FLUIDS AND ELECTROLYTES
Introduction1
Fluid and electrolyte management is paramount to the care of the critically ill patient.
Knowledge of the physiology of fluids and electrolytes is key in understanding the changes to
both fluid volume and electrolyte composition which occurs pre-, intra-, and post operatively
Distribution of body fluids1,2:
Total body fluids + 42
Intracellular fluid (ICF) + 28 Extracellular fluid (ECF) + 14
40% body mass 20% body mass
Plasma + 2.8 Interstitial fluid (ISF) +
5% body mass 15% body mass
The extracellular and intracellular compartments have distinct electrolyte compositions
(table1).
Table 1
EXTRAVASCULAR INTRACELLULAR
Intravascular Interstitial
Na + 142 Na+ 144 K+ 150
Cl- 103 Cl- 114 Mg+ 40
HCO3- 27 HCO3- 30 PO4 – 150
Proteins 16 Proteins 40
The osmolality of fluid in all the compartments is maintained between 290-310mOsm. The
principal determinants of osmolality are Na+, glucose and blood urea nitrogen (BUN).
Calculated serum Osmolality= 2Na + (glucose/18) + (BUN/2.8).
Each compartment is separated by a membrane:
1
, 1. The endothelial membrane separates the plasma from the interstitial fluid
compartment. It allows free movement of electrolytes and water but is impermeable to
protein.
2. The cell membrane separates the extracellular and intracellular compartments. It
allows free passage of water but electrolyte permeability is dependent on active
transport mechanisms. Thus electrolyte differences are maintained by energy
expenditure. Since water is freely diffusible the osmolality of all body compartments
is identical. The understanding of the microstructure and the characteristics of the
vascular endothelium enables the understanding of which regime should be used for
an individual patient.
The glycocalyx6
The recent description and improved understanding the endothelial glycocalyx is a
breakthrough in the understanding of fluid physiology. The glycocalyx is a thin endoluminal
carbohydrate rich layer containing membrane-bound proteoglycans and glycoproteins. Both
endothelium and plasma derived soluble molecules integrate into this mesh.
The classical model used to describe microvascular fluid exchange is that of Starling, stating
that fluid filtration rate across capillary endothelium is determined by the hydraulic and
colloid osmotic pressures in the vascular lumen and in the surrounding tissue. This balance
has been applied across the entire transendothelial barrier, with the different pressures being
assessed globally.
The discovery of a relatively thick endothelial glycocalyx and its influence on oedema
formation has led to a major revision of the Starling principle. Modelling has been performed
by various authors to understand the differences in the permeability of the different regions of
the endothelial layer, such as the glycocalyx, the endothelial clefts, and the tight junctions.
The role of the glycocalyx in leukocyte–vessel wall interactions seems dual as it harbours the
endothelial cell adhesion molecules, such as P-selectin, ICAM-1, and VCAM-1 but it also
attenuates adhesion of leukocytes to these molecules. In pathological states such as shock,
this ability to regulate interactions is altered.
The endothelial glycocalyx also acts as a mechanotransducer, responding to shear stress and
flow. This is important in both normal physiology and pathological states such as
hypertension, fluid over-resuscitation and shock states.
Water and electrolyte balance
Water and electrolyte turnover is considered in terms of external balance and internal fluxes.
External balance refers to the comparison between the water input from and the water output
to the external environment. Over any period of time, input equals output and the body is in
balance.
Internal balance or flux refers to the movement of water across the capillaries of the body
(including the secretion and absorption of the various transcellular fluids) and movement of
water between interstitial and intracellular fluids. Table 2 summarises the water balance in an
average 70kg man.
Table 2.
Intake ml Output ml
2
, Water drunk 1500 Urine volume 1600
Water in food 750 Faecal water content 50
Water from metabolism of food 250 Loss in expired air 850
(Insensible perspiration)
Total input 2500 Total output 2500
Ion balance
Intake mmol/day Output mmol/day
Urine faeces Skin
Na+ 100-200 100-200 <5%
K+ 20-200 20-100 <5% trace
Cl 100-200 100-200ml <5%
Fluid and electrolyte management1,3:
For surgical patients, appropriate selection and administration of fluids can mitigate against
organ failure, whereas improper dosing can exacerbate already injured systems. Fluid and
electrolyte goals and deficiencies must be defined for individual patients to provide the
appropriate combination of resuscitation and maintenance fluids.
Types of fluids:
A. CRYSTALLOIDS
• Resuscitation e.g. MRL/ Plasmalyte/ D.S/ Hypertonic saline
• Rehydration and Replacement 5% D/W and general replacement solution respectively.
• Maintenance – e.g. E2 / Maintelyte
B. COLLOIDS
• Natural, i.e., human albumin, Blood.
• Artificial, i.e., gelatin and dextran solutions, hydroxyethyl starches (HES
The composition of commonly used solutions is shown in table 3, 4 and 5 below.
Table 3. Resuscitation fluids (mmol/l)
Fluid Na K Ca Mg Cl Lact HCo3 Dextrose Kj/L pH Osmolatity
mOSm/l
1. Balanced 131 5 2 1. 111 0 28 0 0 7.4 278
electrolyte
solution.
Plasmalyte B
2. Ringer lactate 130 4 3 0 109 28 0 0 0 6.5 273
3. Ringer lactate 130 4 3 0 109 28 0 50 840 5,5 556
with Dextrose
5%
4. Sodium 154 0 0 0 154 0 0 0 0 5,5 308
chloride 0,9%
5. Sodium 154 0 0 0 154 0 0 50 840 4,0 586
chloride 0,9
With 5%
dextrose
3
Introduction1
Fluid and electrolyte management is paramount to the care of the critically ill patient.
Knowledge of the physiology of fluids and electrolytes is key in understanding the changes to
both fluid volume and electrolyte composition which occurs pre-, intra-, and post operatively
Distribution of body fluids1,2:
Total body fluids + 42
Intracellular fluid (ICF) + 28 Extracellular fluid (ECF) + 14
40% body mass 20% body mass
Plasma + 2.8 Interstitial fluid (ISF) +
5% body mass 15% body mass
The extracellular and intracellular compartments have distinct electrolyte compositions
(table1).
Table 1
EXTRAVASCULAR INTRACELLULAR
Intravascular Interstitial
Na + 142 Na+ 144 K+ 150
Cl- 103 Cl- 114 Mg+ 40
HCO3- 27 HCO3- 30 PO4 – 150
Proteins 16 Proteins 40
The osmolality of fluid in all the compartments is maintained between 290-310mOsm. The
principal determinants of osmolality are Na+, glucose and blood urea nitrogen (BUN).
Calculated serum Osmolality= 2Na + (glucose/18) + (BUN/2.8).
Each compartment is separated by a membrane:
1
, 1. The endothelial membrane separates the plasma from the interstitial fluid
compartment. It allows free movement of electrolytes and water but is impermeable to
protein.
2. The cell membrane separates the extracellular and intracellular compartments. It
allows free passage of water but electrolyte permeability is dependent on active
transport mechanisms. Thus electrolyte differences are maintained by energy
expenditure. Since water is freely diffusible the osmolality of all body compartments
is identical. The understanding of the microstructure and the characteristics of the
vascular endothelium enables the understanding of which regime should be used for
an individual patient.
The glycocalyx6
The recent description and improved understanding the endothelial glycocalyx is a
breakthrough in the understanding of fluid physiology. The glycocalyx is a thin endoluminal
carbohydrate rich layer containing membrane-bound proteoglycans and glycoproteins. Both
endothelium and plasma derived soluble molecules integrate into this mesh.
The classical model used to describe microvascular fluid exchange is that of Starling, stating
that fluid filtration rate across capillary endothelium is determined by the hydraulic and
colloid osmotic pressures in the vascular lumen and in the surrounding tissue. This balance
has been applied across the entire transendothelial barrier, with the different pressures being
assessed globally.
The discovery of a relatively thick endothelial glycocalyx and its influence on oedema
formation has led to a major revision of the Starling principle. Modelling has been performed
by various authors to understand the differences in the permeability of the different regions of
the endothelial layer, such as the glycocalyx, the endothelial clefts, and the tight junctions.
The role of the glycocalyx in leukocyte–vessel wall interactions seems dual as it harbours the
endothelial cell adhesion molecules, such as P-selectin, ICAM-1, and VCAM-1 but it also
attenuates adhesion of leukocytes to these molecules. In pathological states such as shock,
this ability to regulate interactions is altered.
The endothelial glycocalyx also acts as a mechanotransducer, responding to shear stress and
flow. This is important in both normal physiology and pathological states such as
hypertension, fluid over-resuscitation and shock states.
Water and electrolyte balance
Water and electrolyte turnover is considered in terms of external balance and internal fluxes.
External balance refers to the comparison between the water input from and the water output
to the external environment. Over any period of time, input equals output and the body is in
balance.
Internal balance or flux refers to the movement of water across the capillaries of the body
(including the secretion and absorption of the various transcellular fluids) and movement of
water between interstitial and intracellular fluids. Table 2 summarises the water balance in an
average 70kg man.
Table 2.
Intake ml Output ml
2
, Water drunk 1500 Urine volume 1600
Water in food 750 Faecal water content 50
Water from metabolism of food 250 Loss in expired air 850
(Insensible perspiration)
Total input 2500 Total output 2500
Ion balance
Intake mmol/day Output mmol/day
Urine faeces Skin
Na+ 100-200 100-200 <5%
K+ 20-200 20-100 <5% trace
Cl 100-200 100-200ml <5%
Fluid and electrolyte management1,3:
For surgical patients, appropriate selection and administration of fluids can mitigate against
organ failure, whereas improper dosing can exacerbate already injured systems. Fluid and
electrolyte goals and deficiencies must be defined for individual patients to provide the
appropriate combination of resuscitation and maintenance fluids.
Types of fluids:
A. CRYSTALLOIDS
• Resuscitation e.g. MRL/ Plasmalyte/ D.S/ Hypertonic saline
• Rehydration and Replacement 5% D/W and general replacement solution respectively.
• Maintenance – e.g. E2 / Maintelyte
B. COLLOIDS
• Natural, i.e., human albumin, Blood.
• Artificial, i.e., gelatin and dextran solutions, hydroxyethyl starches (HES
The composition of commonly used solutions is shown in table 3, 4 and 5 below.
Table 3. Resuscitation fluids (mmol/l)
Fluid Na K Ca Mg Cl Lact HCo3 Dextrose Kj/L pH Osmolatity
mOSm/l
1. Balanced 131 5 2 1. 111 0 28 0 0 7.4 278
electrolyte
solution.
Plasmalyte B
2. Ringer lactate 130 4 3 0 109 28 0 0 0 6.5 273
3. Ringer lactate 130 4 3 0 109 28 0 50 840 5,5 556
with Dextrose
5%
4. Sodium 154 0 0 0 154 0 0 0 0 5,5 308
chloride 0,9%
5. Sodium 154 0 0 0 154 0 0 50 840 4,0 586
chloride 0,9
With 5%
dextrose
3
