Since its inception in the 1950s, cardiopulmonary bypass has revolutionised cardiac surgery, allowing ever more complex procedures to be undertaken in the knowledge that the bypass circuit is able to provide adequate support to vital organs while the operating surgeon is able to work in a bloodless operative field for several hours. What has changed over the years is the risk profile of the patients for whom cardiac surgery is indicated. The mean age of patients undergoing cardiac surgery in the UK has increased over the past decade with a predictable increase in the range and severity of comorbid conditions observed in patients who come to cardiac surgery today.

The deleterious effects of conventional cardiopulmonary bypass have been known for many years and result primarily from the interaction between the formed elements of blood with the non-physiological surfaces of the cardiopulmonary bypass circuit.1 Platelet and neutrophil activation, consumption of coagulation factors, complement generation and the release of a multitude of pro-inflammatory mediators are recognised consequences of cardiopulmonary bypass.2 A systemic inflammatory response is an inevitable consequence of cardiopulmonary bypass and on occasions may be of sufficient severity as to predispose patients to multiorgan failure. “Off-pump” surgery for the epicardial coronary vessels was driven by a desire to avoid the unwanted effects of cardiopulmonary bypass, but the technique has not proved to be the panacea that was once envisaged3 4 and indeed cannot be applied to intracardiac surgery such as heart-valve replacement where, percutaneous approaches aside, cardiopulmonary bypass is still mandated.

The challenge we face is that of continuing to be being able to offer older and “higher-risk” patients the benefit of safe cardiac surgery. While many of the aetiological factors underlying the cardiopulmonary bypass-related inflammatory response have been the subject of intense investigation, few recent studies have examined those adverse consequences which may be ameliorated simply by miniaturisation of the bypass circuit. This meta-analysis5 of randomised trials comparing the effectiveness of miniaturised versus conventional cardiopulmonary bypass in adult cardiac surgery is, therefore, timely (see page 964).

Miniaturised cardiopulmonary bypass systems may be able to contribute to improving the safety of cardiac surgery in a number of important ways.

MINIMISING HAEMODILUTION DURING CARDIOPULMONARY BYPASS

The unwanted effects of the considerable haemodilution, which is observed when the patient’s blood volume mixes rapidly with the volume of prime fluid in the bypass circuit as cardiopulmonary bypass is instituted, can theoretically be reduced by miniaturisation of the bypass circuit. The prime volume required for a conventional cardiopulmonary bypass circuit is often up to 2 litres of crystalloid. Miniaturised systems exist with a circuit prime volume as low as 150 ml. The use of such systems could, for example, reduce the degree of impairment of red-cell-dependent gas transport associated with haemodilution and facilitate the maintenance of intravascular oncotic pressure during the bypass period. The impairment of platelet and humoral factor-dependent coagulation that occurs with haemodilution could similarly be reduced, and when patients are weaned from cardiopulmonary bypass with a higher haematocrit, a reduction in postoperative blood transfusion can be anticipated.

IMPROVING BIOCOMPATIBILITY AND REDUCING TRAUMA TO BLOOD CELLS DURING CARDIOPULMONARY BYPASS

Another potential benefit of miniaturisation comes from minimisation of the surface area over which blood comes into contact with the elements of the bypass circuit. This is principally achieved by reducing the surface area of the oxygenators used in the circuit. Miniaturised cardiopulmonary bypass systems can incorporate oxygenators with a surface area as low as 1 m² as compared with 1.8 to 2.4 m² for oxygenators used in conventional cardiopulmonary bypass. Reducing the surface area of the oxygenator membrane has been shown to improve the biocompatibility of the bypass circuit with a reduction in cardiopulmonary bypass-related neutrophil, monocyte and complement activation.6

Blood cells are subjected to significant trauma during cardiopulmonary bypass,7 and the mechanical trauma to cells which occurs as a result of cardiotomy suction remains a significant problem.8 Blood which is shed into the mediastinum during surgery is returned to the cardiopulmonary bypass circuit by means of low-pressure suction. This process of continuous aspiration of blood from the surgical field results in haemolysis8 which increases as the duration of cardiopulmonary bypass increases and is a particular concern during complex, long operations in patients with significant preoperative comorbidity. Cardiotomy suction is also implicated in the amplification of the systemic inflammatory response and coagulopathy related to cardiopulmonary bypass9 as well as the transfusion of blood contaminated with particulate debris and mediastinal fat10 back into the cardiopulmonary bypass circuit.

Miniaturised bypass systems which can, for example, incorporate devices that minimise the duration of blood aspiration or which can isolate aspirated blood from the bypass circuit should theoretically reduce some of the adverse effects seen with conventional cardiopulmonary bypass.

REDUCING EMBOLISATION

Gaseous microembolisation is a recognised consequence of conventional cardiopulmonary bypass and contributes to the subtle neurocognitive dysfunction observed in some patients.11 Air usually enters the bypass circuit by entrainment around venous cannulae placed into the right atrium. Effective removal of venous air before it reaches the oxygenator is mandatory for the safe conduct of cardiopulmonary bypass using a miniaturised system, and as a consequence the potential for gaseous microembolisation into vital organs should be reduced if the miniaturised circuit is working effectively.

Biancari and Rimpiläinen report a lower incidence of postoperative stroke and blood loss associated with the use of miniaturised cardiopulmonary bypass in their analysis.5 The trend towards a reduction in mortality in the patients who underwent surgery using minicardiopulmonary bypass did not, however, reach statistical significance. It is unlikely that the wider cardiac surgical community will be persuaded to adopt miniaturised cardiopulmonary bypass technology solely on the basis of this meta-analysis. It is important to note that in spite of all the potential benefits of miniaturised cardiopulmonary bypass described above, it remains the case that such systems have not found widespread usage largely because concerns still exist in the minds of many about the safety of miniaturised cardiopulmonary bypass. The great fear is of massive air embolism if air enters the closed miniaturised bypass circuit and is then pumped into the arterial circulation with catastrophic consequences. Nollert and colleagues,12 for example, discontinued their study comparing miniaturised versus conventional cardiopulmonary bypass in isolated coronary artery surgery following two major air leaks which occurred during cardiopulmonary bypass in a series of 15 cases using a closed miniaturised circuit without a venous reservoir.

Many surgeons still feel that the margin of safety is just too small to justify the risk of major air embolism when the advantages of miniaturisation of the bypass circuit have yet to be comprehensively established.

As ever, what is required in a situation where the risks and benefits to patients are finely balanced is a large-scale prospective randomised trial which will comprehensively examine the utility and safety of miniaturised cardiopulmonary bypass. As Biancari and Rimpiläinen5 point out in their discussion, most studies evaluating miniature cardiopulmonary bypass have been small and have used a plethora of techniques and devices making firm conclusions about the efficacy and safety of miniature cardiopulmonary bypass difficult to reach.

This meta-analysis at least provides some more robust evidence to support the view that the technology can be applied safely and offers the possibility of better cardiopulmonary bypass for the increasing number of elderly and “high-risk” patients who are offered cardiac surgery today.