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MYOCARDIAL PROTECTION
DEFINITION:
It is defined as specific intra op technique designed to protect heart
from ischemic state associated with extra corporeal circulation.
TECHNIQUE:
Most cardiac operations are performed with cardioplegia and a period
of ACC. This technique provides the surgeon still and bloodless field.
Application of ACC isolates coronary circulation from blood volume
circulating through CPB but not all the blood flow to myocardium is
eliminated
Non coronary collaterals in pericardial attachments and pulmonary
vein walls continue to provide some blood flow during period of ACC.
HYPOTHERMIA:
A combination of 3 techniques to achieve to moderate systemic and
profound myocardial hypothermia
 Systemic cooling of blood as it passes through CPB
 Topical cooling of heart with cold solution
 Infusion of coronary artery with chilled CP
SYSTEMIC HYPOTHERMIA / CORE COOLING:
Cooling to 28*c to 32*c is induced at onset of CPB to reduce cellular
metabolic demands of myocardium. When systemic hypothermia has been
achieved, the ventricle fibrillates. At this point, occlusive clamp is applied to
aorta , proximal to aortic cannula used for arterial return from CPB circuit.
TOPICAL / SURFACE COOLING:
Reduce rewarming between the doses of CP , performed by bathing
the heart continuously or intermittently with ice cold saline / RL . Care is
taken to avoid exposing left side of pericardium containing phrenic nerve to
iced solution to avoid nerve injury.
An isolating pad may also be placed between and the left side of
pericardium for this purpose. As an alternative to bathing, a cooling pad or
jacket may be wrapped around the heart .Ventricular temperature is
measured by thermistor probe placed on LV myocardium. With these
measures myocardium temperature to 10*c.
CARDIOPLEGIA:
Induction and maintenance of the heart in an arrested state using a
solution infused into coronary artery circulation.To protect against ischemic
injury during ACC when normal antegrade coronary artery blood flow is
absent.
Categories:
 Autologous blood
 Crystalloids
 Oxygenated crystalloids
Autologous blood cardioplegia has been preferred type in 72% of
cardiac operations , crystalloid CP in 22% , oxygenated crystalloid CP in
6%.
Advantages of blood CP:
 Less systemic hemodilution
 Delivery of oxygen and provision of exogenous buffer
 Free radical scavengers
 Proteins to control oncotic pressure
ADMINISTRATION OF CP
Antegrade:
Infusing through catheter placed proximal to ACC in aortic root. With
a competent AV and aorta clamped distal to catheter , CP solution passes
directly into coronary artery . Aortic root pressure is measured
simultaneously through a separate catheter during infusion of CP.
Cold CP:
Most often,cardiac arrest is induced and maintained with cold
CP.Used when a fast decrease in energy metabolism is preferable.
Warm CP:
CP delivered at normothermic temperature , for patients with acute
ischemia. An oxygenated CP solution at normothermia as initial dose to
induce cardiac arrest increase myocardial oxygen uptake is thought to
provide additional benefit in patients with acute ischemia.
Warm CP induction usually is followed by intermittent doses of cold
CP to prevent myocardial ischemia during ACC.
 Intial doses of 1-1.5lt cold CP – antegrade technique , rate of
administration is adjusted to maintain aortic root pressure
between 80-100mmhg to ensure effective delivery of solution
and achieve rapid diastolic arrest.
 Global cardiac arrest occurs in 30 sec of CP infusion but may
take
1-2min
in
presence
of
stenotic
/
occlude
coronaries.because the blood flow from non coronary collateral
arteries washes away infused CP solution and gradually
rewarms heart
 Intermittent infusion of 500 – 750ml of CP administration every
20mn during period of ACC.
Retrograde CP:
Catheter is introduced through the right atrial wall and guided by
digital manipulation into coronary sinus , then coronary veins and perfuse
myocardium.
Many use a combination of antegrade and retrograde CP.Usually initial
dose is given using an antegrade route with subsequent doses in
retrograde infusion for better myocardial cooling distal to disease arteries.
Retrograde CP is used in AVR with AR.as an alternative CP infused
through small individual catheter / cannulae directly placed into ostio of
coronary artery.
Antegrade CP delivery during bypass grafting is limited due to;
 Jeoparadised myocardium does not receive CP untill distal
anastomosis to supplying artery is constructed.
 Muscle supplied by an artery receiving IMA pedicle graft is not
revascularised until end of ACC period.
CP solution can be injected directly into proximal end of vein graft after
completion of distal anastomosis / infused in retrograde fashion while
proximal anastomosis is being constructed.
If the proximal anastomosis are done first , CP delivered into aortic root
after ACC will flow through graft to myocardium it supplies as soon as distal
anastomosis is constructed.
Administration of CP through graft is not possible when IMA is used as it
proximal attachement to subclavian artery is left intact.
Blood product
Coagulation factor
fibrinogen
FFP
1u/ml
2-4mg/ml
Platelet
2u/ml except factor V,VIII
4-8mg/ml
Cryo
5u/ml
10mg/ml