The Lfppvecco2R technique

LFPPV-ECCO2R is performed using a venovenous bypass. After a bolus of intravenous heparin (100 IU/kg), catheters are usually inserted in femorofemoral sites, even if a jugulofemoral approach is possible. These catheters, which are reinforced with spring-wire and up to 34 French in size, are positioned percutaneously using a modified Seldinger technique.

The extracorporeal apparatus consists of a blood section (artificial lung, blood pump, blood circuit), a gas section (flowmeter, gas lines, humidifiers), and a monitoring apparatus (blood flowmeter, differential pressures across the membrane lung, temperature). A water heater is used to warm the blood flow.

The extracorporeal circuit is assembled and primed with Ringer's lactate after a CO 2 flush to prevent bubble formation. Immediately before connection, the priming solution is substituted with heparinized whole blood at 37 °C. When the patient has been connected to the extracorporeal circuit, the blood flow is started slowly, paying close attention to body temperature and hemodynamic indices. After 20 to 30 min it is usually possible to set the extracorporeal blood flow at the maintenance rate. Clearance of the total CO2 minute production (200-400 ml/min) occurs mainly through the artificial lung and requires a blood flow of 1.5 to 2.5 l/min with a ventilation of 10-20 l/min in the artificial lung. While oxygen transfer by the artificial lung is strictly dependent on extracorporeal blood flow, the CO 2 clearance is less dependent (ECCO2R increases linearly with the logarithm of extracorporeal blood flow). Consequently, the amount of ECCO 2R depends on the surface area of the artificial lung and its level of ventilation. Initially, the ventilator FiO 2 is kept unchanged from the pre-bypass value, while the artificial lungs are ventilated with the same FiO 2. The ventilation of the natural lungs is then decreased to 2 to 4 breaths/min, while PEEP is progressively increased to maintain mean airway pressure at the same level as during the previous period of mechanical ventilation. Oxygen (100 per cent) is then delivered into the carina through a small catheter to provide for the oxygen consumed during the long expiratory pause (apneic oxygenation) (CD Figure 1).









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CD Figure 1. Extracorporeal apparatus in operation.

Bleeding is the main complication during long-term bypass, even though heparin-coated circuits with heparinized artificial lungs, introduced in 1992, have reduced the need for systemic anticoagulation. Heparinized circuits can be used without systemic heparinization; however, we routinely maintain an intravenous infusion of heparin (15 000-30 000 lU/day). The activated clotting time (Hemocron, International Technidine, New Jersey, USA) is monitored hourly and the heparin infusion is modified throughout bypass time to maintain activated clotting times between 180 and 210 s. The range of heparin infusion is usually l0 to 30 lU/kg/h. Systemic heparin is discontinued only if major bleeding occurs or when surgical maneuvers are required.

The weaning process, initially from FiO 2 and then from pressures, starts as soon as the patient's gas exchange improves. When oxygenation increases, the FiO 2 of the ventilator is decreased first using a target PaO2 of 80 to 100 mmHg (10.7-13.3 kPa). When the target PaO2 is maintained with a ventilatory FiO2 setting of 40 per cent, the FiO2 of the gas mixture ventilating the artificial lung is decreased stepwise to 21 per cent. If oxygenation is still maintained at the target value, the PEEP level is decreased. When PaO2 remains stable at 80 to 100 mmHg (10.7-13.3 kPa), with the PEEP level set at 5 to 10 cmH2O and the FiO2 set at 40 per cent on the ventilator and 21 per cent on the membrane lung, the patient's ventilatory mode is shifted to spontaneous-assisted ventilation.

Disconnection from bypass is considered when the patient is able to tolerate spontaneous-assisted ventilation for 6 to 12 h with the same level of FiO 2 in the ventilator and the membrane lung. Firstly, gas flow through the artificial lung is stopped and then, if the patient is able to maintain a viable gas exchange without any extracorporeal support, the catheters are removed. Percutaneous cannulation does not require any surgical repair.

The artificial lung routinely used is the Maxima-Carmeda microporous heparinized lung (Medtronic, Anaheim, CA, USA) which allows a decrease of systemic anticoagulation but tends to leak plasma after a few days' use. This leads to frequent changes of the artificial lung. However, the problem now appears close to being solved as a new model, the Maxima Plus PRF, has not shown any plasma leakage (up to 3 weeks) in the most recent patients treated.

A Biomedicus centrifugal pump (Medtronic, Eden Prairie, MN, USA) with a heparinized pump head is used for pumping blood. The main advantage of this device is that it can be used without a reservoir. The reservoir, which is mandatory with a roller pump, may be considered a weak point of the circuit, particularly when heparinized circuits are used, owing to the low blood flow rate and consequent formation of clots, thrombi, and, possibly, emboli. In contrast, the major disadvantage of the heparinized centrifugal pump is the tendency for deposition of small clots, which may cause hemolysis, near the rotor shaft. To prevent this problem the pump head is changed every 5 days.

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