Extra corporeal circulation

Dr. Arati Mohan Badgandi

Introduction

• A procedure in which blood is taken from a patient's circulation to have a process applied to it before it is returned to the circulation

• All of the apparatus carrying the blood outside the body is termed extracorporeal circuit

Extra corporeal circulation

• Hemodialysis • Hemofiltration • Plasmapheresis • Extracorporeal membrane oxygenation

(ECMO) • Cardiopulmonary bypass during open

heart surgery

CPBCPB

Introduction

• CPB involves the temporary substitution of the functions of the heart and lungs with mechanical devices, placed outside the human body

• Dr. John gibbon - Performed first successful intracardiac operation with the aid of heart lung machine in 1953

Goals of CPB

• To provide a stilled bloodless heart with blood flow temporarily diverted to an extra corporeal circuit that functionally replaces the heart and lungs

• Respiration– Ventilation– Oxygenation

• Circulation• Temperature regulation

Indications

• Surgical correction of congenital, ischemic or valvular heart diseases

• Coronary artery bypass• Valve replacement• Correction of septal defects• Advances of CPB technology have been

utilized in the development of other complex medical devices like– Artificial hearts– ECMO– Ventricular assist devices

Basic CPB Circuit

CPB Circuit in Real Setting

Venous Cannulas

Venous cannulae

• These drain blood from the patient into the CPB circuit

• One or two cannulae are used, depending on the type of cardiac surgical procedure or surgeons preference

• These drain the blood by gravity

Venous Reservoir

Blood reservoir

• This stores the blood• It can be of two types

• Venous reservoir - collects the blood drained from the patient

• Arterial reservoir – collects the oxygenated blood coming from the oxygenator

Pump

Pump

• These are the devices, which are used to generate the pressure required to return the perfusate to the patient

• These are driven by electrical motors• There are two types

– Roller pump– Centrifugal pump

Heat Exchanger

Heat exchanger

• It adjusts the temperature of the perfusate to provide moderate systemic hypothermia during the period of cardiac repair and gradually rewarms the blood during the discontinuation of CPB

• Water circulates within the heat exchanger in a counter current fashion to the flow of blood at temperatures between 1-2°C and 42°C

• The temperature gradient between the water and blood is kept at 10°C or less

Oxygenator

Oxygenators

• It is an apparatus where O2 and Co2 exchange takes place

• There are two types– Bubble oxygenator– Membrane oxygenator

Bubble oxygenator

• This type of oxygenator is used when CPB is needed for less than 2hours

• In this, O2 is directly infused into a column of systemic venous blood through a diffusion plate

• The diffusion plate produces thousands of small O2 bubbles within the blood

• Gas exchange occurs across a thin film at the blood gas interface around each bubble

• Co2 diffuses into the bubble and O2 diffuses into the plasma

Membrane oxygenator

• This type of oxygenator is used when CPB is needed for more than 2-3 hours

• In this type of oxygenator, gas doesn’t come in direct contact with the blood

• A membrane, made up of silicon rubber or polypropylene or Teflon, separates blood and gas compartments

• Blood is made to flow in small streams (hollow fibers) or thin sheets over the membrane

• CO2 diffuses outwards and O2 diffuses into the blood

Filters

Filters

• These are designed to trap particulate matter and gaseous emboli

• In the suction line - Prevents the debris from the operating site from reaching the oxygenator

• Arterial filter - This removes the emboli from the blood in the arterial tubing prior to entering the body

• Venous filler - Remove the particulate matter from the cardiotomy suction devices

Filters

Filters

Aortic cannulas

Arterial cannulae

• These cannulae return the blood from the CPB circuit to the patient

• Commonly ascending aorta is the site for arterial cannulation

Priming

• The composition of the fluid used for priming, varies according to the preference of institutions

• But its composition should be close to that of ECF• The volume depends upon the capacity of the CPB

circuit• The prime for most adult perfusions contains, a

balanced salt solution • Individual recipes add

– Albumin

– Mannitol

– Heparin

– Bicarbonate

– Calcium

Conduct of CPB

• Various steps associated with conduct of CPB

1. Pre bypass preparations

2. Initiating CPB

3. Maintenance of extracorporeal circulation

4. Weaning from CPB

Pre by pass preparations

• The heart lung machine should be partially assembled 45-60min before bypass is scheduled to begin

• The system is primed with priming solution and recirculated for several minutes through a sterile 0.5u filter to remove all air bubbles and particulate emboli from system

• The patient positioned in trendlenberg position & anesthesia induced

• The surgeon prepares the sites for arterial and venous cannulation

Anticoagulation

• As bypass circuits are thrombogenic, appropriate systemic anticoagulation is a must before initiating bypass

• Heparin 300-400Ukg-1 or 2-3 mg/kg given into the central vein / directly into the right atrium

• Activated clotting time is measured 2-5minutes later and additional heparin given as required

• Supplemental dose of heparin given every hourly at the dose of 1/3 of initial dose

Cannulation

• After full heparinization, cannula’s are placed• Arterial cannula is usually placed first

because, it is technically more difficult to do & after placement, perfusionist can give additional volume via arterial cannula to support the patients BP

• The ascending aorta is most often used• Reduction of systemic BP to 90-100mm Hg

systolic facilitates placement of aortic cannula

Cannulation

• The anesthesiologist should palpate both carotids to confirm cannula is not obstructing right-sided carotid blood flow

• Once the surgeon, perfusionist & anesthesiologist are satisfied with functioning & placement of arterial line, venous cannula is then placed

Initiating CPB

• Bypass begins when surgeon unclamps venous lines & blood begins to enter extracorporeal circuit

• As blood starts to fill up venous reservoir of oxygenator, arterial pump is turned on

• The perfusionist gradually increases bypass flow rates until all systemic perfusion is machine generated

• Left ventricular vent introduced through right superior pulmonary vein & tapes around SVC & IVC are tightened

Initiating CPB

• Once machine is at full flow, all venous blood is bypassing cardiopulmonary circulation

• Ventilation at this point is discontinued• Once circulation begins, aorta is cross-

clamped & cardioplegic myocardial protection given before surgical correction is undertaken

Maintenance of bypass

• The primary goal of this period is to obtain desired levels of hypothermia, maintain adequate systemic perfusion, tissue oxygenation & manage arterial blood gases

• Ischemic depletion of high energy phosphate compounds & accumulation of intracellular calcium, depletes the energy stores of myocardium

Maintenance of bypass

• To maintain normal myocardial cellular integrity and function during CPB, the available high energy phosphate compounds have to be spared

• This is accomplished by– Hypothermia – Cardioplegia – Intermittent periods of myocardial perfusion

Hypothermia

• Hypothermia of 20-30°C is routinely used for CPB

• O2 consumption is reduced 5-7% per degree centrigrade decrease in temperature

• 10°C drop in temperature will halve the BMR

Cardioplegia

• A number of different solutions are described

• Most common compounds are– Potassium = 15-40 meq/l– Sodium = 100-120 meq/l– Chloride = 110-120 meq/l– Calcium = 0.7 meq/l– Magnesium = 15 meq/l– Glucose = 28mmol/l– Bicarbonate = 27mmol/l

Cardioplegia

• 14G needle is inserted into the aortic root below cross clamp

• 1 litre of cold (0-40C) cardioplegic solution is rapidly infused through needle

• In presence of aortic incompetence, the infusion is done directly through cannulae inserted into each coronary osteum

Mechanism of action

Increased extracellular potassium

↓

Decrease in transmembrane potential

↓

Impairment of Na+ transport

↓

Abolition of action potential generation

↓

Cardiac arrest in diastole

Intermittent periods of myocardial perfusion

• Intermittent arotic cross clamping• Intermittent coronary perfusion with

arterialized blood from heart – lung machine via 2 small cannulae inserted into coronary ostia

• Alternating ventricular fibrillation with defibrillation to allow heart to beat intermittently

Blood gas management

• Acid base management during CPB is done by two methods– Alpha stat method– pH stat method

Alpha stat method

• It considers alkaline pH seen during CPB is physiological (increased solubility of carbon dioxide seen during hypothermia raises pH)

• Hence no additional measures to correct pH/Pco2 levels is undertaken

• This is more commonly used method & it appears to preserve cerebral autoregulation & improve myocardial preservation

pH stat method

• In this, pH & Pco2 are maintained at normal values regardless of body temperature

• In order to maintain Pco2, Co2 is added to ventilating gas mixture

• This method is not preferred because patients tend to have higher CBF because of increase in Co2 content and there will be loss of cerebral autoregulation

Blood pressure & flow rate management

• A 50 —60 ml/kg of flow rate maintains mean arterial pressure at 40- 60 mmHg

• To preserve CNS function MAP should be maintained >50mmHg

• As the patient is rewarmed, MAP should also be increased accordingly, to ensure adequate CBF

• Renal function is maintained with pump flows of more than1.6L/min

Rewarming

• When the surgeon begins last phase of procedure,perfusionist begins to rewarm patient

• The rewarming should be gradual & is done over a 30 minute period

• A gradient of I0°c is maintained between patient & perfusate to prevent formation of gas bubbles due to their increased solubility as blood gets warmed

• Anaesthesiologist should ensure amnesia of patient by administering additional doses of benzodiazepines

Rewarming

• Upon completion of the surgical repair, any residual air in ventricles to be removed

• The anaesthesiologist vigorously inflates lungs to remove air from pulmonary veins & aids in filling of cardiac chambers

• TEE assesses the effectiveness of deairing process

• The aortic clamp removed to resume myocardial perfusion

• The heart is defibrillated & allowed to beat empty

Weaning from CPB

• Ensure that patients are rewarmed adequately & myocardial contractility rhythms are acceptable

• If necessary, heart rate & rhythm can be regulated either pharmacologically or electrically with appropriate pacing

• Ventilation begun with 100%-O2• Venous drainage lines are gradually

occluded, allowing, arterial return to progressively raise the circulating volume

• Venous cannulas are removed following a satisfactory interval of stable hemodynamic functions

Weaning from CPB

• Reversal of anticoagulation done with protamine

• Dose - 1 to 1.3ml (10mg/ml) for every 1000U of heparin or dose is calculated based on the heparin dose response curve

• Arterial cannulas remain in place for continued transfusion of pump contents

• When this is completed & bleeding is controlled, arterial cannula is removed & chest is closed

Post bypass period

• Patient should be shifted to the ICU, kept on mechanical ventilatory support

• He should be continuously monitored for his haemodynamic stability & cardiac electrical activity

• SBP should be maintained at 90-110mmHg, HR - 70-80bpm & Hct kept at 25-30%.

• Hypertension may be due to pain & should be treated with analgesics & sedatives

• Extubation considered when muscle paralysis has been worn off and patient is hemodynamically stable

• Most patients can be extubated by following morning

Complications

Physiologic• CNS - Intracranial Haemorrhage (14%),

bleeding,seizures• Respiratory - Pulmonary oedema, pulmonary

haemorrhage• CVS - hypo/hypervolemia leading to hypo/HTN• Alteration in R-Angiotensin – aldosterone cycle,

secondary to the non pulsatile perfusion, may lead to Renal complications

• Haematologic - Anaemia, leucopenia,thrombocytopenia (because of consumption in membrane oxygenator)

• Infections

Complications

• Mechanical• Failure of pump• Rupture of tubing• Failure of the membrane • Difficulties with cannulas