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| Guest Commentary |
by James R. Beck, C.C.P.
Columbia Presbyterian Hospital, New York, New York, USA |
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Cannulation and Perfusion Perspective for MICS
Cannulation for minimally invasive cardiac surgery (MICS) poses several challenges for the clinical perfusionist. Use of new, innovative cannulae and products as well as remote cannulation sites requires the perfusion team to embrace a thorough understanding of flow and pressure dynamics with respect to these new techniques.
Several new arterial cannulation techniques are emerging as we embark on thoracotomy approaches for MICS as well as total closed-chest procedures. These include femoral arterial access and axillary artery cannulation. Femoral artery access is certainly not a new concept. However, we must appreciate some of the pitfalls which can occur with this approach. Short arterial cannulae with side-hole designs coupled with percutaneous insertion techniques may increase the risk of femoral artery dissection. Retrograde flow from femoral artery cannulation also carries the risk of plaque embolization from this so called “backward flow”. New cannulae, such as the Estech remote access cannula, allow antegrade flow, balloon endoclamp, and cardioplegia delivery from a femorally inserted cannula. Perfusionists need to be cognizant of parameters associated with these new cannulae. These may include higher line pressure, higher cardioplegia delivery pressures, and the need to assess endoclamp balloon inflation pressure as well as balloon placement to ensure non-occlusion of the innominate artery. The latter involves measurement and comparison of right- and left-sided patient pressures.
Similar cognizance and team communication are necessary when utilizing axillary artery cannulation. Line pressure and patient pressures must be monitored closely to ensure adequate perfusion with minimal hemolysis from mechanical obstruction such as side graft or cannula kinks. Right and left sided (or systemic) pressure must be monitored to ensure adequate perfusion. Right radial pressure is valuable for assessing pressure during periods of antegrade cerebral perfusion, but, in our experience, this tracing is elevated during times of full flow and patient rewarming. This can mislead the clinician into believing systemic perfusion is adequate in the face of very low systemic pressure. This is probably the result of preferential flow to the radial artery. Therefore, right radial pressure should not be used to assess adequacy of perfusion during periods of full flow with axillary artery cannulation.
Venous cannulation techniques for MICS also pose new challenges for the Perfusion team. Cannulation of the internal jugular vein and femoral vein require close monitoring and augmentation of return over conventional gravity drainage methods. The use of kinetic-assisted (venous pump) or vacuum-assisted venous drainage (VAVR) allow the use of smaller venous cannulae and remote access sites. However, the clinician needs to be diligent in monitoring and trouble-shooting when using negative pressure on the venous line. Excessive negative pressure may cause air entrainment from various sites including small suture holes. Remember, small bubbles are not very buoyant and tend to move quickly through the perfusion circuit, including the oxygenator and arterial filter and into the systemic circulation. If you have any doubt about this phenomena, just take a look at the transesophageal echo of the aorta during periods of venous air entrainment. Venous cannula entrapment is not uncommon with MICS retractors coupled with augmented venous return. Surgical, perfusion and anesthesia team communication is essential in assessing and trouble-shooting cannulation and positional issues. The perfusion team needs to be able to recognize this situation and take appropriate action. If venous return slowly disappears, the perfusionist might be inclined to increase the negative pressure. A more appropriate action might be to communicate this loss to the surgical field and briefly remove the negative pressure from the venous line allowing the atria or vessel intima to relax and blood flow to return. The perfusionist should then slowly increase the suction using the least amount of negative pressure to obtain adequate venous drainage. Central venous pressure should be monitored at all times to aid in assessing adequate venous drainage. At our institution, we transduce venous pressure off of the Swan-Ganz introducer when superior and inferior caval snares are used. This technique allows the clinician to evaluate venous return from the head. We can roughly predict cerebral perfusion pressure by subtracting this “head” venous pressure from the mean systemic pressure. Since, the majority of venous return comes from the inferior vena cava, we use flow and volume changes in our reservoir to help assess entrapment of the inferior venous cannula. We also employ a Transonic flow probe on the tubing from the superior vena cava to monitor drainage from this site throughout the procedure. Some centers perform MICS with a single femoral vein cannula passed to the level of the right atrium. This technique has been effective for some applications provided venous return is not impaired by cross-clamp application and trans-sternal retraction techniques.
As we embark on this new era of cardiac care, the surgical and perfusion communities will be challenged with innovative ideas to improve surgical procedures and patient outcomes. I believe that the key to truly improving patient care lies in open communication between teams and careful scientific evaluation of new products, procedures and techniques. We must embrace change and look to the future for new technology while maintaining the highest degree of safety.
Miniaturization of Perfusion Circuits
The movement towards minimally invasive cardiac surgery was in part stimulated by an effort to reduce the complications associated with traditional cardiopulmonary bypass (CPB). Yet efforts to primarily reduce the risks associated with CPB, including inflammation, hemodilution, coagulopathy, and emboli, remained limited during that period. The recent impetus for change has been largely driven by off-pump coronary artery bypass (OPCAB) procedures.
A host of innovative devices, circuit design and clinical technique changes are currently aimed at reducing the pathogenicity associated with CPB. This evolution of technology continues to be a joint effort of manufacturers and clinicians to improve patient outcomes. Manufacturers have recently designed new circuitry to address circuit performance issues such as surface area and type, hemodilution, blood/air interface, cardiotomy suction, and exposure to silicone. Clinicians are actively reassessing their conduct of perfusion including management of blood gases, coagulation, pressure, temperature, drug use, cardioplegia delivery, and shed or suctioned blood handling. Many have identified this multifactorial approach as the key to handling the dilemmas posed by cardiothoracic surgery utilizing CPB.
Early use of improved and miniature devices included clinical application of the Miniature Extracorporeal Circuit (MECC) by Jostra Corp (The Woodlands, Texas, USA) and Cardiovention CORx from Cardiovention Inc. (Santa Clara, California, USA). These devices used closed-loop bypass to reduce the blood/air interface, silicone particle exposure, prime volume, and foreign surface area to mitigate platelet activation and inflammatory response. This was coupled with changed in clinical technique aimed at reducing numerous pathological conditions associated with CPB. Although initial reports looked very promising with these circuits, issues of heat exchange, volume management and venous air handling and the risk of air embolization with associated neurocognitive dysfunction continues to challenge the clinician.
The perfusion and cardiothoracic community have clearly identified a host of CPB-related issues including hemodilution, systemic inflammatory response, anticoagulation, platelet and formed element activation, blood loss, and neurological complications. New device technology must embody circuit performance designs that optimize sterility, sheer stress, hemodilution, air emboli protection, areas of blood flow stasis, gas exchange and bio-friendly surfaces. Clinical practices should address changes in current techniques aimed at an overall reduction in the sequelae associated with CPB. Further analysis may prove useful in elucidating clinical and technical benefits providing additional treatment options to cardiac surgeons while improving patient outcomes.
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