test 5 Flashcards
myocardial protection pre-1955
Systemic hypothermia
1955 myocardial protection
-Melrose advocated the use of high potassium solutions to induce cardiac quiescence. Caused permanent myocardial injury.
1956 myocardial protection
-Lillehei introduced retrograde cardioplegia
1973 myocardial protection
-Gay & Ebert reintroduced hyperkalemic arrest with lower potassium concentrations (<20 mmol), preventing permanent myocardial injury.
1979 myocardial protection
-Buckberg & Follette introduced blood cardioplegia
europe myocardial protection
-Bretschneider HTK (histadine, tryptophan, alpha ketoglutarate)
-Low Calcium, low sodium, procaine with histadine buffers
-Non-depolarizing arrest
NOW – Custodial HTK
-St. Thomas Solution (London)
-Intracellular ionic concentration
-Normocalcemia, hyperkalemia (16mmol/L)
-NOW – Plegiol in US
Coronary Blood flow is determined by hemodynamic factors
- Perfusion pressure
- Coronary Vascular Resistance
-Q = P/R
Delivery of Oxygen (DO2) to myocardium (oxygen supply) is determined by two factors:
- Coronary blood flow (ml/min)
- Oxygen content of the blood (ml O2 /mL blood)
-O2 Delivery = CBF x CaO2
Consumption and Demand often used interchangeably
- Not equal
-Demand = Need
-Consumption = Actual amount of oxygen consumed per minute
how is oxygen used
- Regenerate ATP
*Na/K-ATPase pump
- Myocyte contraction and relaxation
Oxygen consumption (mL O2/min per 100g):
arrested heart\
resting heart rate
heavy exercise
2
8
70
using hypothermia and fibrillation
-reduces workload of the heart
Lowest level of Cardiac Oxygen Consumption
-When heart is arrested
Highest level of Cardiac Oxygen Consumption
- Shortly after weaning from bypass
*Heart is repaying oxygen debt
Ischemia results when oxygen delivery does not meet
-Oxygen demand
-Supply/Demand
*Normally – Supply is greater than demand – Ratio >1
*With Ischemia – Supply is less than demand – Ratio <1
-ANAEROBIC metabolism
-Production of lactic acid
result of the lactic acid buildup
- Decreased intracellular pH
-decreases the stability of the cellular membranes
-Decreases the stability of the mitochondrial membranes.
-Impairs Na-K ATPase
*Leads to calcium influx
*Calcium overload
ATP generated from aerobic metabolism is used preferentially for 1_________, whereas anaerobically produced ATP is used for 2___________
- myocardial contraction (work)
2. Cell survival and repair (work to survive)
increase in heart demand for oxygen, what needs to be done
- increase in coronary blood flow
- can’t extract more
Coronary blood flow is dependent on the and normal flow pressure
- transmural gradient
- Coronary Perfusion Pressure = DBP - LVEDP
- normal 60-80 mmHg
- Pressure gradient of at least 15mmHg may be necessary for survival
coronary blood flow during isovolumetric contraction and ejection compared to diastole
- flow is lower than during diastole
Myocardial Protection: Pre-Ischemic Intervention
- Minimize on-going ischemia *Pharmacology (ie. Nitroglycerin) - Prevent ventricular distension *Vent!!! - Myocardial preconditioning
Myocardial Protection: Preconditioning
- Myocardium that has undergone one or more brief periods of ischemia may be better able to tolerate subsequent prolonged ischemia.
- getting the heart used to ischemia
- for beating heart it might help
- also used when myocardial protection not optimal
Myocardial preconditioning can be achieved by
- ischemia
- drugs
- Cardiopulmonary bypass itself may override these other methods and be the “best” preconditioning tool
Why give Cardioplegia?
- Cardiac quiescence
- Bloodless field
- Preservation of myocardial function
- Induces myocardial hypothermia
Goals of Hypothermic Cardioplegia
- Immediate / sustained electromechanical arrest
- Rapid / sustained homogenous myocardial cooling
- Maintenance of therapeutic additives in effective concentrations
- Periodic washout of metabolic inhibitors
Without cardioplegic arrest
- Irreversible ischemic injury within 20 minutes
With myocardial protection strategies
- Can prolong ischemia to more than 4-5 hours without irreversible damage
Normal Cardiac Action Potential
PHASES: •0 – Na+ influx •1 – Transient K+ efflux •2 – Ca++ influx •3 – K+ efflux •4 – Na/K ATPase
Mechanism of Potassium Arrest
- With a blood potassium of 8-10 mEq/L, depolarization of the cell occurs and sodium rushes into the cell.
- Because the extracellular potassium is so high the cell cannot repolarize and the sodium remains inside the cell.
*sodium gates do not reset: fast-gates remain open; slow gates remain closed
- After AoXC, potassium washes out of the extracellular space (Repolarization) - stops phase 3
Mechanism of Custodial-HTK (“Low Sodium”)
- stops phase 0
- nonpolarizing phase
Mechanism of Del Nido (“Low Calcium”)
- stops phase 2
- stop contraction phase
- still depolarized but arresting in this phase
Components of Myocardial Protection
- Route of delivery
- antegrade / retrograde / both / directly into opstia / conduits
- Composition of solution
- Crystalloid / Blood / Microplegia
- Temperature
- Delivery interval
- Intermittent / Continuous
- Additives
- Monitoring
- Preparation for reperfusion (hotshot)
Antegrade pros
- Simple
* Mimics normal coronary flow
Antegrade cons
- Requires competent Aortic Valve
- Can interrupt surgery
- Advanced CAD
Retrograde pros
- Avoids limitations from AI and CAD
- Doesn’t interrupt surgery
- Augments de-airing
Retrograde cons
• Catheter placement difficult
• Closely monitor
pressure
Integrated (both) pros
• Uniform distribution of cardioplegia
Integrated (both) cons
- Complex
* Closely monitor pressures
Antegrade Delivery
- Initial dose = ~10-15 mL/kg
- Up to 30 mL/kg in pediatric patients.
- 4:1 CPG = 4 parts blood = 800mL AND 1 part CPG = 200mL
- Subsequent doses are
- Less Volume
- Lower potassium concentration
Antegrade Delivery pressures and flow
- Line pressure:
*125-150 mmHg
*Goal : Maintain a ROOT pressure of 50- 100mmHg - Flow is generally 250-400 mL/min
*150 ml/minute/m2
Antegrade Delivery Benefits
- Easy - Physiological flow pattern - Quick arrest - Appropriate distribution to the right and left heart. - Root is tolerant of higher pressures
Antegrade Delivery disadvantages
- Requires competent
aortic valve
- Poor distal perfusion in diseased arteries
- Poor subendocardial perfusion (especially in LVH)
Retrograde Delivery
- Delivered into Coronary Sinus
*Must be vented
- A balloon is inflated on the cannula that provides two functions:
*Prevents backflow
*Holds cannula in place
*Flow is ~200 mL/min
*Flow should be titrated to maintain a coronary sinus pressure of 30-40 mmHg.
Retrograde Delivery benefits
- Ideal for aortic valve regurgitation
- Good distal perfusion of obstructed arteries
- Retrograde flushing of emboli – augments de-airing
- Does not impede conduct of case - can run continuously
*Warm continuous
Retrograde Delivery disadvantages
- Catheter placement can be difficult
- Impaired right heart protection
* Right coronary veins drain into the right atrium
- Surgical skill required for placement of cannula
- Distracting to perfusionist
- Possible coronary sinus rupture
Direct Ostial Delivery
- Not as common as traditional antegrade or retrograde.
*Position dependent
- Hand-held cannula used to directly perfuse ostia
*AVR
*Aneurysm / Dissection
- 250-300 mmHg required (circuit pressure)
*High pressures due to small cannula orifice.
- 150-250 mL/min
*Normal perfusion is 5-8% of cardiac output
Delivery Through Grafts
- After the initial dose for CABG, may infuse cardioplegia directly into the vein grafts.
*Infusion pressure of 50 mmHg
*Flow rate of 50-100 mL/min. - Surgeon may use hand-held syringe
Delivery Through Grafts Allows surgeon to check
- Anastamosis
- Adequacy of flow
- Flow to previously under-perfused areas
Delivery Through Grafts benefits
- Allows antegrade protection of areas of coronary artery disease
- Obviates limitations from aortic insufficiency and coronary artery disease
- Allows delivery without need to pressurize aortic root or interrupt surgery
Delivery Through Grafts disadvantage
- Requires graft placement
- Complexity
- Distribution only to those areas perfused by graft
Integrated Delivery
- It is common to give a large arresting dose of antegrade cardioplegia, followed by a smaller dose of retrograde cardioplegia.
- More likely to perfuse all areas of the heart.
Integrated Delivery benefits
- Benefits of all methods utilized
Integrated Delivery disadvantages
- Complexity
