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The non-occlusive feature of the CP prevents generation of excessive pressure in the extracorporeal circuit, thus preventing circuit rupture. Myocardial protection To provide a dry, motionless, operative area, a cross-clamp is placed across the ascending aorta above the coronary ostia and proximal to the aortic cannula, thus isolating the coronary circulation and preventing blood entering the chambers of the heart.
Therefore, techniques of myocardial protection are used to preserve myocardial function and prevent cell death. Cardioplegic techniques for myocardial protection involve the delivery of cardioplegic solution to the myocardium to provide diastolic electromechanical arrest. This is administered by an anterograde approach via the aortic root or direct coronary ostium access, or by a retrograde manner if severe coronary artery occlusions exist. The latter may not provide adequate right-heart protection; thus, the delivery of both anterograde and retrograde cardioplegia may ensure adequate distribution and subsequent improved myocardial protect.
Intermittent cardioplegia delivery i. Continuous administration would improve for myocardial protection but this is not always practical.
Fast inward sodium channels are inactivated preventing the upstroke of myocardial cell action potentials, causing the myocardium to become unexcitable in diastolic arrest.
Blood cardioplegia is thought to offer the advantage of delivering oxygen at the cellular level via its haemoglobin content, but it may be less efficient during cold delivery because of the left shift of the oxygen-dissociation curve. Blood cardioplegia also provides other benefits including hydrogen ion buffering, free-radical scavenging, reduced myocardial oedema and improved micro-vascular flow.
Various ratios of blood to cardioplegic solution can be administered e. The addition of glutamate and aspartate to cardioplegic solutions may promote oxidative metabolism in energy-depleted hearts. The use of esmolol and nicorandil as cardiac electromechanical arrest agents has also been considered as possible alternatives to potassium. The negative inotropic and chronotropic effects of esmolol result in a reduction in myocardial oxygen consumption.
The potassium channel opener, nicorandil, may offer advantages over potassium-based cardioplegia by providing cardiac arrest near its resting membrane potential, allowing balanced trans-membrane ion gradients and minimal energy requirements. A variation of this technique involves hypothermic systemic perfusion combined with prolonged VF, enabling continuous coronary perfusion.
Unfortunately, this greatly increases myocardial oxygen consumption compared with the empty beating heart; it also reduces subendocardial perfusion. Systemic hypothermia has remained a cornerstone of cardiac surgical practice, with a great deal of evidence demonstrating that it decreases myocardial oxygen consumption.
This ensures protection and increased tolerance for ischaemia of vital organs and allows periods of low blood flow during CPB. Total body oxygen consumption is said to have a Q10 factor of approximately 2. It is also hypothesized that electromechanical work is the main determinant of myocardial oxygen requirement rather than cooling of the heart.
Acid—base management Management of the acid—base status throughout the hypothermic CPB period remains controversial. Arterial blood gas parameters are either corrected for temperature and pH is kept constant i. Autoregulation, which normally keeps cerebral blood flow constant at arterial pressures of 50— mm Hg, is lost during pH-stat and cerebral blood flow becomes pressure-dependent.
Alpha-stat blood gas management seems more appropriate as it maintains autoregulation, offers better control of cerebral blood flow and demand, and limits cerebral microemboli load. However, the pressure-dependent characteristics of pH-stat may improve cooling and oxygen delivery to the brain.
Screen filters comprise a woven mesh material such as polypropylene, with a defined pore size to determine its filtration ability. Depth filters incorporate packing materials such as Dacron wool or polyurethane foam that have no definite pore size; thus, filtration depends on the thickness, tightness and tortuous nature of the packing.
Generation of gaseous e. An arterial screen filter is often used to limit sources of embolic load to the patient. Air emboli retention within the wetted filter is accomplished by surface-active forces, which maintains fluid in the pores and prevents displacement of fluid by gas through the pores.
Bacterial filtration of the gas flow line to the oxygenator and the use of pre-bypass filters to reduce extracorporeal circuit debris are often practiced. There is a current trend to utilize leucocyte depleting filters, as activated leucocytes are considered to have a pivotal role in the inflammatory response to CPB and post-ischaemic injury.
The application of this technique during all or part of CPB i. These devices mainly consist of a hollow-fibre semipermeable membrane to allow the passage of water and electrolytes from the blood to a filtrate compartment. Conventional ultrafiltration haemoconcentration and zero balance ultrafiltration filtrate replaced with equal crystalloid volume can be performed during CPB; modified ultrafiltration can be initiated at the termination of CPB.
Conduct of CPB Components selected for the extracorporeal circuit are determined to minimize haemodilution, while being able to achieve blood flow requirements without excessive resistance or pressure generation. The CPB circuit is typically primed with a crystalloid solution with electrolyte composition and osmolarity similar to plasma and a colloid solution to provide a degree of plasma colloidal oncotic pressure, which is reduced as a repercussion of haemodilution in CPB.
Other prime additives may include mannitol diuresis, oedema reduction, possibly free-radical scavenging ; sodium bicarbonate buffers the prime ; heparin; and blood, if required.
Cardiopulmonary Bypass and Mechanical Support: Principles and Practice
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