Blood is drained from the vena caval filters directly into a reservoir. Normally a single two-stage cannula is used (drains blood from the RA and the IVC), however if a bloodless heart is required, two cannulae (SVC and IVC) can be used instead. Venous return is dependent on the hydrostatic column between the heart and venous reservoir, fluid status, and placement of the cannulas – if venous return is insufficient, the bed height can be increased
Once in the venous reservoir, blood is then drawn from the reservoir by a centrifugal pump (less trauma to RBC and fewer air bubbles than roller pumps, also less risk of line rupture because centrifugal pumps are afterload dependent. The tradeoff is that flow cannot be guaranteed, thus necessitating a downstream flow meter). Note that roller pumps are still sometimes used for cardioplegia and/or vent/cardiotomy suction. Both roller and centrifugal pumps produce constant blood flow – some studies [Driessen JJ et al. Perfusion 10: 3, 1995; Song Z et al. J Extra corpor Technol 29: 170, 1997] have suggested that pulsatile flow is advantageous, but this has not been proven
Centrifugal pumps then send blood to a heat exchanger and then a membrane oxygenator (less traumatic than bubble oxygenators which are highly efficient but which induce hemolysis and platelet destruction, and activate the complement system if bypass times exceed 90 minutes). Thermal control is critical, as each degree C lowers metabolic rate by 8% (at 28C, metabolic rate is 50% of baseline).
Oxygenated flow is then split into two divisions – a left heart division passes through a filter and then into the aorta (post-clamp), and a right heart division passes through a cardioplegia pump (which adds KCl), a cardioplegia heat exchanger, and then is split into a RA (retrograde coronary sinus catheter) and pre-clamp aorta (anterograde cardioplegia). Blood (from suction) is also collected in a cardiotomy reservoir, which then filters and defoams the blood before sending it back to the oxygenator
Provides systemic pressure. Placed distal to the cross clamp. It is thought (although not proven) that placement of the aortic cannula during normal or high blood pressures can lead to dissection, thus cardiac surgeons will often request that SBP < 100 or MAP < 80. The aortic cannula is always placed prior to the venous cannulas (and removed last), because in the event of an emergency, the perfusionist can temporarily initiate cardiopulmonary bypass via a single aortic cannula (i.e. without venous cannulation). “Crashing” onto bypass is short-lived, however, as the CPB machine’s reservoir is quickly depleted, thus the central veins need to cannulated quickly and venous return to the pump established as soon as possible. Note that the aortic cannula does not have to enter directly via the aorta – an arterial cannula can be threaded into the aorta via the femoral artery and in some instances, via the subclavian artery
Importantly, embolization during aortic cannula placement is thought to be a major source of neurologic morbidity, and the traditional means of “palpating” the ascending aorta to determine its suitability for cannulation is probably inadequate [Wareing TH et al. .J Thorac Cardiovasc Surg 103: 453, 1992]. Despite this, it does not appear that routine epiaortic scanning reduces cerebral embolic load (as measured by TCD of the MCA) [Djaiani G et al. Anesth Analg 106: 1611, 2008;FREE Full-text at Anesthesia & Analgesia]
Depending on the needs of the surgeon, one or two venous cannulas may be placed. Most commonly one venous cannula is placed into the right atrium and threaded into the IVC. The major advantages of a single-cannula technique are speed and the use of fewer incisions. The major disadvantage of the single cannula is the inability to stop all blood from passing through the heart and lungs, as well as physical interference with the right atrium. In instances in which a completely bloodless field is desired (ex. valvular surgery), or in which access to the right atrium is necessary, two cannulas are used. When two cannulas are used, blood from the coronary sinus will bypass both cannulae, mandating either an atriotomy or placement of a vent. Note that femoral venous cannulas cannot completely drain systemic venous blood and therefore only partial CPB is possible using femoral access.
Regardless of the cannulation technique selected, all cases of CPB will produce some blood flow return to the left ventricle (ex. thebesian veins, bronchial veins, shunts, aortic insufficiency, etc.). In order to minimize increases in temperature and pressure (both of which are disadvantageous), all CPB cases require an LV vent. Additionally, the bicaval cannulation technique requires the use of a second vent to capture coronary sinus blood that misses the IVC and SVC cannulae.
Initiation of Cardiopulmonary Bypass
The predominant cause of hypotension following initiation of CPB is decreased SVR (secondary to reduced blood viscosity, dilution of endogenous catecholamines in priming solution, and differences in pO2, pH, and electrolyte concentrations between the priming solution and native blood).