Anaesthetics- Fluids & Electrolytes Flashcards
Remind yourself of the normal distribution of fluid in a 70kg man, include:
- % is solid and % fluid
- How fluid is distributed in body
- How ECF fluid is distributed
- 60% TBW is water, 40% TBW is fluids
- 1/3 fluids is extracellular, 2/3 fluids is intracellular
- ECF is disributed as follows:
- 80% interstitial space
- 20% intravascular
- “Third space”
We have said that ECF is divided primarily into interstitial fluid and intravascular fluid (and third space fluid); these two ‘divisions’ make up the functional ECF which is why we often only talk about these tow subdivisions when discussing fluids. However, there is also some ECF that is not functional (available for exchange) known as sequestered ECF. Discuss the distribution of ECF if you are to consider both functioainl and sequestered ECF
ECF can be divided into:
- Transcellular water 2.5%v (e.g. peritioneal fluid, pericaridal fluid, pleural fluid, intraluminal fluid in GI tract- THIRD SPACE!)
- Bone water 7.5%
- Dense connective tissue water 7.5%
- Interstitial fluid 20%
- Intravascualr fluid 7.5%
NOTE: these percentages are percentage of whoel body water!
NOTE: first 3 are referred to as sequestered ECF as not available for exchange, bottom two are referred to as functional ECF.
What is meant by the ‘third space’?
The third space refers to areas of the body that does not normally contain fluid, and where fluid collection is not functional or desirable. This includes areas such as the:
- Peritoneal cavity (forming ascites)
- Pleural cavity (forming pleural effusions)
- Pericardial cavity (forming a pericardial effusion)
- Joints (forming joint effusions)
**NOTE: the third space also refers to the non-functional and excessive collection of fluid in the interstitial space, resulting in oedema.
What is our main concern with ‘third spacing’?
When fluid moves into the ‘third space’ or ecessively accumlates in the interstitum, this may come at the expense of the intravascular space, resulting in hypotension and reduced perfusion of tissues- potential for hypovolaemic shock.
State some sources of fluid input
State some sources of fluid output
Sources of fluid intake include:
- Oral fluids
- Nasogastric or PEG feeds
- Intravenous fluids
Sources of fluid output include:
- Urine output
- Bowel or stoma output (particularly diarrhoea)
- Vomit or stomach aspiration
- Drain output
- Bleeding
- Sweating
What is meant by insensible fluid losses?
Insensible fluid losses is a term that refers to fluid output that is difficult to measure, such as through respiration (breathed out), in stools, through burns and from sweat. This varies a lot and can only be estimated. It may account for a large volume (in excess of 800mls per day) in patients with significant diarrhoea or high fevers and significant sweating.
We have already said that water is lost through urine, faeces, breathing & sweating; discuss how much water is lost through each body system e.g. renal, lung etc..
- Renal 1.5L
- Lung & skin 0.7L
- GIT 0.3L
Therefore total loss per day ~2.5L
Remind yourself what is mean by a positive and negative fluid balance
- Positive: more fluid coming into body than leaving body
- Negative: less fluid coming into body than leaving body
State some signs of hypovolaemia
- Hypotension (systolic < 100 mmHg)
- Tachycardia (heart rate > 90)
- Capillary refill time < 2 seconds
- Cold peripheries
- Raised respiratory rate
- Dry mucous membranes
- Reduced skin turgor
- Reduced urine output
- Sunken eyes
- Reduce body weight from baseline
- Feeling thirsty
State some signs of hypervolaemia
- Peripheral oedema (check the ankles and sacral area)
- Pulmonary oedema (shortness of breath, reduced oxygen saturations, raised respiratory rate and bibasal crackles)
- Raised JVP
- Increased body weight from baseline
What are the 3 main aims/reasons we give fluids?
- Resuscitation (e.g. sepsis or hypotension)
- Replacement & redistribution (e.g. vomiting and diarrhoea)
- Maintenance (e.g. nil by mouth due to bowel obstruction)
State, and explain difference, between two main types of fluid
- Crystalloids: are essentially water with added salts and / or glucose. The contents of crystalloid solutions will redistribute throughout the different fluid compartments of the body
- Colloids: contain larger molecules that stay in the intravascular space longer. Theoretically, this helps to retain fluid in the intravascular space. However, the research suggests there isn’t really much benefit to using them in resuscitation scenarios.
State some examples of crystalloids
- 0.9% saline
- Hartmann’s solution
- 5% dextrose
State some examples of a colloid
- Human albumin solution
- Hydroxyethyl starch
- Dextrans
Colloids are very rarely used; who may you consider prescribing a colloid to?
Colloids such as human albumin solution may be used in patients with decompensated liver disease. Albumin is an important component of plasma, and works to increase the plasma volume. It increases the oncotic pressure of the plasma, drawing in and retaining fluid. In decompensated liver disease, the patient’s liver is not producing adequate albumin, leading to reduced oncotic pressure in the intravascular space, resulting in reduced circulating blood volume. Human albumin solution may be used to help correct this, although the effects are only temporary.
Explain, in terms of Starling’s law of capillary, why colloids are less frequently used now
- In original Starling model for capillary, we thought that there was filtration at the arterial end and reabsorption at the venule end of teh capillary
- However, in revised Starling model for capillary it is thought that there is filtration at both ends; however, filtration is less at the venule end. This is because they found that most of oncotic pressure capillary comes from subglycocalyx layer, not from oncotic molecules plasma, and the oncotic pressure from subglycocalyx layer is much lower than initailly thought. Consequenlty, difference in hydrostatic pressure (between capillary & interstitium) always remains higher than difference in oncotic pressure (between capillary and interstium) so fluid is always filtrered out of capillary; jsut rate of filtration decreases as you move along the capillary.
- The filtered fluid is then reabsorbed by lymphatics and returned to plasma
Remind yourself of normal concentrations of the following extracellulary:
- Sodium
- Potassium
- Calcium
- Magnesium
- Chloride
- Bicarbonate
- Sodium: 145mmol/L
- Potassium: 4mmol/L
- Calcium: 2.5mmol/L
- Magnesium: 1mmol/L
- Chloride: 115mmol/L
- Bicarbonate: 25mmol/L
Remind yourself of normal concentrations of the following intracellulary:
- Sodium
- Potassium
- Calcium
- Magnesium
- Chloride
- Bicarbonate
*Main ones to know are in bold; idea of ballpart figure
- Sodium: 12mmol/L
- Potassium: 155mmol/L
- Calcium: x10-7mol
- Chloride: 4mmol/L
- Bicarbonate: 25mmol/L
- Phosphate: 105mmol/L
- Protein: 55mmol/L
Remind yourself of the daily requirements of the following for a healthy adult:
- Water
- Sodium
- Potassium
- Chloride
- Magnesium
- Calcium
- Glucose
- Phosphorus
- Water: 30ml/kg
- Sodium: 1-2mmol/kg
- Potassium: 0.5-1mmol/kg
- Chloride: 1mmol/kg
- Magnesium: 0.1mmol/kg
- Calcium: 0.1mmol/kg
- Glucose: 50-100g per day
- Phosphorus: 0.4mmol/kg
Summary of fluids
State some examples of hypotonic, isotonic and hypertonic solutions
There are some special circumstances in which we must be cautious/carefully consider what fluids to administer; state these special circumstances
- Renal failure: avoid K+ & careful fluid balance monitoring
- Cardiac failure: volume restriction & give furosemide
- Post-operatively: less Na+ due to ADH secretion
- Liver failure: avoid Na+, give dextrose 5%
- Brain haemorrhage: avoid dextrose, give NaCl
- Alcohol excess: IV pabrinex before dextrose to avoid korsakoff psychosis
- Nutrition refeeding: avoid too much dextrose
Summary of how to prescribe fluids
