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Fluid Management

Basics

Intravenous fluids may be used for:

Maintenance

 provide daily fluid requirement in patients unable to take in adequate fluids or losing increased amounts and correct dehydration

Use crystalloids to maintain volume.

A common regime (for an average 70kg man) to give 3 L fluid with 150 mmol sodium and 60 mmol potassium is:

  • 1L normal saline with 20 mmol potassium over 8 hr
  • 1L 5% dextrose with 20 mmol potassium over 8 hr
  • 1L 5% dextrose with 20 mmol potassium over 8 hr

    Other common regimes are:

    • 1L normal saline with 20mmol potassium over 8hrs each and 1L of 5% dextrose with 20mmol potassium for patients with surgical problems (such as bowel obstruction) that may lead to excessive Na loss. Always check U+Es before prescribing
    • 3x 1L of Hartmann’s solution over 8hrs each.  Hartmann's is physiologically close to plasma but has a lower osmolality. Continuous therapy can therefore reduce plasma osmolality and it should be avoided in patients with head injury to avoid exacerbating cerebral oedema.

       

      Resuscitation

       The replacement of intravascular volume in hypovolemic patients

      Use colloids to replace the intravascular volume fast and effectively. These fluids (such as gelofusin and dextran) are contain large particles osmotically active and remain in the intravascular space. These are often used initially in patient who are hypotensive.

      Use crystalloids to expand volume over longer periods of time. These contain smaller molecules and will distribute over time into the interstitial and interacellular spaces.

      There is no clear benefit in outcome from initial resuscitation with colloids versus crystalloids and either can be justified. Current ATLS guidelines recommend an initial 2L of crystalloid (specifically hartmanns) to restore volume in acute haemorrhage. the amount of crystalloid required to restore circulating volume is roughly 3-4 times the volume lost.

        A common regime to replace fluid loss requires several adjustments and close monitoring is:

        • 250 ml colloid (Gelofusine) or 5% dextrose fluid challenge to maintain CVP at 8-12 cmH2O and BP >120 mmHg (repeat if necessary)
        • 1 ml/kg/hr normal saline as adjunct fluid therapy
        • Additional potassium in 5% dextrose if K+ <3 mmol/L

          Severe hemorrhage requires packed red blood cell infusion.

          Hartmann’s solution at 20-30 ml/kg/hr can also be used for fluid resuscitation

           

          Total body water

          70 kg man has ~42 L (%60) body fluids. Distribution of fluid in the body is: 

          1/3 in extracellular fluid

          • Interstitial fluid                10 L
          • Plasma                           3 L
          • Transcellular fluid              1 L

          2/3 in intracellular fluid

          • Red blood cells                 2 L
          • Other cells                      26 L

          Transcellular fluids include cerebrospinal fluid, synovial fluid, pleural fluid, ocular fluid, etc.

           

          Fluid movement between these compartments are governed by osmotic and Starling forces:

          Osmotic forces:

          Osmotic equilibrium at the cell membrane regulates the water balance between ECF and ICF. Osmotic forces depend on osmolality (Osmoles of solute per kilogram of solvent). Main solutes include charged (Na+, K+, Cl-, HCO3‑) and uncharged (urea, glucose) molecules.

           

          Starling forces:

          Starling equation illustrates movement of fluid across capillaries depending on three factors:

          • Hydrostatic forces that push fluid
          • Oncotic pressure exerted by proteins in fluid that pulls water
          • Permeability of endothelium between plasma and interstitial fluid

          On the arterial side of capillary bed, intravascular fluid moves into interstitial space (higher intravascular hydrostatic force). On the venous side, fluid is reabsorbed into plasma (lower intravascular hydrostatic force).

          When fluid is infused into plasma, hydrostatic forces increase and oncotic pressure decrease (dilution effect) until fluid is evenly distributed in ECF and Starling forces are in equilibrium.

           

           

          Assessing volume status

          History: Patient history, observation and fluid charts, patient notes, etc.

          Examination: Blood pressure, pulse, respiratory rate, skin turgor and capillary refilll time, temperature

          Investigations: urine output (<30 ml/hr for 70kg man), central venous pressure readings, blood tests, chest x-ray

          Response to fluid challenge: Give a 500 ml 0.9% saline or 250 ml colloid through large bore 14G (brown) or 16G (grey) cannula over 5 minutes. Measure response: CVP monitoring, pulse or BP increase and reduction of respiratory stress. Repeat as necessary. Fluid challenge is safe is lungs are clear.

             

            Dry patient

            Nil-by-mouth, increased fluid loss (diarrhea ot vomiting), thirst and dry mouth

            Low CVP, low BP, tachycardia, weight several kg below pre-op weight

            Decreased urine output

            Bloods: urea disproportionately raised to creatinine, high sodium and potassium levels

            Fluid challenge may not be sufficient to raise CVP initially

             

             

            Overloaded patient

            Fluid intake > output

            Raised CVP, pulmonary oedema, weight several kg above pre-op weight

            Blood sodium level may be low

            X-ray may show pulmonary oedema and effusion

            CVP rises and plateaus with fluid challange

             

            Fluid and electrolyte balance

            Daily requirements

            • Water: 40 ml/kg/day (rough estimation, see below for exact calculation)
            • Sodium ~ 100 mmol
            • Potassium ~60 mmol

             

              Daily fluid balance of 70 kg man:

               

              Intake: ~2500 ml

              • 1500 ml – liquid intake
              • 750 ml – food
              • 250 ml – oxidative phosphorylation

               

              Output: ~ 2500 ml

              • 1500 ml – urine*
              • 100 ml – feces
              • 900 ml – insensible loss (skin, lungs)

              * Minimal volume of urine a healthy person needs to produce is 0.5 – 1 ml/kg/hr

               

              Maintenance fluid requirement for healthy nil-by-mouth patient:

              • 4 ml/kg/hr for first 10 kg of patient’s weight
              • 2 ml/kg/hr for second 10 kg of patient’s weight
              • 1 ml/kg/hr for every kg after that
              • + 100 mM sodium and 60 mM potassium

              For example:

               A 70 kg man will require 2650 ml fluid per day: 40 + 20 + 50 = 110 ml/hr --> 2640 ml/day + 100 mM Na+ and 60 mM K+

               A 40 kg woman will require 2160 ml fluid per day: 40 + 20 + 20 = 80 ml/hr --> 1920 ml/day + 100 mM Na+ and 60 mM K+

               

              Daily fluid requirements increase in illness:

              • Fever (500 ml/day for every degree above 37oC)
              • Breathlessness and tachypnoea
              • Diarrhoea and vomiting
              • Haemorrhage
              • Surgical drains, stoma and fistulae
              • Polyuria
              • Third space losses (pancreatitis, bowel obstruction,  and after laparotomy)
              • SIRS – capillary leak

               

              Fluid requirements in resuscitation depends on stages of hypovolemic shock:

              Stage 1 (< 15% or <750ml loss):  Normal blood pressure as compensated by increased systemic vascular resistance --> give Crystalloid

              Stage 2 (15-30% or 750-1500ml):  Tachycardia, postural hypotension, +/- sweating and anxiety – partially compensated by increased systemic vascular resistance --> give Colloid

              Stage 3 (30-40% or 1500-2000ml):  Systolic blood pressure <100 mmHg, tachycardia, tachypnoea, altered mental state (confusion) --> give Colloid + Blood

              Stage 4 (>40% or >2000ml):  Very low blood pressure, bradycardia, weak pulse pressure, depressed mental state, urine output negligible --> give Colloid + Blood

                 

                Fluid overload risk is high in:

                • Cardiac failure patients
                • Chronic renal failure patients
                • Elderly patients

                 

                Post-operation fluid maintenance

                • Aim for urine output of >30ml/hr
                • Maintenance fluid regime (see above)
                • Avoid potassium in the first 24-48 hr post-op
                • Encourage patient to start oral fluids as soon as possible
                • Account for extra fluid losses (drains, fever, etc.)

                 

                   

                  Intravenous fluids

                  Crystalloids are water with electrolytes, which form a true solution and are able to pass through a semipermeable membrane. Crystalloids are lost rapidly from intravascular space into interstitial space (depending on the osmolality), and they remain in extracellular compartment for about 45 minutes. Therefore they require larger volumes than colloids for fluid resuscitation. Eventually water from crystalloids diffuse through intracellular fluid as well (membrane pumps and metabolism alter crystalloid distribution and osmotic forces)

                  • Normal saline is an isotonic solution of 150 mmol/L NaCl
                  • 5% dextrose acts as free water and is hypotonic – distributes to ECF and ICF rapidly
                  • Hartmann’s solution is an isotonic physiological solution with electrolyte composition similar to ECF

                  Advantages:

                  • Cheap and readily available
                  • Safe and free of side effects (if used correctly)

                  Disadventages:

                  • Higher volumes needed to restore intravascular volume
                  • Hartmann’s solution is contraindicated in diabetes mellitus (gluconeogenic), renal failure (risk of hyperkalemia) and liver failure (risk of lactic acidosis)

                  Colloids: Water with large molecules or microscopic particles, which are dispersed through water and do not form a true solution. These particles do not pass through semipermeable membranes. Therefore colloid solutions remain in intravascular space longer depending on molecular weight of colloids.

                  Natural:

                  • Packed red blood cells increase hematocrit by 3-4% and hemoglobin by 1g/dL per 1 unit infused. They efficiently restore intravascular volume in hemorrhage and oxygen carrying capacity.
                  • Human albumin solution with 4.5% albumin reflects normal plasma. It is used in patients with low albumin, such as burn and chronic liver failure patients.

                  Synthetic:

                  • Haemaccel and Gelofusine are water solutions with gelatine (degradation product of animal collagen) and electrolytes
                  • Hydroxyethylated starch (HES) contains different sized hydroxyethylated polysaccharide, which amylase cannot breakdown. Its volume expanding effect is long and depends on molecular size.

                  Advantages:

                  • Restore intravascular volume fast
                  • Improve oxygen carrying capacity (packed red blood cells only)

                  Disadvantages:

                  • High costs and limited supply of natural colloids
                  • Allergic reactions to gelatine
                  • HES cannot be used to replaced >30% of loss due to renal failure risk
                  • Risk of transmitted infections
                  • Risk of haemolytic reaction to unmatched blood

                   

                  Case scenarios

                  • 25-year-old goalkeeper fell on left side of his chest reaching for the ball. Now his ribs hurt a lot. He is alert with pulse of 120 beats per minute, blood pressure of 150/100 mmHg, respiratory rate 24 per minute and apyrexial. He is clammy to touch. What is the ideal initial fluid management for him?

                   

                  This man most likely has injured his spleen and losing blood into abdominal cavity. He is probably in stage II hypovolemic shock and can progress to stage III. Sympathetic response to bleeding causes hypertension, tachycardia, tachypnea and sweating. He needs colloids (Gelofusine) to replace intravascular fluid and achieve adequate hemodynamic response for a long period. Adequate amount of colloids can be determined by series of fluid challenges. Crystalloids (0.9% saline or 5% dextrose) can later be added to further increase fluid input.

                   

                   

                  • 60 year old woman with intestinal obstruction has the following fluid balance chart:

                  - Intake: nil by mouth, 3000 ml IV normal saline

                  - Output: 500 ml urine, 3500 ml from nasogastric tube

                  Last blood results show low potassium. She weighs 60 kg, is apyrexial and has normal BP and heart rate. She complains of a dry mouth. Which fluids are required for the next day before surgery?

                   

                  First we need to calculate her current water deficit. Her total fluid output is 5000 ml (another 1 L from insensible losses) and total input is 3000 ml. This leaves a deficit of 2000ml. For the next 24 hours she needs IV fluids to replace this 2000 ml on top of her normal needs.

                  Her normal daily fluid requirement is 2400 ml (calculated using her weight: (4*10+2*10+1*40)*24ml). Since she is nil-by-mouth, her only water input is ~250 ml metabolic water. Adding all her requirements, she needs minimum of 4000 ml fluids(2000+2400-250). Replacement fluids for anticipated losses from nasogastric tube can be prescribed as well, but caution must be taken to avoid overloading. Possible regimes are:

                  -       4 L Hartmann’s solution over 24 hours

                  -       2 L 5% dextrose (replace deficit) and 2 L Hartmann’s solution over 24 hours

                  In this scenario preferring Hartmann’s solution over 0.9% saline is important to correct hypokalemia. Blood tests should be repeated to re-check electrolyte levels.

                   

                  Electrolyte composition of body fluids

                  This is a reference section for those interested in numbers

                   

                  Electrolyte composition of IV fluids

                  References:

                  1.         McLatchie GR, Borley NR, Chikwe J, Ovid Technologies Inc. Oxford handbook of clinical surgery. Oxford ; New York: Oxford University Press; 2007. 

                  2.         Cooper N, Forrest K, Cramp P. Essential guide to acute care. 2nd ed. Malden, Mass.: Blackwell BMJ Books; 2006.

                  3.         Kumar PJ, Clark ML. Kumar & Clark's clinical medicine. 7th ed. Edinburgh ; New York: Saunders Elsevier; 2009.

                  4.         Goldberg A, Stansby G. Surgical talk : revision in surgery. 2nd ed. London, Singapore ; Hackensack, NJ: Imperial College Press ; Distributed by World Scientific; 2005.

                   

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