Renal - OSCE - Equipment - HF

Question 1

What is ultrafiltration

Answer 1

Movement of water across a semipermeable membrane

Because of a hydrostatic pressure gradient

Question 2

What is convection

Answer 2

When there is ultrafiltration of large volumes of water across of semi permeable membrane, there is a convective current that drags additional molecules.

This is convection and removes small/mid size molecules

Question 3

Define HF

Answer 3

Process where ultrafiltration and convection are used to remove fluid and solutes in blood

Requires pressure gradient across a semi-permeable membrane

Question 4

Define HD

Answer 4

Uses a COUNTER CURRENT flow

Diasylate in one direction across the membrane, blood on the other

Enhances diffusion across the semi permeable membrane due to concentration gradients

Question 5

How do we remove fluid from a patient in HF

Answer 5

Ultrafiltration removes fluid from plasma

We then replace it with less fluid to create a net loss

Question 6

What is pre-dilution

Answer 6

Replacement fluid can either be added pre or post filter.

Pre - dilutes blood and prolongs the lifespan of the filter by reducing tendency to clot

Therefore less anti-coag

BUT

Post dilution concentrates blood in the filter - greater clearance

Question 7

Factors influencing clearance of substances from plasma

Answer 7

Membrane - 
Surface area
Hydraulic permeability
Pore sore 
Charge

Pressure gradient - hydrostatic, colloid, osmotic

Solute properties - size, charge, concentration

Question 8

Methods of anti-coag

Answer 8

• Systemic unfractionated IV heparin.
• Heparinisation of the RRT equipment alone.
• Low-molecular-weight heparin.
• Prostacycline.
• Citrate and calcium.
• Factor Xa Inhibitor — fondaparinux (either answer sufficient for the mark).
• Direct thrombin inhibitor — bivalirudin.

Question 9

Who would you avoid citrate in

Answer 9

Hepatic dysfunction

Question 10

Causes of the filter clotting, and how to manage

Answer 10

Impaired vascular access - adequate flow and positioning

High intra-abdo pressures and high intra-thoracic pressures impede flow

Ineffective anti-coagulation - consider pre-dilution

Ensure optimal haemofilter settings

OR - consider taking a break and assess renal recovery

Question 11

Other things to consider while on HF

Answer 11

Drug dose corrections
Correct electrolytes
Fluid balance
Maintain normothermia
Haemodynamic stability

Question 12

Indications for RRT

Answer 12

Metabolic acidaemia
Hyperkalaemia
Symptomatic uraemia
Fluid overload
Temperature control
Overdose - Lithium, salicylates

Question 13

CVVHF

Answer 13

Convection
Bulk flow
Hydrostatic pressure gradient
Rate of removal proportional to blood flow rate, pressure gradient and membrane area

Question 14

CVVHD

Answer 14

Diffusion
Countercurrent blood and diasylate
Concentration gradient

High diasylate flow and blood flow, sets up maintained pressure gradient

Apply pressure difference- fluid out

Question 15

What is the dose of RRT

Answer 15

Defines how much effluent is produced

25-35ml/kg/hour

Question 16

Prescription factors

Answer 16

Intermittant or continuous
HF or HD
Dose/rate of iltration (higher flow rates do nothing)
Pre and post fluid (usually as a ratio)
Fluid balance
Anti-coagulation
Blood Flow rates up to 250ml/min