Pediatr Nephrol (2005) 20:636–643
DOI 10.1007/s00467-004-1785-5
Nicholas L. Everdell · Malcolm G. Coulthard ·
Jean Crosier · Michael J. Keir
A machine for haemodialysing very small infants
Received: 27 April 2004 / Revised: 13 October 2004 / Accepted: 19 November 2004 / Published online: 17 March 2005
IPNA 2005
Abstract Babies weighing under 6 kg are difficult to
dialyse, especially those as small as 1 kg. Peritoneal dialysis is easier than haemodialysis, but is not always
possible, and clears molecules less efficiently. Two factors complicate haemodialysis. First, extracorporeal circuits are large relative to a baby’s blood volume, necessitating priming with fresh or modified blood. Second,
blood flow from infants’ access vessels is disproportionately low (Poiseuille’s law), causing inadequate dialysis,
or clotting within the circuit.
These problems are minimised by using single lumen
access, a very small circuit, and a reservoir syringe to
separate the sampling and dialyser blood flow rates. Its
manual operation is tedious, so we developed a computercontrolled, pressure-monitored machine to run it, including adjusting the blood withdrawal rate from poorly
sampling lines. We have dialysed four babies weighing
0.8–3.4 kg, with renal failure or metabolic disorders. The
circuits did not require priming. Clearances of creatinine,
urea, potassium, phosphate and ammonia were mean (SD)
0.54 (0.22) ml/min using one dialyser, and 0.98 (0.22) ml/
min using two in parallel. Ammonia clearance in a 2.4 kg
N. L. Everdell ())
Department of Medical Physics and Bioengineering,
Malet Place, Engineering Building, University College London,
London, WC1E 6BT, UK
e-mail: [email protected]
M. G. Coulthard
Department of Paediatric Nephrology,
Royal Victoria Infirmary,
Newcastle, NE1 4LP, UK
J. Crosier
Department of Paediatric Nephrology,
Royal Victoria Infirmary,
Newcastle, NE1 4LP, UK
M. J. Keir
Department of Medical Physics,
Royal Victoria Infirmary,
Newcastle, NE1 4LP, UK
baby had a 9 h half-life. Ultrafiltration up to 45 ml/h was
achieved easily. This device provided infants with immediate, effective and convenient haemodialysis, typically delivered for prolonged periods.
Keywords Dialysis · Ultrafiltration · Preterm baby ·
Renal failure · Renal clearance · Renal replacement
Though improvements in neonatal management have increased the survival of preterm babies, acute renal failure
affects up to 8% of babies requiring intensive care, and its
treatment remains problematic. Most cases are secondary
to other medical conditions rather than inherent kidney
disease [1]. If renal replacement becomes necessary,
peritoneal dialysis is preferred [1], but sometimes this is
not feasible. Some infants require haemodialysis because
they have a congenital defect of the abdominal wall, or
necrotising enterocolitis or abdominal surgery. Other
neonates with inherited metabolic disorders require
haemodialysis to augment the removal of toxic chemicals
(such as ammonia) that are generated more quickly than
the kidney or peritoneal dialysis can clear them.
The minimum haemodialysis circuit volume [2] (typically up to 49 ml) is large in proportion to a baby’s blood
volume of approximately 85 ml/kg. This makes it
preferable to prime the circuit with blood for larger infants, and essential for smaller ones, to prevent a dramatic
dilution of the baby’s blood with saline. However, stored
blood is unsuitable for priming, having a high potassium
concentration, an excess of citrate which chelates calcium
and magnesium ions, and a low pH. This necessitates
another approach. The circuit blood may be dialysed before use [3], or the red cells washed, resuspended in albumin, and chemically modified, or heparinised blood
may be drawn from walk-in donors, tested and used fresh
Also, Poiseuille’s law dictates that the baby’s access
blood flow rates are disproportionately low compared to
Fig. 1 Diagram of the dialysis circuit
their dialysis requirement. This factor alone makes it
about four times harder to dialyse a term baby than an
adult, and around seven times harder to dialyse a 1 kg
neonate [4]. Resultant poor flows may prevent effective
dialysis, and lead to clotting within the dialyser. Though
haemodialysis has been reported in infants of around 2 kg,
it is certainly technically difficult [5], and has been considered impossible in smaller preterm infants [6].
Because of these problems we developed a novel
manual dialysis system for tiny infants which employed a
much smaller circuit, and separated the blood flow requirements of the sampling and dialysis phases of the
cycle [7]. Single lumen access was used to improve access efficiency [4], and the circuit consisted merely of two
syringes, a small dialyser, and a series of three-way taps.
The first syringe had a reservoir function which allowed
blood to be sampled slowly from the child’s line, but to be
passed quickly across the dialysis membrane. Ultrafiltration was achieved by generating a positive transmembrane pressure by fixing an elastic band on the distal
syringe to resist opening. Though effective, this manual
system was extremely tedious to perform, continuously
requiring an extra nurse. We therefore developed an automated device which would drive the system, and allow
us to provide practical haemodialysis and ultrafiltration
for babies continuously over prolonged periods of time.
Materials and methods
Overview of the circuit
Figures 1 and 2 show a diagram and a photograph of the circuit.
Apart from the syringes, it is constructed from standard sterile
disposable clinical equipment with screw locks, including threeway taps (Connecta Plus 3, Beckton Dickinson, SE-251 06, Sweden), a pressure transducer (Transpac IV, Abbott, Ireland), a 0.042
m2 surface area hollow-fibre dialyser (Miniflow 10, Hospal Ltd,
CV2 1PB, UK), and connecting lines. Standard clinical polypropylene syringes with neoprene bungs stiffened after prolonged use
with blood, so we used 25 ml gas-tight syringes with glass barrels
and Teflon-tipped bungs (Hamilton Ltd, LA5 9EA, UK).
Unlike conventional circuits, the haemodialysis blood flow is
independent of a continuous supply from the child. Instead, blood is
first aspirated from the patient at a rate determined by the vascular
access, and then passed repeatedly through the dialyser to allow
dialysis and ultrafiltration, before being returned to the baby. This
cycle is then repeated.
The circuit extracorporeal blood volume is small, at the syringe
stroke volume plus 5 ml (3.5 ml within the dialyser). If the stroke
volume was set at 3 ml for a 1-kg baby, the maximum extracorporeal circuit would be 8 ml, or 9.4% of its blood volume (assuming 85 ml/kg). For larger babies, any particular sample volume
would provide a proportionately greater stroke volume. For example, sampling 20 ml from a 3 kg baby would give an extracorporeal circuit volume of 7.8% of the blood volume, and a 6.7 ml/kg
stroke volume. Because the syringe volume is limited to 20 ml, the
stroke volume/kg and percentage extracorporeal volume would fall
for larger babies, reaching 3.3 ml/kg and 4.9% in a 6 kg baby. Two
dialysers can be used in parallel for larger infants, increasing the
circuit volume from 5 to 13 ml. Babies of 4–6 kg with 20 ml
sampling volumes would then have extracorporeal circuits between
6.5% and 9.7% of their blood volumes.
The operating cycle
The flow within the circuit is governed by the three-way taps. The
circuit is initially primed with heparinised saline, and cycled to
simulate clinical use, drawing fluid from a reservoir bag and returning it to waste, through tap 1, and a Y-connector with one-way
valves. To commence dialysis, tap 1 is turned manually to disconnect this fluid system, and connect the baby’s blood access line
instead. Taps 3 and 5 remain permanently open in all directions,
and taps 2 and 4 are turned by the machine. During blood withdrawal, tap 2 connects the baby to syringe A and obstructs the flow
of intravenous fluid infusion, and tap 4 disconnects syringe A from
the dialyser.
Next, the blood is passed back and forth between the syringes,
through the dialyser. Tap 2 turns to disconnect the baby’s access
line from the circuit, and allows the intravenous infusion to resume.
Fig. 2 Photograph of a circuit
for a larger baby with two
dialysers in parallel. Note that
extra syringes were used with
this particular circuit to enable
easy removal of clots, and that
extra taps were used compared
to Fig. 1 for ease of connection
of infusion and heparin lines,
An initial bolus of fluid flushes the line because the infusion pump
continues to operate against the closed tap. Tap 4 also turns to
connect the circuit between the two syringes. Syringe A’s plunger
is then depressed and syringe B’s is simultaneously withdrawn,
until the blood has been transferred between syringes through the
dialyser. Their directions are then reversed, and so on for a predetermined time. If ultrafiltration is required, the receiving syringe
opens less than the volume being emptied from the driving syringe.
This generates a rise in transmembrane pressure which drives
plasma water out of the circuit. For chemical clearance, the flow of
a commercially available bicarbonate-based dialysis fluid is controlled through the dialyser cylinder using an inflow intravenous
infusion pump, and passive outflow. Finally, taps 2 and 4 return to
their sampling positions, and the processed blood is returned to the
baby, and another volume aspirated.
During treatment, clotting within the circuit is prevented by a
slow heparin infusion into tap 5. This provides relatively higher
concentrations within the dialyser than in the baby’s circulation
because some effect is lost with passage through the dialyser. The
infusion is adjusted to maintain the glass-activated clotting times,
measured on 0.1 ml blood from the patient line, between 180 and
220 s.
Recirculation is inevitable in single lumen circuits because
dialysed blood is left in the sampling line at the end of each cycle.
For this circuit, the impact of this is best expressed by calculating
the effective cycle volume for any particular aspirated volume, and
will depend critically on the volume of the connecting tubing and
intravenous line lumens. These totalled about 1.0 ml (0.4 ml in taps,
0.4 ml in connecting line, 0.2 ml in venous line) for our two smaller
infants, and 1.4 ml in the two larger babies who had longer connecting lines. In our clinical practice, the 5 ml stroke volume used
for the smallest baby was therefore reduced by 20% to an effective
cycle of 4 ml, and the 20 ml aspirate for the largest baby was
effectively cut by 7% to 18.6 ml. Recirculation due to aspirating
freshly returned blood from within the vein is likely to be minimal
because the returning and sampling phases are sequential, unlike
conventional dual lumen dialysis where they occur simultaneously.
To prevent babies becoming clinically unstable during periods
of fluid removal or dialysis, treatment is performed either continuously throughout the 24 h, or at least slowly over prolonged periods, and not in short treatment runs. At the end of a treatment
period, the machine is stopped with both syringes empty, and the
blood remaining in the circuit returned to the baby. The taps are
positioned to allow heparinised saline to flush the blood, under
visual control, from the distal end of the circuit to the patient line.
The circuit can then be disposed of, or preserved by cycling with
heparinised saline until flushed completely clear, and used later (for
example, the next day). This saved the time of assembling new
circuits (which are not yet available as pre-packed units) and was
not associated with any bacterial contamination, judged by negative
blood cultures.
This prototype device has neither air nor blood leak detectors
built in. We have relied upon the fact that air leaks would be likely
to result in the accumulation of bubbles within the clear glass operating syringes to monitor these, and have observed the clarity of
the effluent dialysate to diagnose blood leaks.
Description of system
The syringe plungers are driven by 1 W stepper motors, similar to
those used in a clinical syringe pump, but allowing the plunger to
be driven considerably faster. We initially used powerful motors to
overcome the stiffness that polypropylene/neoprene syringes developed with use, but this was an unsatisfactory solution because
ultimately the bungs lock within the barrel, and the force of the
motor detaches them from the plungers. Glass/Teflon gas-tight
syringes were slightly stiffer than standard syringes initially, but
were unaffected by use.
The torque required to turn taps 2 and 4 between two positions,
90 apart, during each operating cycle is relatively high because
they also stiffen as traces of blood seep into their bearings. This
problem was also overcome by the use of more powerful electric
The system is controlled by a desktop PC. The software controls
the desired volumes and flow rates for blood sampling, dialysis and
ultrafiltration. It monitors the blood circuit pressures (user interface, Fig. 3). It also displays cumulative information, such as the
total treatment time and ultrafiltrate volume.
The system is classified as class I equipment by IEC 60601 (BSI
1990) [8], and is powered through an isolation transformer to significantly reduce the risks of electric shock. Much of the software
and hardware is concerned with providing safe single fault conditions, and a single fault analysis was performed in accordance with
IEC 60601 part 1, and part 2 which deals specifically with the
safety requirements of dialysis machines (BSI 1990; BSI 1998) [8,
9]. Each time the machine is switched on, an initialisation procedure tests the entire system’s functions.
Fig. 3 Graphical user interface
The circuit pressure is continuously monitored using a disposable sterile transducer. The circuit pressure and the phase of the
dialysis cycle are constantly displayed on the monitor. If the
pressure limits are exceeded, an audible and visible alarm is triggered. If necessary the operating cycle is halted.
The transducer position enables all the phases of the operating
cycle to be monitored. During blood withdrawal it senses the
negative pressure generated by the resistance to flow. With adequate access, this drop is minimal, but obstructions produce a
marked fall. As blood is returned, obstructions generate a pressure
rise. During dialysis, the transducer monitors the patency and resistance to flow through the lines and hollow fibres, registering a
pressure rise as blood is passed from syringe A to B, and a fall as it
is returned. Gradual changes in pressures with time could indicate
problems such as clot accumulation within the dialyser.
The computer adjusts the blood sampling rate if babies have
poorly flowing lines. Typically, blood withdrawal at 20 ml/min
produces minimal negative pressure, but may generate a marked
fall and lead to a complete obstruction. This is probably because
rapid sampling from a vein or the right atrium virtually empties it in
a small infant, occluding the line by sucking its sampling hole
against the vessel wall. This is a familiar experience when taking
blood with a syringe, when clinicians release the suction, and then
sample more slowly. In this computer controlled algorithm, the
syringe stops if the negative pressure reaches a trigger value, briefly
reverses, and restarts 2 ml/min slower. This sequence repeats until
the rate allows sampling without reaching the trigger pressure.
After 15 min, the sampling rate increases by 2 ml/min each cycle in
case the sampling conditions have improved.
Ultrafiltration, dialysis and haemolysis testing
The system was tested in vitro on fresh heparinised blood. Using
syringe stroke volumes of 3–20 ml, eighteen ultrafiltration rates up
to 50 ml/h were each tested over 1 h, and dialysis clearances were
measured to determine optimal combination of working blood
volume and dialysate flow rates. To increase measurement precision, urea and creatinine were added to the blood to produce concentrations seen in renal failure. Clearances were measured by the
removal of chemicals from blood, and their accumulation in the
effluent dialysate.
Red blood cells may be haemolysed by mechanical forces during
dialysis. Fresh heparinised blood was gently agitated in a reservoir
by a mechanical stirrer, dialysed, and free plasma haemoglobin
concentrations measured hourly for 12 h. Simultaneous controls
were measured from blood that was agitated, but not dialysed.
Clinical use of the system
We have used this device only for babies where conventional options were clearly inferior or non-existent (Table 1). Parents were
always fully aware that this device was a prototype. The Research
Ethics Committee did not consider the individual clinical decision
to use this device to be within their area of jurisdiction. The Hospital Trust deemed treatment appropriate if the clinicians considered it to be the child’s best option.
Machine reliability
The device was run for frequent prolonged periods during
development, and clinically almost continuously for
2 months, and had no failures of the computer control, the
machine, or any circuit components. We had no air or
blood leaks. Some awkward computer programme control
sequences need modification.
In vitro testing
This was consistently achieved to within €4.5% of the
prescribed rate, which compares well with other pub-
Table 1 Details of four babies dialysed using the automated syringe driven device
Case number, sex, gestational
age, weight and age at dialysis
Clinical details
Time on dialysis
Lactic acidosis from a mitochondrial cytopathy, anticipated from family history
and antenatal testing. Died of cardiomyopathy.
End stage renal failure from dysplasia, treated initially with peritoneal dialysis.
Also had a protein-losing enteropathy, and developed an ileal perforation, so
required her Tenckhoff to be removed. Planned to haemodialyse until abdomen
recovered to allow PD again, but enteropathy shown to be due to a severe intestinal
abnormality, so treatment withdrawn.
Very preterm baby who had had a perforated bowel from necrotising enterocolitis, 1.0
and developed acute renal failure secondary to liver abscess. Died of overwhelming
Hyperammonaemia due to methyl-malonic acidaemia, presenting after 4 days
of feeding. By start of dialysis her ammonia levels were already 2,300 mmol/l
and she was unconscious and fitting. Good metabolic clearances, but definite
evidence of having sustained brain damage, so treatment stopped.
Fig. 4 Mean plasma clearances
of creatinine, urea and potassium, over a range of syringe
stroke volumes and dialysate
flow rates, measured in vitro on
whole blood, and in baby 2
lished data [10, 11]. To avoid producing hyperviscosity,
the ultrafiltration rate was always adjusted to avoid producing packed red cell volumes greater than 70%. Rates
of 50 ml/h were achieved without a discernible increase in
operating pressures.
Clearances of creatinine, urea and potassium were similar,
and absolute values achieved using from 3 to 20 ml syringe stroke volumes ranged from 0.15 to 1.05 ml/min.
Assuming a clinical stroke volume of 5 ml/kg, the
clearances equated to 0.21 to 0.36 ml/kg/min (Fig. 4).
Clearances increased by 17% as the dialysate flow rates
were increased from 10 to 20 times the blood volume
(paired t -test, P =0.001), but increased little beyond that
(Fig. 4). In practice, there is little to gain by reducing the
dialysate flow rate below 400 ml/h. At this rate, a 5-l bag
of dialysate will last for 12 h, which is the stability limit
advised for commercially available bicarbonate-based
Clearances followed single exponential kinetics, described by the equation C t= C 0 (1– e kt ), where C 0 and
C t are the concentrations in the plasma at the start and
time t, and k is the clearance. This was used to predict
how poor blood sampling would affect clearances. Prolonged cycles become relatively inefficient as the concentration gradient falls, but in short cycles a greater
proportion of the time is taken up with returning and
sampling blood. Figure 5 shows examples of this. In
practice, we therefore used 4–5 min dialysis times.
Fig. 5a–c Calculated clearances (ml/min) using a 10 ml syringe stroke volume for various values of k assuming a sampling speed from the
patient of a 20, b 10, and c 5 ml/min
The free haemoglobin concentrations remained around
50 mg/l in the undialysed control blood, but increased to
920 mg/l during 12 h dialysis. With a haemoglobin of
14 g/dl, this would represent destruction of < 1% of the
cells, and is therefore an acceptable complication of an
essential clinical procedure.
Clinical use
Four babies were dialysed continuously (Table 1), two
newborns with inborn errors of metabolism, an extremely
small infant who was unable to be peritoneally dialysed
and too small to be conventionally haemodialysed, and a
larger baby with end stage renal failure and peritoneal
dialysis failure who was predicted to need prolonged
haemodialysis. She was continuously dialysed whilst
clinically very unstable for 3 weeks, and for 12 h daily
Fig. 6 Clearances achieved in four babies dialysed with the syringe
driven device
pressure alarms even after the machine reduced the
sampling rate to 2 ml/min. This led us to successfully
change access to a 1 mm internal diameter central line.
Dialysis and ultrafiltration
Access and sampling
Baby 1 was initially dialysed through a 6 French (1.9 mm
external diameter) umbilical venous line which sampled
only intermittently, and then through a 5 French (1.6 mm
external diameter) umbilical arterial catheter which gave
excellent flows. Two babies had sialastic, surgically
placed, 1 mm internal diameter superior vena cava lines.
In one of these, the standard sampling rate of 20 ml/min
produced negative pressures of more than 200 mmHg,
which initiated the automatic slowing process, and subsequent attempted increase. The access had spontaneously
improved, and the standard rate was subsequently
achieved. One baby had a percutaneous long line of
0.6 mm internal diameter initially which caused negative
The clearances obtained clinically were similar to those
achieved in vitro. The three smaller babies treated with
one dialyser had mean (SD) clearances for creatinine,
urea, phosphate, potassium, and in one case ammonia, of
0.54 (0.22) ml/min. Baby 2 was nearly 4 kg and had two
dialysers in parallel which gave clearances of 0.98
(0.22) ml/min (Fig. 6). She maintained stable electrolytes,
a mean creatinine concentration of 220 mmol/l, and a
mean urea of 13 mmol/l over several weeks. She required
continuous phosphate supplements to compensate for
high clearances. Varying the dialysate flow rate conferred
little extra benefit once its flow rate in ml/h was 10 times
greater than the sampling volume of the syringe (Fig. 4).
The machine required little nursing input, apart from
Fig. 7 Clearance of ammonia
from the plasma of case 4 using
syringe driven automated
haemodialysis, plotted a on a
linear, and b on a log scale.
Clearance half-life was 9 h.
Shaded areas show the period
of dialysis
Fig. 8 Theoretical clearances
produced by dialysis with the
syringe driven device, compared a to the predicted normal
GFR [15] expressed in ml/min,
and b expressed as a percentage
of normal
measuring clotting times and adjusting the heparin infusion. Baby 2 was mostly dialysed for 12 h during the day,
and the circuit repeatedly cleaned and reused. Despite the
theoretical possibility that the babies’ blood would tend to
cool towards room temperature whilst in the extracorporeal circuit, none had a discernible fall in central body
temperature, probably because the rate of blood removal
from the babies was relatively slow, and perhaps because
they were mainly nursed under overhead heaters.
Ammonia was cleared efficiently from baby 4 using
one dialyser. The plasma concentration quadrupled in the
14 h before dialysis, and with treatment it fell with a halflife of 9 h (Fig. 7). Unfortunately, the pre-treatment
concentration had already caused brain damage.
Babies 1 and 4 were not in renal failure, so no attempt
was made to ultrafilter them. Babies 2 and 3 were ultrafiltered without any rise in dialysis working pressure,
baby 2 for many days. It was easy to precisely remove
fluid to maintain clinical balance regardless of her fluid
intake, producing rates of up to 45 ml/h.
We have shown that this previously described manual
dialysis system can be safely and effectively automated,
and used for prolonged periods in babies between 800 g
and 4 kg. The constant extra nursing required for the
manual technique made it exceptionally tedious to perform for prolonged periods without error. By contrast, this
device requires minimal nursing input, even when run
24 h/day. This blood circuit design is unique, enabling
sampling from the baby to be at much slower rates than
the flow needed through the dialyser. Other circuit designs with a superficial resemblance differ fundamentally
[12, 13, 14].
The babies we treated did not have satisfactory alternative treatment options. The two with inborn errors of
metabolism would have needed blood-primed circuits for
conventional haemodialysis, which generates significant
delay since either walk-in donors or donor blood modification is needed. Our system meant no delay. Unfortunately, both of our babies died, one of her mitochondrial
cardiomyopathy, and the other because her pre-treatment
blood ammonia had already reached damaging levels. The
clearances achieved were similar to those previously described for conventional haemodialysis [15].
The two babies with renal failure could not be peritoneally dialysed because of recent laparotomies for intestinal perforations. Baby 3 was too small to allow
conventional haemodialysis to be considered, and died of
overwhelming sepsis. We used our device instead of a
standard circuit for baby 2 because it required considerably less nursing time, and avoided the need to prime the
lines. This is a major advantage in prolonged and therefore repeated use. We now consider this device to be the
treatment of choice for haemodialysing infants up to 6 kg.
Both biochemical and fluid volume control were easily
maintained extremely stable. It is unfortunate that she also
had an intrinsic abdominal pathology which led to withdrawal of treatment.
The device achieved clearances of around 0.5 ml/min
with one dialyser, and 1 ml/min with two. To avoid blood
priming, and minimise the extracorporeal circuit volume,
we anticipate using one dialyser in babies under 3 kg, and
two for larger infants. Although recirculation is inevitable
with single lumen vascular access, this is not a major
problem and can be readily deduced by subtracting the
sample line dead space from the cycle volume. Recirculation is also likely to be relatively significant when using
conventional double lumen circuits in small sick neonates, because relatively low flows in small vessels will
allow greater re-aspiration of recently returned blood,
even when the return lumen exits distally. Because the
glomerular filtration rate (GFR) in babies does not increase linearly [16], the proportion of the GFR achieved
by this device varies with body size. Its clearances equal
the GFR in babies of 1 kg, falling to about 15% of this by
3 kg (Fig. 8), and then range from 30% to 10% using two
dialysers in babies weighing from 3 to 6 kg. Clearances
produced by peritoneal dialysis vary with the molecule,
but are typically about 10%.
This device is a prototype. We believe its simplicity,
ease of use, safety and reliability of dialysis and ultrafiltration, and the fact that it does not require blood priming,
make it the method of choice for treating children under
6 kg with inborn errors of metabolism, and renal failure if
peritoneal dialysis is not feasible. We think it opens up the
possibility of haemodialysing very tiny preterm babies,
even those below 1 kg. We are therefore currently producing a second prototype with a more clinician-friendly
interface, and to conform to commercial and internationally recognised standards, including having reliable
automated air and blood leak detectors.
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A machine for haemodialysing very small infants