Haemodialysis during cardiopulmonary bypass using

135

Haemodialysis during cardiopulmonary bypass using a

haemofi Iter

Carole C Hamilton, Stephen J Harwood, Kathy A Deemar, Steve Juhasz and Eldon Sharpe Cardiovascular Perfusion Services, Sunnybrook Health Science Center, Toronto, Ontario

Introduction Traditional haemodialysis involves the use of a renal dialysis machine in which ’tap water’, after a series of processes, is purified and chemically altered. The problem of availability, coupled with cumbersome dialysis equipment and cost of personnel, limits the accessibility of conventional haemodialysis for fluid and solute removal during

cardiopulmonary bypass (CPB). A simplified technique of haemodialysis is presented in which a sterile, potassium free, peritoneal dialysate is applied to the effluent side of a haemofilter, normally used to haemoconcentrate blood. The components of these haemofilter devices are discussed along with related terminology and theory of operation.

Haemodialysers, haemofilters, haemofilter/dialysers

haemodialyser has a very small pore size of only 6000-10 000 daltons’ and is used in conjunction with a dialysate that allows solute and toxin removal while restricting the amount of plasma A true

for Carole Hamilton, correspondence: Ludwig-Maximilians-Universitaet, Klinikum Grosshadern, Herzchirugische Poliklinik, Marchioninistrasse 15, 81377 Muenchen, Germany.

Address

(filtrate) escape. A haemofilter has a larger pore size of up to 55 000 daltons’ to allow a greater amount of filtrate removal. This unit is used to remove excess fluid from a patient during CPB. A haemofilter/dialyser also has a large pore size of up to 55 000 daltons and is therefore considered to be a haemofilter.2 It is called a haemofilter/dialyser because it is used in conjunction with a dialysate. The above units consist of porous hollow fibres encased in a rigid plastic shell. Blood flow is directed to the inside of the hollow fibres and plasma water is removed to the outside filtrate path. This filtrate path also serves as the channel for the dianeal during dialysis. The hollow fibre material can either be cellulose based or synthetic that differs in permeability, blood compatibility and mechanical properties.2 The original hollow fibres are of cellulose material which is a natural cotton and very strong allowing the bundles to be wound tightly and orderly without fear of breakage. The fibre wall thickness can be kept to a minimum improving solute transfer and the dialysability of the membrane.2 Synthetic fibres such as polyacrylonitrile

water

(PAN)

or

polymethylmethacrylate (PMMA)

are

softer, and hence weaker resulting in thicker walls to provide support. These membranes provide water clearance and are well suited for haemofiltration techniques.2 The synthetic units are considered to be more biocompatible.3 There

superior

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136

approximately 6000 hollow fibres per unit with length that generally varies between 163 mm to 290 mm. The fibre internal diameter is usually 200 Am with a wall thickness varying from 8 Am to 45 Am depending on the membrane material. Fibre dimensions have important consequences on overall function depending on whether the

viscosity (haematocrit and protein concentration), colloid osmotic pressure (total protein concentration in blood g/dl), length and diameter of blood lines, and blood flow.5 The major force working against the UFR is the oncotic pressures As the protein concentration increases there is a greater boundary layer between the blood and the mem-

blood source to the haemofilter is from a separate pump (constant flow, pressure-dependent) or a positive pressure source from the arterial line (constant pressure, blood flow-dependent). In the latter, any increase in resistance, such as increased fibre length, leads to a reduced blood flow and hence reduced function.

brane, which then reduces haemofiltration.

are a

Haemofiltration The main purpose of haemofiltration is to remove excess fluid from a patient. Haemofiltration has been used successfully as an adjunct to CPB in the management of hypervolaemia secondary to haemodilution. Haemofiltration is the process describing the removal of plasma water and solutes by a pressure gradient and is the established term in the scientific literature.4 In perfusion this is known as haemoconcentration and in dialysis this is referred to as ultrafiltration. Blood is driven into the hollow fibres and this hydrostatic pressure overcomes the oncotic pressure of the blood and literally pushes water through the pores. The solutes that are dissolved in the water are dragged along, all at the same speed with the water, if they are small enough to permeate the membrane pores. Haemofiltration is specified for each filter product by the ultrafiltration rate (UFR). The UFR is the speed at which the plasma water is removed4 from the blood at a given transmembrane pressure (TMP) and is measured in ml/min. There is a linear increase in UFR with increased TMP to a point where it levels off as a function of surface area.4 The TMP, measured in mmHg, is determined by the positive pressure applied to the blood side of the membrane and the negative pressure applied to the ultafiltrate side. The distance between the ultafiltrate outflow line to the waste receptacle adds a gravity pull of 0.74 mmHg/cm. This becomes significant when used in a low pressure system. Other factors affecting the UFR include blood

ultrafiltration coefficient (KUF) is the permeability of a particular membrane to water and is measured in ml/hr/mmHg. This term provides a guideline in designating the membranes into low flux or high flux devices. Low flux units have a KUF of less than 10 ml/hr/mmHg and high flux units have a range of 10-60 ml/hr/mmHg.7 The higher the coefficient, the faster water will go through the membrane. For example, if the KUF is 26 ml/hr/mmHg (high flux) and the TMP is 100 mmHg then 26 divided by 60 = 0.43 ml/min/mmHg times 100 mmHg = 43.5 ml/min water flux possible at a TMP of 100 mmHg. It is important to note that the process The

of haemofiltration alone does not effect the potassium concentration in the blood. Although potassium is removed, it occurs in the same proportion as the blood. The overall concentration remains unchanged if the blood is not diluted afterwards.

Haemodialysis The

use

of

open-heart

haemodialysis on patients undergoing surgery has been reported in the

literature. 6-10

Haemodialysis is principally used to remove solutes from the blood. Solute removal occurs in two ways during haemodialysis. First, as the hollow fibres are bathed in a dianeal that differs from the blood concentration, a solute gradient is created. The solutes diffuse from an area of high concentration to an area of low concentration and small solutes such as urea are removed rapidly. Given the inherent property of the membrane, there is always some degree of haemofiltration allowing solute removal by way of convection, albeit to a lesser extent than the diffusive process. Solute removal is expressed as clearance and is measured in ml/min at certain flowrates. Clearance is defined as the volume of blood from which

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137 solute has been haemofiltration is a

completely eliminated. When taking place the clearance is

higher.2 Solute molecular size Solute molecular size plays an important role in whether the molecule is easily haemofiltered or

haemodialysed. Small molecules are compounds that are easily and range up to 300 daltons.2 Potassium is a small molecule with a molecular weight of 39 daltons and is therefore easily removed by the diffusive process of haemodialysis. Middle molecules are compounds that are poorly dialysed and range from 300 daltons to 5000 daltons.2 Most drugs have a molecular weight of less than 5000 daltons. il Larger molecules have a molecular weight of greater than 5000 daltons and are not dialysed to any significant extent but are more efficiently removed by the convective process of haemofiltration.2 Heparin is a large molecule with a molecular weight ranging from 6000-20 000 daltons.12 It is, therefore, more effectively removed by haemofiltration but it also has a high ratio of protein binding and usually does not require supplemental doses. However, the ACTs should be closely monitored.

dialysable

Once the decision is made to use a haemofilter, the unit is set up according to institutional protocol. CPB is conducted in the usual fashion. There are no changes employed in the operative process of the patient. The haemofilter unit is rinsed with 1000 ml of sodium chloride. With the proper priming technique blood compatibility is improved.2 This includes not only rinsing the internal blood path but also slight haemofiltration before it is incorporated into the blood path of the patient. The recirculation line or the purge line from the top of the arterial filter may be used as the driving force to the haemofilter alleviating the need for an extra pump. Most haemofilters have a tubing connection size of 3/16 inch. The size and length of the tubing connected to the haemofilter should be kept to a minimum; an inlet line that is too long and narrow greatly adds to the resistance of the system and a line that is too wide only increases the priming volume. The outlet of the haemofilter is connected to the cardiotomy reservoir. In this way air removal is easy and the blood is also filtered. A two-litre bag of 0.5% peritoneal dialysate (dianeal) is attached to the effluent side of the haemofilter. The flowrate of the dianeal is regulated through a roller clamp against gravity running countercurrent to the blood flow. A waste line is attached and connected to a waste receptacle. There are no extra pumps required which adds to the simplicity of the set-up procedure.

Simplified technique of haemodialysis The system designed by the Sunnybrook Health Science Center perfusion department consists of haemofilter/dialyser (HFD) (Hospal AN 69 Filtral 10) and a sterile 0.5% peritoneal dialysate (Baxter

dianeal). The Filtral 10 is made of a synthetic material, with a pore size of 55 000 daltons. There are 6000 hollow fibres with a fibre length of 200 mm. The fibre width is 45 gm dry and 50 gm wet. The effective surface area is 0.85 m2 with a priming volume of 64 ml. The pressure drop is 30 mmHg at a TMP of 100 mmHg. The KUF is 31 ml/mmHg/hour. Therefore, at a transmembrane pressure of 100 mmHg the UFR is 51 ml/min. At a blood flowrate of 300 ml/min and dialysate flowrate of 500 ml/min the urea clearance is 189 ml/min.

Blood flowrate The average blood flow ranges from 300 ml/min to 500 ml/min. This can vary depending on the

pump flowrate, arterial line pressure, and the size of tubing incorporated into the haemofilter. The higher the blood flowrate the greater the solute and water removal up to a point where the efficiency levels off as a function of the surface area. In general, flows greater than 500 ml/min should not be used.

Dialysate flowrate The dialysate flowrate is regulated through a roller clamp against gravity. The optimal flow is the same as the blood flowrate and the most practical flow is half the blood flow. Typical dialysate flows range from 100 ml/min to 500 ml/min. The faster the dianeal flow, the

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138 of acid base status and administration of bicarbonate is necessary to treat

greater will be the clearance of diffusible sub-

Frequent monitoring

stances and hence the faster the rate of solute removal.2 It may be tempting to increase the flow of dianeal but clearance does not increase to any

these cases.

significant degree

when it is faster than the blood

flow. The flow of dianeal can be reduced, stopped and restarted as solute levels change. If the removal of a solute such as potassium is no longer necessary but fluid removal is, then it is important to clamp the dianeal out of circuit proximal to the inflow of dianeal. The haemofilter/dialyser may then be used solely as a haemofiltrator. As with any added system to the CPB circuit, special attention should be made to any precautions. The occurrence of metabolic acidosis is higher when the patient is haemofiltered or haemodialysed.13 The preferential movement of anions especially with rising protein concentration is the suggested cause of bicarbonate depletion.

Although this system is easy to set up and run, from a safety point of view the use of a second person is necessary as attention should not be diverted from the main pump console. The type of dianeal used may vary depending on the end result required. Normally this system is used to remove excessive amounts of potassium and so a potassium-free solution is used. The reason the first choice was the 0.5% peritoneal dialysate, is the solution is physiologically balanced, potassium free and available in a two-litre sterile bag. However, the level of glucose may rise depending on the patient’s own blood level. We have alternated this solution with one-litre bags of sodium chloride with very good results.

Figure 1 Haemodialysis set-up with dianeal in CPB circuit

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139

Summary The decision to employ haemofiltration and/or haemodialysis is based on various criteria depend-

ing on institutional protocol. Cardiac surgical patients, especially those with renal failure, often require fluid and electrolyte intervention. In the past haemodialysis patients were closely monitored and often delayed for surgery depending on their electrolyte status. Operative technique was changed to accommodate the impending sequelae of cardioplegic solutions, blood transfusions and fluid adminis-

Although haemofiltration has been used successfully in the management of hypervolaemia

3 Jacob AI, Gravellos G, Zarco R et al. Leukopenia, hypoxia and complement function with different 18: hemodialysis membranes. Kidney Int 1980; 505-509. 4 Holt DW, Landis GH, Dumond DA et al. Hemofiltration as an adjunct to cardiopulmonary bypass for total oxygenator volume control. J Extra14: 3. 1982; Corp Technol 5 Golper TA. Continuous arteriovenous hemofiltration in acute renal failure. Am J Kidney Dis 1985; 6: 373-86. 6 Murkin JM, Murphy DA, Finlayson DC.

Hemodialysis during cardiopulmonary bypass: report 7

hemodialysis during cardiopulmonary bypass. Dialysis

tration.

and anaemia due to haemodilution, the rate of uraemic toxins and solute removal may not be adequate. The use of haemodialysis helps in the treatment of these difficult and often unpredictable cases. The type of dialysate and method of administration has simplified the technique of haemodialysis, during CPB, allowing effective solute and toxin removal while being able to control the amount of fluid removed.

of 12 Cases. Anesth Anal 1987; 66: 899-901. Beckley PD, Erlich LF. The use of concurrent

and Transplant 1980; 9: 768-71. 8 Williams JS, Crawford FA, Riley JB. Cardiac surgery for patients maintained on chronic hemodialysis. J SC Med Ass 1991; 12. 9 Kirby D, Aplegate B, Gabrehel W et al. Renal dialysis during coronary artery revascularization: a

Technol 14: 424-27. study. J Extra-Corp 1982; Finalyson DC. Intraoperative hemodialysis during cardiopulmonary bypass in chronic renal failure. J Thorac Cardiovasc Surg 1979; 5: 789-91.

case

10 Soffer O, MacDonnell RC,

11 Dickson

DM, Hillman KM. Continuous renal

replacement in the critically ill. Anesth Intensive Care 12

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of therapeutics, fourth edition. London: Macmillan 1 Cross DA. Use of the hemoconcentrator during cardiopulmonary bypass. Mechanisms of Perfusion VIII. Orlando, Florida, May 1993. 2 Nissenson AR, Fine NR, Gentile DE. Clinical dialysis, second edition. Norwalk, CT: Appleton and

1970: 1447. 13 Hakim M, Wheeldon D, Bethune DW. Hemodialysis and hemofiltration during cardiopulmonary bypass. Thorax 1985; 40: 101-106.

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