Myocardial protection facts

Question 1

What is the benefit of systemic hypothermia on myocardial oxygen consumption

Answer 1

For every 1 degree of systemic hypothermia the myocardial oxygen consumption tissue decreases by 7%.

Question 2

Benefits of Cooling for myocardium

Answer 2

Decreased metabolic rate
decreased enzyme function
decreased membrane stability 
increase calcium sequestration
increased glucose utilization 
decreased ATP generation 
decreased tissue oxygen uptake
decreased osmotic homeostasis
increased hyperviscosity with rouleaux formation

Question 3

What are options for myocardial protection during cardiac surgery

Answer 3

Cardioplegic arrest
Aortic cross-clamping with electrically-induced fibrillation
Hypothermic arrest with fibrillation (where the aorta remain unclamped)
Off pump coronary bypass surgery

Question 4

What are advantages and disadvantages of using retorgrade cardioplegia

Answer 4

Advantage
Useful in patients with severe aortic regurgitation
Significant proximal coronary disease
Useful in patients with patent previous bypass grafts

Disadvantage
concerns regarding protection of the right ventricle
less predictable distribution then antegrade cardioplegia

Question 5

What are methods of delivery for cardioplegia

Answer 5

Antegrade cardioplegia (down the root) 
usually at a pressure of 60 to 100 mmHg at 250ml/min 

Retrograde cardioplegia (in coronary sinus) 
Usually at a pressure of 30 to 50mmHg at 150 ml/min. Usually takes longer (2-4 minutes)

Question 6

How does cardioplegia cause arrest

Answer 6

Regardless of solution they alter the resting potential and cause a diastolic arrest

Extracellular solution— St. Thomas solution—prevents cardiomyocyte repolarization by increasing the potassium concentration in the extracellular fluid.

Intracellular solution–Bredtschneiders solution–blocks deploraization by lowering extracellular sodium concentrations

Question 7

Main components of cardioplegia solution (Crystalloid cardioplegia contains)

Answer 7

Sodium 110 mmol/L
Potassium 16 mmol/L
Calcium 1.2 mmol/L
Magnesium 16 mmol/L
Chlordie 160 mmol/L

Blood cardioplegia contains st.thomas solutaion at a ration of 4:1 in addition it has
1) Procaine 2) Glutamate 3) aspartate 4) oxygen free radical scavengers 5) adenosine

Question 8

Potential benefits of blood cardioplegia over crystalloid

Answer 8

Provides oxygen and nutrients
Buffering capacity
minimises intracellular edema due to its oncotic proteins
Scavengers of oxygen-free radicals as it contains superoxide dismutase, catalase, gluthione and vit C and E.

Question 9

What are myocardial protection techniques for redo cardiac surgery with a patent LIMA

Answer 9

Dissect out the LIMA and temporarily occlude when giving cardioplegia
moderate hypothermia and fibrillatory arrest (without occluding the LIMA)
cooling to 28C with either continuous retrograde cardiopplegia or intermittent cold blood caridoplegia without isolation of LIMA

Question 10

What is principle of terminal warm blood cardioplegia

Answer 10

Dose of warm cardioplegia immediately before removing cross clamp

allows washout of the products of anaerobic metabolism
provides substrate resuscitation of ischemic myocytes with oxygen and ATP

has been shown to improve myocardial metabolism and contractility once ventricular contractions resume.

Question 11

What is Buckberg technique for protection of myocardium during acute MI

Answer 11

Establish extracoporeal circulation as quickly as possible with venting of the left ventricle
initially antegrade cardioplegia using either a warm buckberg solution or cold high potassium to achieve rapid diastolic arrest
Temperatures of anterior and inferior wall of the ventricles
after each distal anastomosis–cold cardioplegia is infused into each graft and the aorta at 200ml/min over 1 minute
followed by retrograde through the coronary sinus for 1 minute
After final distal warm substrate-enriched blood cardioplia is given at 150ml/min for 2 minutes into each anastomosis and the aorta.
Cross clamp removed-18 minutes of blood cardioplegia is given at 50ml/min into the grafts
The proximal vein grafts are then completed
Heart allowed to beat for 30 minutes (empty)
then come off

Question 12

What percentage of retrograde cardioplegia returns to left coronary ostia and to the right coronary ostia

Answer 12

2/3 of total retrograde provides nutritional support (think 70%)
80% returns via LC (so total of 55%)
20% returns via RC (so total of 15%)
1/3 is non nutrictive cardioplegia (30%) and this just returns to RA/RV/LV via thebesian veins, ven-ven collaterals

Question 13

What is dose of cardioplegia induction needed

Answer 13

10ml/kg to 15ml/kg

The typical aortic root perfusion pressure should be 60 to 80 mmhg

Question 14

What are the two types of crystalloid cardioplegia

Answer 14

Intracellular—low sodium based solution
Extraceullular—high concentrations of sodium
most have K+ concentrations of < 40mmol/L

Question 15

What is a potential negative of continous warm blood cardioplegia

Answer 15

one study showed worsening neurological function (3.1 vs 1.0 percentage)

Question 16

What are goals of cardioplegia

Answer 16

Protect against ischemic injury
provide a motionless, bloodless field
allow effective post-ischemic myocardial resuscitation

Question 17

What is most common form of cardioplegia

Answer 17

Blood
Greater oxygen content
superior buffering–b/c of blood protein histidine
reduced oxygen-mediate injury from erythrocyte free radical scavengers during reperfusion
reduced coronary vascular resistance and edema
improved oxygen extraction and energy delivery

Question 18

List advantages of retrograde cardioplegia

Answer 18

benefit for high-grade coronary stenoses or obstruction (left main, acute coronary syndrome) 
More convenient during aortic and mitral valve operations 
Redo CABG (particularly with diseased SVGs or with patent LIMA to occluded LAD

Question 19

Disadvantages of retrograde

Answer 19

various degrees of maldistribution to the RV
unrecognized persistent LSVC
Coronary sinus rupture
maybe less satisfactory in hearts with severe LV hypertrophy

Question 20

What are benefits of intermittently(every 15 to 20 minutes) giving cardioplegia

Answer 20

Maintains arrest
restores desired levels of hypothermia 
buffers acidosis 
washes away metabolites 
replenishes high-energy phosphates 
restores depleted substrates 
counteracts edema with hyperosmolarity

Question 21

What are differences with neonates and adults in terms of cardioplegia

Answer 21

Usually single dose cardioplegia with ischemic times of 65-85 minutes
with stand hypoxia better
greater glycogen stores
more amino acid utilization
slower ATP breakdown
amino acid substrate enhancement is beneficial
DHCA is more commonly used

Question 22

What is protection strategy in Acute LAD occlusion

Answer 22

Initial administration of warm blood cardioplegia
Retrograde and antegrade
can use a vein graft to LAD
Warm hot to come off

Question 23

Which chamber is prone to hypoperfusion with RCP

Answer 23

Right ventricle

Question 24

When RCP is administered roughly what percentage of nutrient flow drains by the routes indicated below

Answer 24

Left coronary ostium (30 +/- 10&)
Right Coronary Ostium (3 (+/- 3%)
Ventricles 67 +/- 10%

Question 25

By what anatomical pathway does RCP perfusion drain into the ventricular cavities

Answer 25

Thebesian veins

Question 26

What’s one surgical modification that can improve retrograde perfusion

Answer 26

purse string suture surrounding the coronary sinus and direct placement

Question 27

What are cardiac metabolic demands at various conditions

Answer 27

Beating loaded heart 10ml of oxygen per 100 mg myocardium per minute

Unloaded heart 6ml of oxygen per 100 mg myocardium per minute
(On CPB)

Arrested heart 1ml of oxygen per 100 mg of myocardium per minute
(cardioplegia)