7.9.2 Conventional ECC

Conventional ECC has major disadvantages: operating room congestion, large priming volume (800-1500 mL) causing significant haemodilution, bleeding, risk of gas embolism, systemic inflammatory syndrome, haematological and neurological disorders. These phenomena can be reduced by using Mini-ECC systems that do not have a venous reservoir and operate with a centrifugal pump (priming volume: 400-800 mL) (see Mini-ECC and Figure 7.16). The miniaturisation of heparinised circuits and the possibilities of percutaneous femoral puncture (instead of open cannulation) have recently made possible the export of supportive ECC outside the operating theatre as an emergency haemodynamic support for patients in cardiogenic shock or cardiopulmonary arrest [1]. The results are encouraging: in a series of 46 patients, 61% were successful in weaning from the bypass and 28% had long-term survival; bypass did not achieve enough flow in only 11% of cases [5].

Most of the time, preheparinised and biocompatible circuits are used to reduce the amount of systemic heparin: while the normal dose is 300 IU/kg, only 100 IU/kg of heparin can be injected and an ACT of 300 seconds can be sought instead of 450 seconds. The heparin is given before the cannulas are inserted and the ACT is checked 2-3 minutes later. Regardless of the dosage, heparinisation immediately worsens bleeding in the surgical field. For this reason, one tries to perform the entire surgical dissection before heparinisation and before cannulation of the vessels. With low heparinisation, machine flow cannot be interrupted once the circuit is filled with blood because of the risk of thrombosis, which usually takes place in the oxygenator. Therefore, there is a shunt system to maintain minimal circulation within the machine if the flow to the patient is interrupted.

Supporting haemodynamics and gas exchange with a supportive ECC provides the possibility to work comfortably in an operating field including organs crucial for survival. In addition ECC allows the patient to be cooled or warmed.

The possibility of inducing moderate hypothermia (30-34°C) by femoral-femoral bypass surgery appealed to neurosurgeons as early as the 1960s, especially for the treatment of very large or strategically misplaced intracranial aneurysms and hemangioblastomas located in the brainstem. Unfortunately, mortality has remained high and the technique is rarely used. In addition to the difficulties inherent in neurosurgery, bypass surgery and hypothermia under these conditions presents major risks of ventricular fibrillation as soon as the temperature falls below 30°C and LV dilation due to the lack of ventricular drainage. In addition, it greatly increases the incidence of acidosis, profuse bleeding and cerebral oedema [4].

Some renal tumours may extend into the inferior vena cava (IVC) and up to the RA. By using ECC, the IVC can be occluded and the risk of tumour embolisation limited [3]. Intraoperative trans-esophageal echocardiography (TEE) is used to diagnose the position of tumour masses in the IVC and to monitor the risk of emboli continuously (Video). In oncological surgery, it is usual not to use a CellSaver™ because of the risk of systemic embolisation of tumour fragments, although this risk is probably only real for tumours with haematological metastasis. In any case, the almost routine use of bypass aspirates is already responsible for autotransfusion; there is no evidence that venous and arterial circuit filters are effcient to intercept any tumour embolisation.

In liver transplantation, a veno-venous shunt is used to maintain flow from the IVC to the RA when the surgeon is forced to clamp the vena cava and portal vein during the recipient's hepatectomy, resulting in a collapse of cardiac output with massive mesenteric and renal stasis. In this case, a pre-heparinised veno-venous circuit and a centrifugal pump are usually used, but not an oxygenator or reservoir; the flow rate must not fall below 1 L/min, otherwise the preload of the heart is no longer maintained and the circuit may thrombose. The risk of embolism and accidental decanulation is real. Developments in liver surgery have made this technique more or less obsolete.

Occasionally, tumours or tracheobronchial trauma require reconstruction that prevents ventilation for more than an hour.  ECC (femoral-femoral cannulation, or ascending aorta and RA in the usual way) provides the possibility to maintain gas exchanges during this period and to operate on a trachea freed from the oro-tracheal tube. At present, ECMO is mainly replaced by veno-venous or veno-arterial ECMO (see Extracorporeal Assistance). In lung transplantation, there is unfortunately no reliable preoperative test to predict the need for ECMO, so the pulmonary artery clamp test in the operating field is the only way to decide. In restrictive syndromes, poor exercise tolerance and a lowered LV ejection fraction have some prognostic value [2]. If pulmonary artery clamping causes right ventricular failure with or without pulmonary hypertensive crisis and if the situation is not pharmacologically under control, ECC is required to maintain cardiac output. ECC also gives the operator freedom to manipulate the heart, and allows pulmonary vein anastomoses to be performed under better conditions.

Severe comorbidities associated with very proximal coronary artery stenoses and ventricular dysfunction (EF < 0.25) may be a contraindication to coronary artery bypass grafting. However, the risk of ventricular fibrillation and circulatory collapse is too high to perform percutaneous coronary intervention (PCI) dilation and stenting. In this case, PCI with circulatory support via intra-aortic counterpulsation or veno-arterial ECMO can be performed.

The application of ECC to thoracoabdominal aortic surgery is covered in the chapter on aortic surgery (see Chapter 18 Spinal cord protection); medium- and long-term haemodynamic support is described in the chapter on heart failure (see Chapter 12 Ventricular support).

Monitoring

For the anaesthetist, the use of an ECC circuit requires the usual invasive monitoring.

• Arterial catheter: the radial route is generally preferred for femoral-femoral bypass surgery, but the choice of catheter position depends on the surgical strategy.
• Central venous line: is necessary for the administration of vasoactive substances and anaesthetic agents during the bypass procedure, as the flow of peripheral veins is uncertain.
• Swan-Ganz pulmonary catheter: it is mainly useful in the postoperative period to manage fluid administration and monitor haemodynamics; intraoperatively, it provides information on the proportion of flow provided by the heart, and allows monitoring of SvO₂ .
• TEE: very useful to control intrathoracic cannula placement (RA positioning), it ensures optimal monitoring of ventricular performance and blood volume; TEE is necessary to exclude or monitor aortic insufficiency (risk of ventricular dilation) and PFO (risk of paradoxical embolism)
• As the risk of bleeding is high, it is a good idea to have large peripheral venous lines accesses.

In supportive ECC, gas exchange is provided by both the bypass oxygenator and the patient's lungs, depending on the relative flow rate of each component. Four data points are available to assess oxygenation.

• SpO₂ : Peripheral saturometry reflects the amount of O₂ actually circulating in the periphery, but it is subject to a lot of artefacts due to variations in peripheral arterial resistance and temperature. It is only reliable if the monitor displays a correctly pulsed curve.
• ScO₂ : Cerebral saturation (NIRS, near-infrared spectroscopy) provides information on peripheral organ oxygenation. It measures a mixture of venous blood (2/3) and arterialized blood (1/3). Its normal value is 65%.
• SvO₂ : central venous saturation is measured continuously in the venous return of the ECC machine; it is the best index of the adequacy of the O supply₂ (DO₂ ) in relation to the body's needs (VO₂ ). Its accuracy depends on where the blood is collected by the venous cannula (RA, IVC, iliac vein). Below 60%, the patient is in the risk zone; below 50%, the effective DO₂ is insufficient: cardiac output and ventilation must be improved. The equivalent of a central venous gasometry is obtained by taking a sample from the venous reservoir.
• SaO₂ : placed on the arterial circuit of the ECC, the SaO sensor₂ only measures the efficiency of the oxygenator of the latter; for the anaesthetist, it is a false security because its value is normally around 98-100%.
• Arterial gasometry: this should be performed at the peripheral arterial catheter, not in the bypass circuit, where it would only monitor the oxygenator’s efficiency.

It is not uncommon for the various SO sensors₂ to give different values. One should always be alerted by the lowest value and not take comfort in the SaO₂ provided by the perfusionist. If in doubt, perform peripheral arterial blood gas (ideally in the radial artery) and measure the SvO₂ of the venous return from the bypass. The relevance of the SvO₂ measured at the Swan-Ganz pulmonary catheter is a function of the actual blood flow through the lungs, i.e. the ratio of heart to pump flow.

Anaesthesia

The start of partial bypass is always a moment of instability because of the sudden haemodilution by the pump priming volume and the risk of emptying the patient by taking too much blood in the venous return to the reservoir. Supportive ECC has special characteristics.

• Two separate arterial pumps: the patient's heart and the bypass machine;
• A common preload: the volume of the RA or IVC;
• Two parallel ventilation systems: lungs and oxygenator.

The result is a more or less stable balance between the patient-system (heart and lungs) and the machine-system (pump and oxygenator). It depends on the proportional contribution of each to arterial flow and ventilation. This ranges from complete unloading of the heart to partial assistance to normalise temperature or oxygenation, and results in a near-depressed blood pressure curve in one case or a simple increase in diastolic in the other. It is wise to maintain residual ventricular ejection to prevent intraventricular thrombus formation. Insufficient LV unloading results in life-threatening ventricular dilation. The patient is ventilated by both the bypass oxygenator and the ventilator, with the proportion of each system depending on the relative flow of the bypass and pulmonary artery. When using a halogen as an anaesthetic agent, it is important to set the same inspired concentration on the oxygenator and the ventilator, otherwise gas entering one system will exit the other.

The sharing of preload leads to massive fluid transfers and constant drug exchanges between the two circuits, while the arterial pressures are provided by two separate and independent pumps. A certain degree of "rhythmic rolling" between the pressures measured in the radial and femoral arteries must be allowed for, and any over-correction which worsens this rolling must be avoided. The most reliable method is to play on the capacity of the ECC to store blood: the perfusionist increases its venous return and temporarily stores volume in the reservoir in case of hypertension, which decreases the preload of the heart. The perfusionist increases venous return and temporarily stores volume in the reservoir in case of hypertension, which decreases the heart's preload. The preload of the bypass machine is regulated by the output of the venous reservoir.

The afterload (systemic arterial resistance) is common to both systems, but the pressure generated depends on the respective involvement of the heart and the pump. Administration of intravenous hypo- or hypertonic agents is only required when the pressure cannot be balanced by the venous reservoir or by volume addition. In general, the machine flow rate should not fall below 1.0 L/min because of the risk of thrombosis in the oxygenator. The adequacy of the overall flow rate is judged by SpO₂ , ScO₂ , peripheral arterial blood gas, SvO₂ and diuresis, which should ideally be 0.5-1 mL/kg/hour.

It is wise to store an adequate supply of blood and clotting factors because profuse bleeding is common when anticoagulating a patient during major surgery or during circulatory support for more than 24 hours. Bleeding up to 1000 mL/hr may occur in 20% of cases [1,5].

  © CHASSOT PG, GRONCHI F, April 2008, last update, December 2019 

References

1. BIRNBAUM  DE. Extracorporeal circulation in non-cardiac surgery. Eur J Cardiothorac Surg 2004; 26:S82-5
2. DE HOYOS A, DEMAJO W, SNELL G, et al. Preoperative prediction for the use of cardiopulmonary bypass in lung transplantation. J Thorac Cardiovasc Surg 1993; 106:787-96
3. LIMATHE J, ATMACA N, MENGHESHA, D, KRIAN A. Combined procedures using the extracorporeal circulation and urologic tumor operation - experiences in six cases. Interact Cardiovasc Surg 2004; 3:132-5
4. MURRAY MJ, COOK DJ. Noncardiovascular applications of cardiopulmonary bypass. In: GRAVLEE GP, ed. Cardiopulmonary bypass, principles and practice. Philadelphia: Lippincott, Williams & Wilkins, 2000, 704-23
5. SCHWARZ B, MAIR P, MARGREITER J, et al. Experience with percutaneous venoarterial cardiopulmonary bypass for emergency circulatory support. Crit Care Med 2003; 31:758-64