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Maintaining an accurate fluid and electrolyte balance

Case study: Postoperactive fluid and electrolyte balance; urea and electrolytes

Author

VOL: 97, ISSUE: 23, PAGE NO: 40

Mandy Sheppard, RGN, is an independent training and development consultant

 

Arthur Foyle, 62, has had an elective repair of an abdominal aortic aneurysm. He spent one night in the postoperative recovery unit and has now transferred back to the surgical ward.

On returning to the ward, Mr Foyle is warm to the touch and appears to be well perfused. He has patient-controlled analgesia. His pulse is 85 beats per minute and regular, blood pressure 135/90mmHg, temperature 37.2°C and respiratory rate 14 breaths per minute.

He has a triple lumen central line in situ via the right internal jugular vein with the following separate infusions:

  • Lumen one dextrose saline running at 125ml/hr via a volumetric pump;
  • Lumen two dopamine running at 3µg/km/min via a syringe pump;
  • Lumen three central venous pressure (CVP) measurements are performed using a water manometer.

To avoid the potential risk of any of the lumens blocking it is preferable, whenever possible, to keep infusions running through each of the three lumens.

While in the postoperative recovery unit, Mr Foyle’s CVP was measured with a transducer that was attached to the monitor. When this method is used, the CVP is measured in units of mmHg, with normal values ranging between 0-8mmHg. When he left the unit, his CVP was 6mmHg.

The surgical ward does not have the facilities to measure the CVP with a transducer so it uses a water manometer instead. When this method is used, the CVP is measured in units of cm/H2O, with normal values ranging between 3-12cm/H2O. Once settled on the ward, Mr Foyle’s CVP is 10cm/H2O.

Observations of pulse, blood pressure and CVP are continued every two hours. Mr Foyle also has a urinary catheter. The urine output is measured each hour and so far postoperatively he has been passing 50-100ml/hr. He is nil by mouth with a nasogastric tube in situ which is on free drainage. Both of his wound drains have minimal amounts in each.

The fluid balance chart accompanying Mr Foyle on transfer is continued on the ward. A blood sample for urea, electrolytes and a full blood count was taken early in the morning before he left the postoperative recovery unit. The results have been telephoned through to the ward and are all within normal limits.

 

Postoperative fluid and electrolyte balance

There are a number of reasons why Mr Foyle’s fluid and electrolyte balance needs to be closely monitored at this stage in his postoperative recovery.


Reduced intake

Mr Foyle was nil by mouth preoperatively and remains so on his first postoperative day. He is therefore dependent on intravenous therapy to maintain his hydration. An accurate fluid balance chart is essential to keep a record of intake.

The use of a volumetric pump may help to administer constant volumes of fluid each hour. The patient, who is nil by mouth, is completely reliant on the intravenous fluid therapy to maintain hydration. Any interruptions in that therapy should be addressed as quickly as possible.

 

Fluid loss

  • Mr Foyle would have lost some fluid during the operation. This may have been the result of bleeding or evaporation from his peritoneum. The fluid balance records for the perioperative period, showing any recordable loss and fluid replacement, are an essential part of Mr Foyle’s fluid balance history, particularly if it should become necessary to undertake a full assessment of fluid status.
  • Mr Foyle’s nasogastric tube is on free drainage, which is a source of both fluid and electrolyte loss. The fluid balance chart should record this loss and the intravenous fluids infused. Daily blood tests for electrolyte levels will monitor his electrolyte balance.

In this case intravenous therapy will provide a volume of fluid to maintain Mr Foyle’s hydration and electrolyte balance while he remains nil by mouth and to replace the electrolytes currently being lost via the nasogastric drainage.

  • Mr Foyle has two wound drains that drain minimal amounts but should be accounted for on the fluid balance chart.


Specific fluid balance considerations

The type of surgery Mr Foyle had has influenced his immediate postoperative management. To apply a graft to the aneurysm site, the aorta had to be clamped for a period of time above the renal arteries. In addition, there was a period of about five minutes during the operation when Mr Foyle was hypotensive (his blood pressure fell to 70/40mmHg but responded to fluid replacement). In combination, these factors may have resulted in a severe decrease in renal perfusion which can, in some cases, progress to acute renal failure.

Many aspects of Mr Foyle ‘s postoperative management aim to prevent the development of acute renal failure or enable the early detection of any renal dysfunction.

  • An adequate intravenous fluid intake is essential. This is monitored by fluid balance chart records and any clinical measurements that may suggest dehydration, such as hypotension, cool peripheries, oliguria and tachycardia.

Additional clinical assessment tools can be used to detect dehydration and may include loss of skin turgor, dry mucous membranes or delayed capillary refill time.

In addition, Mr Foyle has a triple lumen central line, one lumen of which has been dedicated to CVP measurement. In combination with other clinical measurements, trends in CVP readings can provide a useful fluid balance indicator.

  • Another reason for the central line is the safe and effective administration of dopamine. At low doses, such as 3µg/kg/min, dopamine can be used to improve renal perfusion, particularly in situations where kidney function may be at risk. At higher doses, dopamine can exert effects on both heart rate and blood pressure, both of which are not required for Mr Foyle.

For these reasons the dose has to be accurately administered, and in this case this is achieved using a syringe pump. In addition, dopamine has a short half-life, so to maintain its clinical effect it has to be given as a continuous intravenous infusion and not as bolus doses.

  • An important indicator of renal function is urine output. Hourly measurements and recording of urine volumes are essential at this stage. A normal urine output for an adult is 0.5ml/kg/hr. So for an adult weighing 70kg, a urine output of 35ml/hr would be considered normal. Mr Foyle is passing in excess of this amount (50-100ml/hr). This may be caused by the dopamine infusion.

The electrolyte potassium can be lost via urine and a sustained urine output of this nature may cause hypokalaemia (a low plasma potassium level). To guard against this, potassium is added to his intravenous fluid regime and his plasma potassium levels are checked on a daily basis.

  • The plasma levels of urea and creatinine are two other important indicators of renal function. Both substances will rise above their normal plasma values (2.5-6.6mmol/L and 55-120µmol/L respectively) in acute renal failure so both will be monitored daily.

 

Conclusion

Any postoperative patient can be at risk of fluid and electrolyte imbalance. This risk can be heightened if the patient is nil by mouth postoperatively with additional sources of fluid and electrolyte loss. In addition, certain types of surgery or complications during surgery may increase any risk of fluid or electrolyte imbalance.

 

- The patient’s name has been changed

 

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