillary leak syndrome
  • Clotting of blood in the circuit - can block the circuit or send a clot into the patient.
  • Air embolism
  • Leakage - loss of blood perfusion of tissue if a line becomes disconnected.
  • 1.5% of patients that undergo Heart-lung machines are at risk of developing Acute Respiratory Distress Syndrome.

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