Renal replacement Flashcards

1
Q

What % of cardiac output to the kidneys receive? What % of body weight do they comprise?

What % of blood entering the kidney supplies the cortex? Where does the remainder go?

A

The kidneys receive 20–25% of the cardiac output but account for only 0.5% of the body weight.

Of the blood to the kidney, >90% enters via the renal artery and supplies the renal cortex. The remainder of the renal blood supply goes to the capsule and the renal adipose tissue.

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2
Q

Which vessels does blood pass through to reach the cortex?

A
  1. Renal artery
  2. Interlobar arteries
  3. Arcuate arteries
  4. Interlobular arteries
  5. Afferent arterioles
  6. Glomerular capillaries
  7. Filtration
  8. Efferent arterioles
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3
Q

How do efferent arterioles branch after leaving the glomerulus depending on their location?

How does blood eventually leave the kidney?

A

The efferent arterioles from nephrons in
the outer two-thirds of the cortex branch to form a dense network of peritubular capillaries, which surround all the cortical tubular elements.

The efferent arterioles in the inner one-third of the cortex give rise not only to some peritubular capillaries but also to capillaries that have a
hairpin course into and out of the medulla, to form the vasa recta.

Vasa recta and peritubular capillaries eventually drain into the renal vein which leaves the kidney at the hilum.

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4
Q

How much lymph per minute is produced by the kidneys?

Why is it important?

A

The volume of lymph draining into the renal hilum per
minute is about 0.5 ml – i.e. the kidney produces almost as much lymph per minute as urine.

Its function is probably to return protein (reabsorbed from the tubular fluid) to the blood.

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5
Q

What is the definition of clearance? And therefore what are the units?

What is the equation for clearance?

A

The clearance of any substance excreted by the kidney is the volume of plasma that is cleared of the substance in unit time. Thus, the units of clearance are those of volume per unit time (usually ml/min).

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6
Q

What are the classical indications for starting RRT (5)?

What other indications exist (2)?

A

Classical indications for RRT:

  • Rapidly rising urea and creatinine
  • Hyperkalaemia unresponsive to medical therapy
  • Severe metabolic acidosis
  • Fluid over load
  • Oligo/anuria (0–0.2 mL/kg/hour)

Other uses of extracorporeal therapy:

  • Drug removal
  • Adjunct in the management of severe sepsis
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7
Q

When is it generally accepted that RRT can be discontinued?

A

There is no hard evidence on how and when RRT should be stopped.

It is generally accepted, however, that re-appearance of urine output (>500 mL/day) in a previously anuric patient is a signal that the kidneys are improving.

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8
Q

Broadly, how does RRT work? Which physical process underlie it?

A

Renal replacement uses semi-permeable membranes
to achieve filtration. The membrane may be artificial, as in a filter, or autologous, as in the peritoneum. Many molecules, including water, urea, and solutes of various molecular weights, are transported across the membrane by variable combinations of the processes of diffusion (dialysis) and convection (ultrafiltration).

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9
Q

How does diffusion allow filtration?

Which characteristics of the membrane affect diffusion (3) ?

Why is a counter-current used?

A

During diffusion the movement of solutes depends on their tendency to reach the same concentration (equilibrium) on each side of the membrane: this results in the passage of solutes from the compartment with the higher concentration to the compartment with the lower concentration.

Diffusion is affected by characteristics of the semi-permeable membrane including thickness, surface area, temperature, and diffusion coefficient.

Diffusion is provided by dialysis, in which a solution (the dialysate) flows on the other side of the membrane, counter-current to blood flow, in order to maintain a solute gradient.

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10
Q

Which physical process underlies ultrafiltration?

Which equation describes the rate of flow across the membrane in UF?

Which factors determine the pressure gradient across the membrane?

A

In convection, the movement of solute across a semi-permeable membrane is a result of transfer of water across the membrane. In other words, as the solvent (plasma water) crosses the membrane, solutes are carried with it if the pore size of the membrane allows such passage. Convection can be achieved by ultrafiltration (UF), which creates a transmembrane pressure (TMP) gradient. UF depends on the rate of flow (Qf), the membrane coefficient (Km) and the TMP gradient between the pressures on both sides of the membrane:

Qf = Km * TMP

TMP gradient is the difference between the pressure in the blood compartment which is directly related to blood flow (Qb), and filtrate compartment pressure which is modulated by a pump.
in modern RRT machines.

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11
Q

Which modality is the usual first choice in ICU patients?

A

CVVH will clear solutes and correct acidosis and for most patients is adequate therapy – a few will need CVVHDF (see variations) but for most CVVH is the default starting mode

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12
Q

How does transmembrane pressure change as a filter ages?

A

Modern RRT machines are designed to maintain
a constant filtration rate (Qf): when the filter is ‘fresh’ and highly permeable, the pumps retard UF production, generating a positive pressure in the filtrate compartment (TMP is initially dependent only on blood flow). As the membrane fibres become degraded, a negative pressure on the filtrate side is necessary to achieve a constant Qf. With time, TMP progressively increases up to a maximum level at which solute clearance is compromised and clotting of the filter or membrane rupture is possible.

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13
Q

What is filtration fraction?

What range should it be kept in and why?

A

During UF, plasma water and solutes are filtered from blood, leading to a decrease in blood hydrostatic pressure and increase in blood oncotic pressure. The fraction of plasma water that is removed from blood during UF is called filtration fraction and should be kept in the range of 20–25% to prevent excessive haemoconcentration within the filtering membrane. Otherwise, the oncotic pressure gradient could neutralize the TMP gradient resulting in equilibrium.

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14
Q

What are pre- and post-filter dilution in haemofiltration?

What is the benefit of each?

A

Replacing plasma water with a substitute solution completes the haemofiltration (HF) process. The replacement fluid can be administered after the filter (post- filter dilution HF, often called simply
‘post-dilution’), before (pre- filter dilution HF, often called simply ‘pre-dilution’), or both. Post-filter dilution leads to a higher urea clearance, but pre-filter dilution may prolong circuit life by reducing
haemoconcentration and protein build-up in the filter fibres.

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15
Q

What does SCUF stand for?

How does V-V SCUF work?

Circuit flow rate (Qb)?

Ultrafiltrate flow rate?

Main use?

A

Slow continuous ultrafiltration (SCUF): blood is driven by a pump (P) through a highly permeable filter via an extracorporeal circuit, using veno-venous (V) access. The ultrafiltrate produced during membrane transit is not replaced and it corresponds to weight loss.

Circuit blood flow: (Qb) 100–250 mL/min

Ultrafiltrate flow (QUF): 5–15 mL/min.

It is used only for fluid removal in overloaded patients particularly in patients with diuretic-unresponsive cardiorenal syndrome

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16
Q

What does CVVH stand for?

How does it differ from SCUF?

What is Qb?

What is QR?

What is QUF?

A

Continuous veno-venous haemofiltration (CVVH): CVVH is similar to SCUF above except that the ultrafiltrate produced during membrane transit is partly or completely replaced (R) to maintain intravascular volume. Replacement fluid may be delivered before (R PRE), after (R POST) or both sides of the filter (pre- or post-dilution).
Clearance for all solutes is convective and equals UF rate.

Qb: 100–250 mL/min;

QR: 15–60 mL/min;

QUF: QR+ 5–15 mL/min.

17
Q

Which physical process underlies haemodialysis?

Which physical process underlies haemofiltration?

Which clears small molecules more reliably?

A

Diffusion = haemodialysis

Convection = haemofiltration

Middle molecules are preferentially cleared by convective methods, rather than smaller molecules that are more reliably cleared by diffusion.

18
Q
A
19
Q

What is solute drag? How is it also known?

Which CRRT modality makes use of it?

What determines solute clearance? And what doesn’t?

How can pressure across the membrane be adjusted?

A

Haemofiltration is a convective process whereby a hydrostatic pressure gradient is used to filter plasma, water, and solute across a membrane. This is analogous to the process within the renal corpuscle. The underlying mechanism is that of ‘solute drag’ where appropriately sized molecules are pulled along with the mass movement of solvent, traditionally termed ultrafiltration (UF).

Clearance is independent of solute concentration but depends on transmembrane pressure.

Increasing blood flow rate or increasing effluent removal rate (i.e. negative pressure across the membrane).

20
Q
A
21
Q

What are the possible advantages of continuous renal replacement vs intermittent (4)?

A
  • Enhanced haemodynamic stability.
  • Superior management of fluid balance.
  • Enhanced clearance of inflammatory mediators, which may provide benefit in septic patients.
  • Better preservation of cerebral perfusion among patients with acute brain injury or fulminant hepatic failure.
22
Q

What are some possible risks of RRT(6)?

A

Cannula insertion

The side effects of biocompatibility

Fluid shifts

Altered drug metabolism.

Increased nursing workload

Significant cost.

23
Q

For continuous techniques, what is meant by dose of RRT?

What dose is suggested by international consensus guidelines?

A

For continuous techniques, dose is the sum of all effluent fluids expressed as millilitres per kilogram body weight per hour. It is important to note that the addition of dialysate and the targeting of negative fluid balance both add to the summative dose.

20–25 ml kg-1 h-1

24
Q

How does protein binding of a drug affect CRRT clearance?

How does volume of distribution affect CRRT clearance?

A

Drugs with low protein binding are removed by CRRT more readily.

Drugs with a higher volume of distribution have lower clearance by CRRT.

25
Q

Estimated creatinine clearance of modern CRRT?

A

Modern CRRT treatments result in a creatinine clearance of around 25–50 ml min-1.

26
Q

What are the preferred sites for vascular access catheter insertion suggested by KDIGO (in order)?

What are the optimal site for catheter tips for IJ lines? And femoral lines?

A
  • First choice – right internal jugular.
  • Second choice – femoral.
  • Third choice – left internal jugular.
  • Fourth choice – subclavian.

Optimal flow rates are obtained with the catheter tip in the right atrium for internal jugular lines and within the inferior vena cava for femoral access

27
Q

What is the most commonly used method of anticoagulation in RRT?

What are alternatives and when are they used?

A

Systemic heparin anticoagulation is the most commonly used method. It is ubiquitous, cheap, and its activity is routinely tested as part of coagulation screening. Because the treatment is systemic, patients are at risk of haemorrhage, and often a balance between the risk of bleeding and risk of circuit clotting must be addressed in several patient populations.

If patients develop HIT and associated thrombosis then systemic anticoagulation with either the heparinoids (such as danaparoid) or direct thrombin inhibitors (such as argatroban) is required.

28
Q

What does CVVHD stand for?

By what mechanism does it work?

Which size molecule is it best suited for?

Qb?

A

Continuous veno-venous haemodialysis (CVVHD):

In CVVHD, blood is driven through a low permeability dialyser via an extracorporeal circuit in veno-venous mode and a countercurrent flow of dialysate (Di-Do) is delivered on the dialysate compartment. The ultrafiltrate produced during membrane transit corresponds to patient’s weight loss.

Solute clearance is mainly diffusive and efficiency is limited to small solutes only.

Qb: 100–250mL/min;

QDi: 15–60 mL/min;

QDo: QDi + QUF.

29
Q

What does CVVHDF stand for?

By what mechanism does solute clearance occur?

Qb?

A

Continuous veno-venous
haemodiafiltration (CVVHDF): Technique where blood is driven through a highly permeable dialyser via an extracorporeal circuit in veno-venous mode and a countercurrent flow of dialysate is delivered on the dialysate compartment. The ultrafiltrate produced during membrane transit is in excess of the patient’s desired
weight loss and replacement solution is needed to maintain fluid balance. Solute clearance is both convective and diffusive. CVVHDF has the main advantage of getting the most solute clearance with the lowest blood flow rate and should be
reserved for heavier adult patients.

Qb: 100–250 mL/min; QDi: 15–60 mL/min;
Do: Di + QR + QUF.

30
Q

What is dose range of unfractionated heparin for RRT?

How can regional anticoagulation be achieved?

A

The dose ranges from 5 to 10 IU/kg/hour. In patients in
whom systemic anticoagulation is to be avoided, heparin can be
used in combination with post-filter administration of protamine
(regional anticoagulation); with a 1 to 1 ratio (1mg of protamine
per 150 IU of unfractionated heparin)