ECMO: pitfalls

During ECMO venous blood is drained from the venous system through catheters percutaneously inserted either peripherally, via cannulation of a femoral vein, or centrally, via cannulation of the right atrium. The blood is pumped through an artificial lung to oxygenate it and to extract carbon dioxide. The circuit is warmed and heparinized. Blood is then returned back to the patient either in an arterial (aorta, VA) or venous access (right atrium, VV). The VA mode substitutes both heart and lung function, and can be achieved by either peripheral or central cannulation, the VV configuration provides respiratory support only and it is preferred in case of respiratory failure as it leaves normal hemodynamics, it is achieved by peripheral cannulation, usually of both femoral veins. The pumps inserted in the circuit may be of centrifugal or roller type. The centrifugal pump is gravity independent and the inflow is generated by negative pressure at the pump head. Negative pressure should not exceed -20 mmHg to avoid excessive hemolysis, cannula displacement, thrombus at the inflow and inadequate inflow pressures in the patient. The roller pump is gravity dependent, so it must be positioned below the patient level. The inflow is regulated by a bladder regulated by a servomechanism while the forward flow is generated by the compression of the tubing by the heads of the pump and the back plate of the housing. The flow depends on the rotations per minute of the pump, on the occlusion degree and on the tube diameter. Problems may be due to rupture at the tube/heads interface and blow out in the arterial line. New generation circuits are characterized by all surfaces coated with covalently bound heparin and the catheters are wire-reinforced. The system is primed first with a balanced crystalloid and protein coating then the blood is inserted into the circuit using with packed red blood cells and fresh frozen plasma. The support starts quite slowly to allow an adequate mixing of the prime with patient’s blood, the gas flow into the oxygenator is set at an appropriate rate and pressure to avoid apparatus rupture and it is set to maintain adequate CO2 tension. Oxygenation is obtained by the combination of minimal mechanically ventilating the patient natural lung. Several ventilator approaches have been described in association with ECMO. We believe that the most convenient are the ones which try to minimize the potential harm of mechanical ventilation. Therefore, whatever approach is used, attention should be paid to minimize FiO2, plateau pressure and frequency. In our experience, immediately after starting ECMO we keep FiO2 and mean airway pressure as before the bypass. As we decrease the ventilation down to 4-5 bpm this implies an increase of PEEP in order to maintain mean airway pressure. During the bypass, as soon as the patient improves, we first decrease the FiO2 down to 0.4, afterward we decrease PEEP at a rate not greater than 1 cmH2O every 2 hours. When reasonable ventilator set is reached, as an example FiO2 equal to 0.4 and PEEP between 10-15 cmH2O the formal weaning begins by decreasing the gas flow throughout the membrane lung. We decannulate the patient when he is able to tolerate mechanical ventilation without any extracorporeal support (gas flow in the membrane lung equal to 0). Several aspects must be considered evaluating the institution of ECMO. The first one, as previously told, is the likelihood of organ recovery with therapy and during ECMO. Accepted exclusion criteria include contraindication to anticoagulation, (despite the use of surface heparinized apparatus requires reconsideration of this criterion), multiple organ failure, advanced age or poor final prognosis of the underlying pathology, left ventricular failure, immunosuppression, unwitnessed cardiac arrest or cardiac arrest of prolonged duration, aortic dissection or aortic incompetence, sever damage of the central nervous system. The inclusion criteria depend on the centers that performs ECMO [10]. Patients should have been mechanically ventilated for less than 14 day, although some centers exceeded this limit, maximal medical management must have been failed, the disease must be reversible and the mortality risk must be high, although its definition is not easy. Centers usually apply a set of criteria that are modification of the criteria reported by Zapol et al. They include oxygenation, shunt, compliance and sometimes Murray score. Complications are related to technical aspects and to patient complications. Technical aspects include tubing rupture, pump/heater malfunction, oxygenator failure, cannula related problems. Patient related problems are bleeding, neurological complications, additional organ failure due to non-pulsatile perfusion at end-organs, barotrauma, infection and metabolic disorders. The major complication is bleeding which occurs in 10-30% of the patients and that can be reduced reducing heparinization o f the circuit. It must be noted that whatever maneuver, which is usually without risk, as an example the insertion of naso-gastric tube, may be, in these patients, a source of bleeding. Great attention, therefore, should be paid to all the maneuvers which potentially may damage the tissue surface. It worth to underline, however, that the real nightmare of this treatment is the occurrence of intracranial bleeding.