Fluid and Electrolyte Prescribing

Question 1

What are the major fluid compartments and what is the normal volume in these in a ‘70kg male’

Answer 1

Extracellular fluid - Plasma - 3L, Interstitial fluid - 11L. (total 14L) Between these two is capillary wall.
Intracellular fluid - 28L. Between intracellular and extracellular is the plasma membrane

Question 2

Explain the exchange of fluids across the capillary membrane

Answer 2

At the arterial end there is a larger hydrostatic pressure than osmotic pressure (osmotic pressure stays the same throughout capillary of 25mm) and so there is a net flow of fluids out of the cell.
At the venous end there is a smaller hydrostatic pressure (smaller than osmotic pressure) and so there is a net flow of fluids into the capillary.

Question 3

What are the major gains and losses of total body fluid?

Answer 3

Gains - Food and water intake.

Losses - Urine (1500ml), faeces, sweat and insensible losses/ Total losses = 2550ml a day

Question 4

What is the average urine output?

Answer 4

half a millilitre per kilogram body weight per hour

Question 5

What is an insensible water loss?

Answer 5

Solute free water losses that you cannot control. This can be via;

• Transepidermal diffusion (water passed through skin and evaporated),
• Evaporative loss from respiratory tract

Question 6

Describe the relation between osmolality, sodium and volume.

Answer 6

The key driver of total volume is sodium. If sodium drops then total volume falls to ensure osmolality stays the same. If sodium levels rise then total volume will rise to ensure osmolality stays the same

Question 7

What are the major gains and losses of sodium chloride and how are sodium levels controlled?

Answer 7

Gains - food and water.
Losses - sweat, faeces and urine.
Levels are controlled via volume sensors as there are no receptors detecting sodium. The amount of sodium filtered depends of GFR and the amount of sodium reabsorbed changes via flow rate, aldosterone, ANP.

Question 8

What occurs if osmolality rises or osmolality falls?

Answer 8

Rises - Increase in thirst, increase in release of ADH causing increase in water retention/intake causing an increase in volume.
Falls - Decrease in thirst, decrease in ADH, decrease in water intake/retention so decrease in vol

Question 9

How are changes in volume detected?

Answer 9

• If there is an increase then there is stretch of vascular system detected by baroreceptors causing decrease in renin and aldosterone release and an increase in ANP so decreases sodium and water retention.
• If there is a decrease in volume then there is decreased stretch of vascular system, detected by baroreceptors which causes ADH release, an increase in renin release, increase in angiotensin 2, increase in aldosterone release and a decrease of ANP. Leading to sodium and water retention.

Question 10

What are the gains and losses of potassium

Answer 10

Gains via food and drink.
Losses mainly via urine, under normal conditions little is lost in swear and faeces. (vomiting, diarrhoea and use of diuretics can be another cause).

Question 11

Explain the control of potassium ions

Answer 11

Tightly regulated and secretion is linked to sodium reabsorption. When there is an increased activity of the basolateral sodium pump, it causes more potassium ions to enter the cell. Causing increase in simple diffusion at the apical membrane. Basically the more sodium you reabsorb, the more potassium you loose into the urine.

Question 12

What is the effects of aldosterone on the distal convoluted tubule?

Answer 12

• Increases activity of sodium pump at basolateral membrane,
• Increases the number of sodium pumps at basolateral membrane,
• Increases the number or sodium and potassium channels in apical membrane.
  This leads to increased reabsorption of sodium and increased secretion of potassium.

Question 13

What is Conn’s syndrome?

Answer 13

Hyperaldosteronism leading to hypertension due to increased fluid volume and hypokalaemia

Question 14

What are the risks associated with IV fluids?

Answer 14

• A peripheral venous catheter is required. Risk of hospital acquired infection so check everyday and change every 72 hours.
• Easy to give too much fluid,
• Errors in prescribing.
  Oral route is much safer so you have to be able to justify the use of IV fluids

Question 15

What are the signs of hypovolaemia?

Answer 15

• Systolic BP below 100mmHg.
• Heart rate above 90bpm.
• Capillary refill over 2 seconds.
• Resp rate of 20+breaths/min.
• Urine output less than 0.5mls/kg/hr.
• Dry mucous membranes,
• Decreased skin turgor,
• postural hypotension is a sensitive marker

Question 16

What are the signs/indications of a fluid overload?

Answer 16

• History of cardiac or renal problems,
• Raised JVP,
• Peripheral oedema,
• Inspiratory crackles at lung bases,
• Hypertension

Question 17

What investigations are useful in the assessment of volume status?

Answer 17

• FBC,
• Urea and electrolytes,
• Chest X-ray,
• Lactate,
• Urine biochemistry

Question 18

What are the electrolyte requirements?

Answer 18

Sodium of 1mmol/kg/24hrs. Potassium of 1mmol/kk/24hrs (or a little bit less than 1)
Minimum of 400kcals/24hrs

Question 19

What are maintanence fluids?

Answer 19

• This is for when patient does not have excess losses. If there is no oral intake then approx 30mls/kg/24hours.
• Adjust if there is some oral intake
• Use pump,
• 0.18% saline 4% dextrose

Question 20

What are replacement fluids?

Answer 20

Replacement of previous and/or current abnormal losses. This is additional fluid to be given along with maintenance fluid.

Question 21

What are resuscitation fluids?

Answer 21

Occurs when the patient is hypovolaemic and requires urgent correction of intravascular depletion.

Question 22

Name some IV Crystalloids

Answer 22

• 5% dextrose (glucose). Think as water. Distributes into ISF, plasma and cells.
• 0.18% NaCl 4% dextrose (maintenance fluids),
• 0.90% NaCl (isotonic saline),
• Plasmalyte

Question 23

What are some IV colloids?

Answer 23

• 4.5% albumin (supplied in 0.9% NaCl) - stays in plasma and doesn’t enter cells.
• Hydrolysed gelatin (supplied in 0.9% NaCl), protein metabolised overtime so is then the equivalent to 0.9% NaCl.
• Blood, stays in vasculature and increases blood volume (consider if suspect Hb has fallen)

Question 24

What are features of Hartmann’s and plasmalyte?

Answer 24

Their composition is very similar to extracellular fluid

Question 25

Describe features of a fluid challange

Answer 25

• consider when oliguria/hypotensive and no signs of fluid overload.
• Want to give 500mls balanced salt solution (such as Hartmann’s) within 15mins. Then re-assess. Can repeat up to 2000mls.
• Caution with obese patients, elderly, renal impairment, cardiac failure or those who are malnourished or at risk of refeeding syndrome.

Question 26

What is the management of a patient with DKA?

Answer 26

ACTRAPID;

• Airway, breathing, circulation,
• Commence fluid resuscitation (1000mls 0.9 saline over 1st hour),
• Treat potassium,
• Replace insulin (infusion of 6 units per hour),
• Acidosis management,
• Prevent complications,
• Information for patients,
• Discharge

Question 27

What are the clinical features of DKA?

Answer 27

Hyperglycaemia which causes - Dehydration (caused by hyperglycaemia, vomiting, kaussmaul respiration and altered conscious level which leads to reduced intake), tachycardia, hypotension and clouding of consciousness.
Acidosis - Kussmaul’s respiration, acetone on breath, abdominal pain and vomiting
- Think about features which are precipitating to the cause such as sepsis