This post outlines the steps in traditional, central cannulation for cardiopulmonary bypass (CPB). Depending on the nature of the surgery and patient’s comorbidities, other sites are often cannulated (left heart bypass, femoral-femoral peripheral cannulation, etc.)
Cardiothoracic surgery is a complex art requiring precision and coordination among various teams. To facilitate surgery, CPB allows the surgeons to have a bloodless, motionless field to operate in. The lungs and heart are both arrested while the CPB circuit handles systemic perfusion and gas exchange.
Once our surgical colleagues are ready to begin CPB cannulation, cardiothoracic anesthesiologists will work with perfusionists to administer systemic heparinization. This is usually 400 units/kg targetting an activated clotting time (ACT) of greater than 450 seconds. This ACT target varies at each institution. Heparin is arguably the most important drug in cardiac anesthesia, so whenever I heparinize, I ensure that the medication is going through my most reliable access (ie, central line that I can aspirate blood from). If we were to expose the patient to the CPB circuit without heparinization, the pump and oxygenator would clot leading to a catastrophic outcome. As a side note, heparin boluses tend to decrease ionized calcium resulting in transient vasodilation and hypotension. Nothing a small bolus of norepinephrine can’t fix. 😉
Basic CPB Circuit
In very simple terms, the CPB circuit takes blood drained from the venous cannula into a reservoir usually by gravity, runs it through a pump (centrifugal is more popular than roller pumps these days), then to an oxygenator where gas exchange occurs and halogenated volatiles like isoflurane are added. Finally, the blood passes through an arterial filter on its way back to the body through the arterial cannula.
Arterial and Venous Cannulation
The surgeons begin cannulation by placing one or two concentric purse-string sutures on the anterior surface of the ascending aorta (Ao) for the smaller arterial cannula and on the right atrial appendage (RAA) to access the right atrium (RA) and inferior vena cava (IVC) for the larger venous cannula. Once the sutures are in and the ACT is at goal, the aorta is cannulated first. We want to get the systolic blood pressure down to ~100 mmHg to avoid creating a dissection flap during aortic cannulation. An aortotomy is performed through the purse-string sutures, the cannula is directed upwards into the aortic arch, and the purse-strings are cinched down to tightening the opening around the cannula. This same process is repeated for the venous cannula in the RAA.
Once bypass is initiated, the patient will be cooled and an aortic cross-clamp is placed between the aortic cannula and the antegrade cardioplegia cannula (see below). At this point, the heart is still beating until cardioplegia is administered. We can also stop ventilating the patient’s lungs as the CPB circuit will handle gas exchange.
Why is the aorta cannulated first? With an arterial cannula, it’s possible to immediately crash onto CPB in the event of hemodynamic instability. The venous cannula basically supplies “fuel” for the CPB circuit and can be substituted with, for example, pool suctions that siphon hemorrhage from the chest cavity.. When decannulating, the aortic cannula is also the last to be removed to allow transfusion of remaining blood within the CPB machine into the patient as well as “insurance” just in case the patient needs to crash back on pump.
Cardioplegia and Vents
Cardioplegia comes in many forms, but it’s basically a high potassium solution intended to achieve diastolic arrest of the heart. Why do we need to arrest? Once the aortic cross-clamp is placed and the heart is drained of blood, the heart’s supply of oxygen is significantly diminished. To prevent ischemic damage, we need to also drive down the myocardium’s metabolic needs for oxygen. This is why we arrest in addition to cooling the heart.
Cannulas are placed in the aortic root (antegrade) and in the coronary sinus (retrograde). Antegrade cardioplegia perfuses coronary arteries the same way that blood normally perfuses them. Solution is infused into the aortic root where the left main and right coronary arteries come off just above the aortic valve’s leaflets. It relies on a competent aortic valve, otherwise hundreds of milliliters of cardioplegia solution would drop back into the left ventricle (LV) increasing wall tension, myocardial demand for oxygen, and result in ischemic damage. This is why vents are placed. An LV vent is typically advanced from the right superior pulmonary vein (RSPV) into the left atrium, through the mitral valve, and into the LV to keep it empty. The antegrade cardioplegia cannula also doubles as a vent for the aortic root too. Vented blood rejoins the venous reservoir at the beginning of the CPB circuit.
Retrograde cardioplegia is infused through a cannula in the coronary sinus – the common venous drainage for most of the coronary network. This cannula is placed within a small venous structure, so we transduce a pressure off it to a.) ensure proper placement and b.) keep the infusion pressures low to avoid coronary sinus damage.
Surgeons will work with the perfusionists to achieve a good diastolic arrest with antegrade and retrograde cardioplegia before going further. As you can see, mitigating the oxygen supply/demand mismatch is accomplished by cooling, venting, and arresting the heart.
As always, drop me a comment below with any questions or suggestions! 🙂