Fluid & Electrolyte Balance

Normal Physiology
Maintenance Fluids
Replacement Fluids
Perioperative Fluids
Electrolyte Imbalance
The Critically Ill Pt
Example Cases

Normal Physiology
Know the compartment from which fluid is being lost
Know to which compartment infused fluids will be administered.
Two thirds / one third (ICF:ECF) and three fourths, one-fourth (ECF interstitial vs Plasma).

Total Body Water Calculation
~45-60% of body-weight in an average person.
- More if lean / muscle
- Less if old or fat
- Men more, women less.

Compartments of Water
Intracellular
2/3 of all body water
Maintained by cell membranes
K+ predominates.
Extracellular
1/3 of all body water
Na+ predominates

Extracellular Compartments
Intravascular space water (IVS)
1/4 of extracellular
Held in by Starling's Forces
Interstitial (ISS) fluid
3/4 of extracellular

Average Examples
(70kg man)
Total body water = 42L (60%)
Intracellular fluid = 28L
Extracellular fluid = 14L
Plasma fluid = 3L
Interstitial fluid = 11L

Distribution of Water
Osmotic forces determine distribution of fluid between ICF and ECF.
Na+/K+ pumps chiefly responsible for difference (3:2 exchange)
ICF
Na+10, K+150, Mg+4
HPO4--, SO4--, HcO3-, protein- = 150
ECF
Na+140, K+4
Cl-114, HCO3-30.

Osmolality

Equal across the membrane
- any change results in rapid equilibration
Normally 280-290 mosm/kg
- hypothalamic osmoreceptors detect osmolality and stimulate thirst and ADH secretion
--> aquaporons in distal kidney to reabsorb water.
--> urine osmolality varies greatly from 100-1200 mOsm/kg.
mOsm/kg=2x(Na+K)+(BUN/2.8)+(Glucose/18)

Starling's Forces

Between the ISS and IVS is the capillary membrane
- freely permeable to water and electrolytes but not proteins.
- The solute composition is the same but protein higher in plasma
--> colloid osmotic pressure exerted.
At arteriole end, net force is to ISS.
- Hydrostatic ~32mmHg
- Oncotic ~25mmHg
At venule end, net force is to IVS
- Hydrostatic ~12mmHg
- Oncotic ~25mmHg
Difference carried away by lymph

Control of Water Volume
Osmoregulation is distinct but related to volume regulation.
- Osmoregulation describes TBW regulation to maintain osmolality
- Volume regulation describes sodium changes to maintain osmolality.
There is primacy of volume such that ADH will be secreted even in hyponatraemia and low osmolality.

Volume and pressure receptors control sodium retention & excretion.
- sequestered fluid in stress / post op states can be considerable, esp in sepsis
--> may cause overload as it is remobilised during recovery.

Daily Electrolyte Losses
[1mEq = 1mmol/L/L for a monovalent element (eg Na+), but 2mmol/L for a divalent element (eg Ca++))]
(typical post-operative patient)
Na+
1-2 mEq/kg/day (1-2 mmol/kg/day)
K+
0.5-1 mEq/kg/day (0.5-1 mmol/kg/day)
Cl-
100-250mmol/day


Maintenance Fluids

Daily Water Losses
Averages 2500mL for the average 70kg person.
- or, better, 30-35ml/kg/day
800-1500ml in urine
250ml from stool
Insensible = 8-12 mL/kg/day
- i.e. ~750mL/day for a 70kg person.
- 75% from skin, 25% respiratory
Losses vary with ambient T.
- average intake usually matches this: 1500ml ingested, 700ml eaten, 300ml metabolic.
- higher insensible losses in fever, hyperventilation, burns, tachycardia, hypermetabolic states.
- e.g. 10% higher per 1oC above 37.2oC
- insensible losses from thoracotomy / laparotomy are 1L/hr.

Typical Maintenance Regimen
4:2:1 rule

Body Mass
mL/kg/hr
mL/kg/day
First 10kg
4
100
Second 10kg
2
50
Each kg>20kg
1
20
60kg
100 ml/hr
2300 mL

Replace sensible (measurable) and insensible losses
Include dextrose to maintain osmolality and prevent short-term proteolysis

Typical maintenance regimens
- 2.5 L 4%dextrose / 0.18% S
- or 2L 5%D + 500mL 0.9%S
- Add 40 mmol K+ to dex bags.
Replacement Fluids

Composition of Typical Fluids

Na+
K+
Cl-
Vol
Saliva
15
19
40
1.5
Stomach
50
15
140
2.5
Bile, sm bowel
130-145
5-12
70-100
4.2
Sweat (insens.)
12
10
12
0.6

Crystalloid Fluid Composition
Solution
Electroytes
Osmolality
0.9% Saline
Na+ 154
Cl- 154
308
4%Dex0.18%S
Na+ 31
Cl- 31
Gluc 40
284
5%Dex
Nil
Gluc 50
278
Hartmann's
Na+ 131
Cl- 112
K+
HCO3- 29
Ca++4
278
--> fluid with isotonic concs of Na+ will expand the ECF, distributing over several minutes
--> Dex-only solutions will distribute through TBW (glucose is readily metabolised)

Estimating Dehydration

%Weight
Vol L/70kg
Signs
Mild
>4
2-3
Thirst, dryness
Mod
5-8
4-6
Oliguria,
Post hypoT
Tachycardia
Severe
8-10
>7
Thready pulse
Low BP
Cool periph,
- see also shock notes for true hypovolaemia

Pure Water Loss
Usually depletion is associated with electrolyte deficits.
However this is not always the case, eg if not drinking, tachypnoea, fever.
Leads to thirst, dehydration.
May progress to tachycardia, hypotension, prostration.
Hypernatraemia and coma if severe.
--> Replace w/ 5% dex 0.18% saline.

Water+Electrolyte Loss
Replace ECF losses.
Skin: sweating, burn.
Renal: diabetic ketoacidosis.
Gut: vomiting, ileus, fistula, diarrhoea.
The source determines the type of electrolyte loss.
--> Manage on a daily basis.
--> Crystalloids.
--> If sick, needs close monitoring.
--> Regular biochem (us plasma, sometimes loss fluid).

Restoration of normal acid-base balance is the most reliable indicator of adequate resuscitation

Perioperative Fluids

Daily estimation should be performed

- except when losses are large or complex: twice daily.

Preoperative
Most pts presenting for minor/moderate elective surgery don't need pre-op fluids
- loss suffered in starvation can be replaced intra-operatively.
This does not hold for pts losing fluid to an underlying pathology.

Intra-operative fluids
1.5ml/kg/hr for duration of pre-op starvation
1.5ml/kg/hr normal maintenance
5ml/kg/hr operative insensible loss
Blood loss - >15% blood volume.
--> usually using a combo of Hartmann's colloid and blood.

Post-operative
1.  ADH and aldosterone increase
--> water and Na+ are retained.
--> difficult to excrete free water for 72 hrs
- restrict maintenance to 2L / day.
- giving water fluids will result in hyponatreamia.
2.  K+
- Generally avoided on day 1 as post-op renal function is uncertain.
- thereafter 1mmol/kg should be prescribed
- altered as appropriate to losses / plasma levels.
3.  Chart for ongoing fluid losses.
- taking account of type of fluid lost.
4.  Ensure renal perfusion is maintained.
- recognise any deterioration in renal function early.
5.  Pts with cardiac failure, sepsis syndrome, liver failure and renal dysfunction need consideration.

EGFR
60+ = Normal range
15-60 = Renal impairment
<15 = Renal failure
Less accurate if: pregnant, overweight, high muscle mass

Fluids : Evidence and Modern Strategies
Traditionally, post-op care aimed for urine outputs of 0.5ml/kg/hr
- not evidence based and supposes that post-op patients are in some sort of shock-like state, which is not the case
Post-op patients actually have an increased volume of distribution
- due to decreased systemic vascular resistance from GA, epidural, or stress response
Restriction of liberal fluid administration and aiming for normal weight (no excess fluid) is now preferred
- avoids pulmonary oedema, ileus, coagulopathy, AF; every organ system is impaired by excess fluids


Electrolyte Imbalance
Hyponatraemia
Hypernatraemia
Hypokalaemia
Hyperkalaemia
Hypercalcaemia
Hypocalcaemia
Hypomagnesaemia
Hypophasphataemia

Hyponatraemia
Na<135.
- either sodium depleted, sodium-replete, or sodium-overloaded
--> hence the importance of clinical as well as biochemical assessment.
Urinary Na+
ECF Low
ECF Normal
ECF High (Dilution)
High (>20mmol/L)
Diuretics, renal salt-loss
Addison's
Glucocorticoid deficit
Hypothyroidism, SIADH
Renal failure
Low (<20mmol/L)
Extrarenal loss
- (or sequestration)
TURP syndrome Cirrhosis, CHF
Nephrotic syn.

Symptoms
Sodium concentration is the major determinant of plasma osmolality.
- in hyponatraemia, fluid moves into the ICF
--> overhydration of cells
--> headache, lethagy, nausea, vomiting, convulsions, coma
- may not be evident until <120mmol/L, especially if chronically reducing.

Treatment

Depends on chronicity and cause
--> treat at the same rate as to which it occured.
--> hypertonic saline is not necessary
Chronic
- seldom symptomatic
- just treat the underlying cause.
--> sudden restoration of eunatraemia can provoke massive fluid shifts and severe brain consequences.
Acute
- if symptomatic, it is an emergency.
- aim only for safe level (120mmol/L) at first
- aim to raise by 1mmol/hr until symptom free, or until plasma level increased by 20-25mmol/L
--> ie serial measurement is reqd
Post-operative
(For acute water overload)
- a diuretic and 0.9% saline is effective.

image

Hypernatraemia
Na+>155.
Excess sodium may act as a cardiac depressent
Causes:
H20 Loss
Fever, hot climate, thyrotoxicosis, diabetes insipidus or mellitus, nephrogenic
Hypotonic Loss
GI loss, sweating, osmotic diuresis
Salt gain
Iatrogenic administration, steroids.
Treatment
Replace with water orally
Or with 5% dextrose IV
- occasionally 0.45% saline is needed to prevent sugar loading in DM II pts.

image

Hypokalaemia
1.  diet --> Plasma K+ --> faeces, urine, sweat
2.  plasma K+ <=> cells
- remember plasma K+ is a poor reflection of bodily K+ as most is inside cells
- acute changes may occur as K+ enters cells, eg after treating acidosis, from catecholamine release, salbutamol use or anabolic activity
--> keep the plasma level above 3.5

Causes of true loss:
GI: vomiting, suction, diarrhoea, laxatives, fistulae, villous adenoma
Renal: hyperaldosteronism, diuretics, steroids, CHF, nephrotic syndrome, cirrhosis.
Iatrogenic: failing to add K+ to IV fluids.

Features:
Muscle weakness, fatigue, paralytic ileus, cardiac conduction deficits.
ECG: increased PR interval, ST-depression, reduced T-wave height, and U waves and widened QRS if severe.

Treatment:
Most of K+ loss occurs from ICF
K+ deficit = (normal-measured K+) x 0.4 body weight in kg.
- mild hypokalaemia is usually tolerated in the absence of digoxin therapy.
Or for every 1 mmol/L deficit, 200-300mmol/L is required (CCrISP)
--> Rapid administration is dangerous
- infusion rates >10-20mmol/hr should be used only in life-threatening situations.
- often given in bags of 10 mmol in 100 ml of fluid per hour.
- could give 20 mmol/hr via a central line



Hyperkalaemia
Dangerous as it induces respiratory weakness and cardiac arrest (usually at >7)
--> it is extremely difficult to resuscitate these patients.
--> there is no absolute dangerous level; a level of 6 may be chronically tolerated, but if suddenly rising from 4, may be fatal.

ECG
: tall T-waves, wide QRS

Causes
Increased intake

- oral or iatrogenic
Failure of excretion
- renal failure, hypoadrenalism, distal nephron disease (eg obstructive), ACE inhibitors and spironolactone.
Redistribution
- when K+ moves rapidly out of cells (mostly a problem for pts with impaired renal excretion)
- trauma, drugs (eg sux), ischaemic/hypwoxic damage
--> classically the pt who is hypovolaemic with metabolic acidosis and resp compensation, induced with sux, then underventilated, causing a fall in pH, and quickly after intubation suffering a cardiac arrest.

Treatment
Requires urgent correction.
Calcium gluconate
-
10-30ml of 10% solution
- membrane stabiliser
- rapid effect, but short action so stay vigilent
  Insulin - Dextrose IV
-
50ml 50% dextrose bolus, then 10% solution plus insulin 10-20units/100g dextrose over 30-60mins
- transfers K+ into cells
- rapid / intermediate acting but can be corrosive to veins, CVL preferred.
Salbutamol
- 5-10ug/min IV infusion
- transfers potassium into cells
- rapid short action
- tachycardia and vosodilation can occur
ECG monitoring is mandatory.



Hypercalcaemia
If severe, affects nerves and damages renal tubular function.
Diminishes salt retention, meaning Ca secretion is more difficult (vicious cycle)

Causes

Hyperparathyroidism
Vitamin D intoxication
Paraneoplastic syndromes
- eg breast Ca, myeloma

Treatment
Establish a saline diuresis
Intravenous bisphosphonates may help
Treat the primary cause
- if associated with a primary solid tumour, prognosis is worse.

Hypocalcaemia
Ensure it is referenced to albumin level

Causes

Pancreatitis, vitamin D deficit, rhabdomyolysis
Tollowing thyroid surgery when parathyroids removed as well.

Treatment
Administer calcium
Treat the primary condition
In post-parathyoidectomy or Vit D deficit
- administer vitamin D analogues
If pt is critically ill, restrict use to situations of clinical evidence of hypocalcaemia.

Hypomagnesaemia
The most important intracellular cation after K+
--> depletion is common; causes confusion, seizures and dysrhythmias.
--> excess causes paralysis and CNS depression
Plasma levels reflect Mg++ poorly, but <0.6mmol/L is likely to by problematic

Causes
Excess
- almost always iatrogenic, eg MgSO4 use in pre-eclampsia.
Deficit
Usually poor diet, plus bowel & kidney (rarely congenital tubule disorders)
- long-term loop diuretics
- chronic malabsorption
- alcohol abuse
- during recovery from faecal peritonitis

Treatment
IV is best as MgSO4 may be purgative
- do not exceed 1.5mmol/L.

Hypophosphataemia

Phosphate is present in protein-containing food
- kidney excretes it under control of PTH
Deficiency is common in the critically ill usually once healing begins
--> as PO4 is taken into cells, levels fall
When <0.6mmol/l, respiratory, immune and skeletal muscle effects are demonstrable
Treatment
Replacement will come with refeeding.
But if <0.6, IV supplements are a good idea.


Managing the Critically Ill

Differences
Large GI or renal losses may exist
Greater insensible losses occur from open wounds and resp tract.
Great sequestration of water (up to 20L) may occur.
- contributed by capillary permeability and hypoalbuminaemia

Basic Fluid Requirements
Children
100 ml/kg for first 10kg
50ml/kg for next 10kg
25ml/kg for each subsequent 1kg
Adults
30ml/kg/day.

Assessment
Follow the CCrISP plan
- note charts, clinical status, hx, meds.
Daily weight can be very helpful for overload

Managment
ECF-type loss
- (eg bld loss, vomiting, diarrhoea, third spacing, sepsis, burns, DM II, pancreatitis)
--> replace with isotonic fluid

Replace GI losses
--> replace ml:ml with Hartmann's

H20-type loss
- (eg increased RR, fever, water deprivation, diabetes insipidus)
--> replace with water.

General Points
Keep good fluid balance charts and monitor key ions
--> keep these in balance and success will be yours.
--> reassess because the pt is constantly changing.

Example Cases

Case 1
60-yr-old female, 70kgs. 
- 2d hx of vomiting, abdo pain.
- dry membranes, tachycardic
- abdo distended, XR=sm. bowel obstruction

What fluid should you use?  How fast?
N saline to rexpand the ECF.
6% of the body weight has ben lost (4-5L)
- the presence of post. hypotension would confirm this.
Correct gradually in the absence of hypovoaemic shock.
Replacement of K+ should be included.
Adequacy is noted by clinical status.

Case 2
A 45 yr-old-man for elective cholecystectomy.

What are his daily fluid & electrolyte requirements?

2.5L of water per day
1mmol/kg/day of Na+
1mmol/kg/day of K+

Case 3
17 yr-old-male MVA
Alert.  BP 70 systolic.
Pulse 140
Complaining of pelvic pain on palpation.

Why is his BP low?

He is hypovolaemic
What is the blood volume and how much has he lost?
~30% - See shock card (signs)
What fluid to use?
Controversial
If crystalloid used, follow the 3:1 rule (300ml for every 100ml blood loss).
The ability of colloids to stay in the compartment varies by type.
Remember to concentrate on tissue perfusion and oxygenation.

Case 4
58yr old 70kg man 3/7 post cystectomy.
AF, rate 118 (on dig), low urine output.
-ve fluid balance 1L over last 24 hrs.
NG losses 1450; Drain 720, Urostomy 640.

What further info do you need?
Electrolytes (Na+138, K+3.1,Urea 5.2)
Full clinical assessment
What would you prescribe?
Needs ~2L of H20, 1.5L of 0.9%NaCl, 80mmol K+ in next 24hrs
Review bd.


References
Cameron 10th
Tjandra and Clunie Chapter 1.
Toouli et al. Chapter 2.2.