Blood versus Crystalloid Cardioplegia
Blood cardioplegia resembles normal physiology and causes an improved
postoperative cardiac outcome(23). It allows for
rapid cardiac arrest in an oxygenated environment, intermittent
reoxygenation of the myocardium when cardioplegic solution is delivered
so anaerobic substrates like glucose and insulin are no longer
needed. In a meta-analysis by Guru et al., blood cardioplegia was
associated with decreased low-output syndrome (LOS) and creatine
kinase-myocardial band (CK-MB) release therefore, a decreased mortality
rate(23). Additionally, blood cardioplegia preserves
ventricular performance and systolic function. A meta-analysis by Zeng
et al. found that cold blood cardioplegia also caused a significantly
lower rate of perioperative MI compared to cold crystalloid
cardioplegia(24). Blood cardioplegia’s oxygen carrying
capacity is advantageous(23). Hypothermia during
cardiac surgery may offset this advantage by causing a left shift in the
oxyhaemoglobin dissociation curve- so less oxygen is available for
myocardial tissue(24). At 20⁰C only 50% of the total
oxygen is released from blood cardioplegia, falling to 37% at 10⁰C.
This is a stark contrast to crystalloid cardioplegia which releases all
of its oxygen at all temperatures.
There is, however, myocardial uptake of haemoglobin bound oxygen from
the blood cardioplegic solution during hypothermia. This is due to the
acidotic environment developing during cardiac arrest which shifts the
oxyhaemoglobin dissociation curve to the right, along with a higher
tissue affinity for oxygen in hypothermia. Delivering oxygen to the
ischaemic myocardial tissue in blood cardioplegia may produce oxygen
free radicals which cause ischaemia and reperfusion injury.
However, blood also contains endogenous oxygen radical scavengers which
protect against this(24).
It is important to note that haemoglobin has six times higher buffering
capacity than plasma proteins. Whole blood also contains physiological
oncotic pressure constituents. This minimises myocardial oedema,
although this is not a significant issue with crystalloid cardioplegia
either. Red blood cells present in blood cardioplegia are also critical
for perfusion of the capillary bed due to improved rheologic effects
which leads to better oxygen delivery.
With blood cardioplegia there is also avoidance of the cost and need to
prepare a complex pharmacological mixture as in crystalloid.
However, apparatus to deliver blood cardioplegia is more complicated and
expensive than crystalloid. Due to these higher costs and impaired
visualisation of the operating field some surgeons choose not to use
blood cardioplegia(23). A study by Gundry et al.
concluded that blood cardioplegia was also associated with a higher rate
of perioperative and postoperative conduction disturbances such as right
bundle-branch block, than crystalloid(25). On the
contrary, crystalloid cardioplegia is widely used due to its simplicity,
lower costs and good efficacy in the majority of
patients(26). It allows for good visualisation of the
operating field due to being clear.
A strong argument against the use of crystalloid cardioplegia is it
contains only one-fourth as much oxygen as blood
cardioplegia(19). It also decreases the oncotic
pressure, increasing risk of oedema(26). Some studies
have found crystalloid cardioplegia causes intracellular oedema,
depletion of glycogen stores and higher release of CK-MB. It also causes
a higher degree of intra-operative haemodilution which is associated
with increased blood transfusion requirement, significantly greater
intensive care requirements, longer hospital stays, higher operative
costs and higher mortality rates and postoperative organ failure e.g.
renal failure(26). Mullen et al. concluded in a study
that although crystalloid cardioplegia was associated with higher
incidence of MI and higher CK-MB release postoperatively, it lead to
better right ventricular systolic function compared to blood
cardioplegia(27). The advantages and disadvantages of
blood and crystalloid cardioplegia are summarised in Table 3.