Renal replacement Flashcards
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?
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.
Which vessels does blood pass through to reach the cortex?
- Renal artery
- Interlobar arteries
- Arcuate arteries
- Interlobular arteries
- Afferent arterioles
- Glomerular capillaries
- Filtration
- Efferent arterioles
How do efferent arterioles branch after leaving the glomerulus depending on their location?
How does blood eventually leave the kidney?
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.
How much lymph per minute is produced by the kidneys?
Why is it important?
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.
What is the definition of clearance? And therefore what are the units?
What is the equation for clearance?
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).
What are the classical indications for starting RRT (5)?
What other indications exist (2)?
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
When is it generally accepted that RRT can be discontinued?
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.
Broadly, how does RRT work? Which physical process underlie it?
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).
How does diffusion allow filtration?
Which characteristics of the membrane affect diffusion (3) ?
Why is a counter-current used?
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.
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?
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.
Which modality is the usual first choice in ICU patients?
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
How does transmembrane pressure change as a filter ages?
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.
What is filtration fraction?
What range should it be kept in and why?
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.
What are pre- and post-filter dilution in haemofiltration?
What is the benefit of each?
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.
What does SCUF stand for?
How does V-V SCUF work?
Circuit flow rate (Qb)?
Ultrafiltrate flow rate?
Main use?
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
