Fluid Management Flashcards
Total percentage of body water in an adult?
45 L and 60%
Two main Fluid compartments
There are two main fluid compartments:
- 2/3rd – Intracellular fluid compartment = 30L
- Composed of high potassium/ low sodium, maintained by Na-K-ATPase
- 1/3rd – Extracellular fluid compartment = 15L
- ¼ Plasma = 3L – non cellular component of blood
- Interstitial fluid – bathing cells in extracellular spaces e.g. pleural cavity, joint space
How does the body increase body water?
- ADH
- aldosterone/ renin/ angiotensin
- thirst mechanism
How does the body decrease water?
- ANP
Normal electrolyte requirements of water, sodium and potassium per day
- 30 ml/kg/day water
- 1.0-1.4 mmol/kg/day sodium
- 0.7-0.9 mmol/kg.day potassium
Composition of 1000ml 0.9% Saline
- 150 mmol SODIUM, 150mmol CHLORIDE
- 0 mmol POTASIUM
- 0 CALORIES
Compositon of Hartmann’s solution 1000ml
- 131 mmol sodium
- 111 cholride
- no calories
- 5 mol potasium, calcium, lactate and chloride
composition of 5% dextrose
- no potassium
- no sodium
Who is at risk of peri-operative fluid balance disturbance?
- The elderly
- patients with CV diorders
- patient with CVD
- Renal disease
- Major fluid losses
- Septic patients
Anticipation of fluid balance
In certain circumstances it is possible to anticipate that there may be potential fluid and electrolyte disturbances and so institute measure to minimise these disturbances. These situations include:
- patients that have receive bowel preparation preoperatively can lose large amounts of fluid and electrolytes, therefore additional fluid may need to be administered
- patients with epidurals – as well as blocking the pain carrying nerves, they also block the sympathetic nerves where they leave the spinal canal. This causes vasodilatation in the territory blocked by the epidural and for this reason they may need additional fluids. However, other causes of hypotension should be excluded (eg. Haemorrhage, sepsis). When the epidural wears off the patients are at risk of fluid overload as the vasodilatation decreases
Possible causes of post-operative fluid depletion
- Causes of unrecognised or inadequately treated preoperative or intraoperative hypovolaemia (eg. Prolonged vomiting in bowel obstruction, chronic poor intake in a cachectic patient with a malignancy)
- Losses into 3rd space, including peritoneal cavity, pleural cavity, subcutaneous tissues due to low oncotic pressure, leaky capillaries (sepsis) etc
- Losses via drains, nasogastric tubes or fistulae
- Pyrexia
- Haemorrhage
- Inappropriate high urine losses due to polyuric renal failure or inappropriate use of diuretics
- Unexpected delay in oral intake due to eg. nausea and vomiting.
- It is very rarely appropriate to treat postoperative oliguria with diuretic unless there is clear evidence of fluid overload and a high intravascular fluid volume
Consequences of fluid depletion
If left untreated, fluid depletion can lead to:
- Renal failure
- Multiple organ failure
- Breakdown of bowel anastomoses
- Cerebral infarction
Signs of fluid depletion
- Thirst, not always a reliable sign but it is useful in conjunction with others.
- Dry mucous membranes. Look in the patients mouth
- Reduced skin turgor, sunken eyes. In young children, a sunken fontanelle is another useful sign.
- Evidence of poor peripheral perfusion. For example, cold peripheries, delayed capillary refill time, raised serum lactate.
- Tachycardia. This has many potential causes including fluid deficit, pain, myocardial ischaemia, pulmonary embolism – it always needs investigated. However, patients on beta-blockers may fail to mount a tachycardia in response to a fluid deficit so its absence does not rule out hypovolaemia.
- Hypotension, this should be considered in relations to the patients normal blood pressure, a patient who is normally hypertensive may have signs and symptoms of hypovolaemia at a much a higher blood pressure than a fit patient. This is normally a late sign, particularly in younger fitter patients, but a postural blood pressure drop may be seen at lesser levels of hypovolaemia.
- Oliguria. Urine volumes should be at least 0.5 ml/kg/hr. Oliguria should always be investigated. In addition to checking fluid status, fluid balance and the general condition of the patient it is always necessary to rule out post renal causes of oliguria or anuria, the most common of which is a blocked urinary catheter or urinary retention (most commonly in older men with large prostates).
- Confusion has many causes in the perioperative period, especially in the elderly. It is important to rule out hypoxia or hypovolaemia as a cause as these are easily correctable.
Invasive monitoring
The central venous pressure is frequently used to assist in the assessment and management of the potentially unstable surgical patient. The measurement requires central venous cannulation. This is invasive and carries a small but nonetheless measurable risk. The central venous pressure is affected by fluid status and by right ventricular function. In the unwell patient there is poor correlation between central venous pressure and fluid status. It is therefore more useful to look at trends in CVP rather than individual readings. In most patients, fluid balance can be accurately estimated without the need for a CVP.
What is normal urine output in an adult?
0.5-1 ml/kg/hr
