- fistula vascular access is preferred for long-term chronic hemodialysis
- in young children, less than 15 kg, the time needed to develop a fistula before it can be used could be some months
- the double-needle technique is the standard, but single needle with double pump system is an alternative
- a single lumen catheter with clamps offers for small children an acceptable compromise between a very low extracorporeal blood volume and valuable dialytic efficacy
- total extracorporeal blood volume (needles, tubing and dialyzer) should, approximately, be less than 10% of patient total blood volume
- anticoagulation in the extracorporeal circuit is achieved either with conventional heparin or with low-molecular-weight heparin
- an extracorporeal blood flow rate (QB) of 150–200 mL min−1 m−2 or 5–7 mL min−1 kg−1 is often sufficient
The success of chronic hemodialysis depends on good vascular access: internal arteriovenous fistulae (AVF), shunt (AVS), graft (AVG) or central venous catheter. The type of access used is variable depending on factors in different units and countries, for example surgical experience, patient age and size, the time available before dialysis must be started, and the presumed waiting time before transplantation. Patient choice plays a major part, especially with adolescents.
A catheter is more commonly used in the USA than in Europe [7
]. A catheter can be a primary access particularly in acute renal failure or chronic renal failure with acute presentation, in small children and in the case of a presumed short period on chronic hemodialysis. Internal jugular vein catheter access is superior to subclavian vein; it admittedly preserves the future arteriovenous fistula implantation on the arm. Femoral catheter access should be used only for “rescue and transient” access if intensive care is needed: it is easy to perform but with a higher risk of infection and thrombosis. A double lumen cuffed catheter, at least 8 French, is mostly preferred for children and has been reported to have a survival rate as high as 60 to 85% in one year [33
], or as low as 30% [34
]. Nevertheless in small infants a single lumen catheter used with the alternative clamps technique offers an acceptable compromise between recirculation and both the amount of extracorporeal blood volume and the achieved blood flow [35
]. Thrombosis, a major cause of catheter failure, is reported to be between 9 and 46% [34
]. Thrombosis causing poor flow can be corrected to salvage the catheter by different methods: catheter replacement over guidewire, systemic oral anticoagulation and local urokinase or tissue plasminogen activator instillation [36
]. Loss of catheter access related to infection has decreased during the last decade; the aggressive use of antibiotics and perhaps antibiotic lock therapy, although not universally accepted, account for this lower rate of infection related catheter loss [34
Microsurgery enables creation of a functional AVF at the wrist in most children, even small ones [8
] but only a few surgeons are trained for vascular microsurgery, which therefore is rarely used. Creation of a fistula at the elbow is a second-choice vascular access. With a non functional cephalic vein, a basilic vein transposition, i.e. superficialization, is possible [38
]. Synthetic grafts should be reserved for children who have exhausted autologous veins and should be used in children only very rarely. For all these reasons preoperative evaluation of the vessels to determine the correct choice of vein before the operation is mandatory. The non-dominant arm should be regarded as first choice of fistula implantation. The survival rate for a AVF is higher than the survival rate for a catheter, with more than two thirds of the children having a functioning AVF at four years [8
]. With a basilic vein superficialization the fistula should not be used before full healing (2 to 6 weeks) to avoid a dissecting hematoma. Otherwise the time needed for venous development before use depends on the age of the patient and the place of the AVF (distal or proximal). In small children this period of time is often a delay of months. Before surgery it is essential to avoid venopuncture of the selected arm in the weeks before AVF creation. It is of interest to protect the dominant arm from the beginning of taking care of a child with “chronic dialysis risk” to enable, if necessary, implantation of a fistula. Such venoprotection should not be forgotten for peritoneal dialysis children, even babies/infants. For a period of time before surgery, especially for small children, [8
] dilatation of the veins by immersion of the forearm in hot water is advantageous, a maneuver enhanced by placement of a tourniquet. A proximal AVF with a high blood flow, usually close to 1000 mL min−1
, is a risk factor for cardiac failure. Nevertheless, the major complication is thrombosis, consequent to local stenosis. Therefore, follow up of the access flow is essential, on the one hand clinically: auscultation (the sound of the AVF is maximum at the surgical site and decreases with distance from the fistulae), observation (elevation of the forearm should induce emptying of the previous dilated veins, and on the other hand by Doppler ultrasound or vascular access flow monitoring [9
]. Application of regular access flow monitoring can be used to detect vascular stenosis before complete AVF thrombosis [9
]. But it should be remembered that “Transonic” access flow monitoring can only be performed with double-pump dialysis and is not available for pediatric-sized blood lines.
The extracorporeal blood flow rate is achieved through venous puncture, most often via two needles, one for blood aspiration called the arterial needle, one for venous reinjection called the venous needle. The distance between the needles should be sufficient to limit recirculation, which is best prevented by opposite orientation of the needles: the arterial one toward the fistula, the venous one in the opposite direction. Usually the needle size is 17-gauge at initiation of dialysis; thereafter considering patient need and fistula development 16 or 14-gauge needles, particularly in adolescents, can be used to achieve a sufficiently high blood flow rate. Pain related to the puncture should be prevented by anesthetic cream (Emla or Amelop); this advance is important for both the children and nurses [39
An extracorporeal blood flow rate (QB) of 150–200 mL min−1 m−2 , 5–7 mL min−1 kg−1, is often sufficient to achieve the targeted goals with double needle dialysis; in small children QB is determined using body weight (BW, kg): (BW+10)×2.5=QB (mL min−1). The arterial blood aspiration pressure should be monitored if possible and kept between 150–200 mmHg to limit endothelial trauma.
For single-needle dialysis in children the highest blood flow rate is obtained with a double pump system (venous flow higher than arterial flow) monitored by the pressure, system called time pressure regulation. The risk of recirculation is important with the latter; some machines limit this risk more than others, especially with the addition of clamps. Conversely for small infants a single lumen catheter used with the alternative clamps technique is an acceptable compromise between recirculation and both the extracorporeal blood volume and the achieved blood flow [35
The total extracorporeal blood volume (needles, tubing, and dialyzer) should preferably be less than 10 % of patient total blood volume. This is essential for small children; however, the relative normal hemoglobin level obtained with erythropoïetin therapy enables this volume to be exceeded slightly without significant hypotension at the end of dialysis session when the patient reaches dry body weight. Nevertheless, it should be kept in mind that the higher the extracorporeal blood volume, the higher the volume of returned fluid, which will load the patient with fluid at the end of the dialysis session. (In very small children the substitution by air may be necessary to limit blood loss on one side and high substitution volume on the other side, but is very dangerous and should be strictly monitored.) System priming with saline, albumin, and sometimes blood should be applied in the first dialysis sessions with babies or small infants.
Anticoagulation of the extracorporeal blood volume is performed either by use of conventional, heparin with continuous infusion of 20 to 30 IU kg−1
, or with low-molecular-weight heparin at 1 mg kg−1
as a bolus at the beginning of the dialysis session. If the hematocrit is over 35%, the risk of clotting is increased. Regional citrate anticoagulation is sometimes used especially when acute dialysis is needed [2
]. Predilution treatment, feasible in either hemofiltration or hemodiafiltration, reduces the risk of clotting and even enables dialysis without anticoagulation in some circumstances. In the presence of thrombopenia heparin-toxicity is to be suspected.
The venous blood line has a pediatric size air-trap chamber to limit extracorporeal blood volume. The dialysis membrane is protected by an arterial chamber of expansion which in small children is often not incorporated in the line to reduce the extracorporeal blood volume. Prevention or treatment of ethylene oxide allergy is possible by using steam sterilization of needles, lines, and membranes; this is becoming the preferred option throughout Europe.