An man in his early 80s with a background of Type I diabetes is day 1 post above knee amputation.

You are called to see him at 5pm because of a dropping urine output. He has had no urine output at all for the past 4 hours. In the 12 hours before this, he has passed 180ml of urine in total. He weighs 63.5kg.

On examination, he is haemodynamically stable (BP 130/80, pulse 80). His mucous membranes are a little dry, but you also notice pitting ankle edema in his remaining right leg. The JVP is not visible and his chest is clear. He is a bit more drowsy than when you saw him on the ward round that morning.

You flush his catheter, but this does not make a difference.

Of note, his Hb was noted earlier to be 7.6 g/dL, and he has been receiving a unit of blood at a 3 hourly rate for the past 2 hours, with another unit due immediately afterwards. He has not been eating post operatively, and so has been put on an insulin sliding scale, receiving 1 litre of dextrose every 10 hours for the past 5 hours. Before this, he was on Hartmann’s every 10 hours since the operation.

You also note his sodium to be 116 that morning. This was down from 125 the day before, and from 135 a few days before the operation.  His eGFR was seemingly better than ever, at >90mls/min/1.73m² compared to 75 90mls/min/1.73m² prior to surgery.

Medications wise, he was normally on insulin, furosemide 40mg, and ramipril 2.5 OD. He was not on any other heart failure medications (in particular, no beta blockers) and had no documented PMH of heart failure. He had been started on paracetamol 1g QDS, gabapentin 300mg OD (due to increase the next day to 300mg BD, then 300mg TDS), diclofenac 50mg TDS and PRN oramorph.

He had also started to feel a bit nauseous for the past hour, but had not vomited anything (BM 11.1, capillary ketones 0.1).

What do you make of the fluid used for the sliding scale?

The term ‘variable rate intravenous insulin infusion’ (VRIII) should replace the ambiguous term ‘sliding scale’.

Sorry, what do you make of the fluid used for the VRIII?

5% dextrose is not recommended as the fluid of choice for a VRIII. It is associated with hyponatremia. The first line fluid for a VRIII is 0.45% saline with 5% glucose and 0.15% KCl.

Fluid	Drug added	Dose	Volume	Route	Rate
0.45% Sodium Chloride with 5% Dextrose	Potassium Chloride	20 mmol	1 litre	IV	12 hours

Huh? I don’t think my hospital even has that.

They probably don’t. It’s about three times as expensive as plain old 5% dextrose. Once there is greater appreciation of the guidelines published by Diabetes UK it should be ordered more, leading to a drop in price.

 So what do I use in the meantime?

If you can get two cannulas in, you can give 10% glucose with 0.15% potassium chloride at 60 ml/hr with a continuous VRIII in one cannula, and 0.9% saline at 60 ml/hr in the other.

Fluid	Drug added	Dose	Volume	Route	Rate
0.9% Sodium Chloride	–	–	1 litre	IV	18 hours
10% Dextrose	Potassium Chloride	20 mmol	1 litre	IV	18 hours

If you can not get two cannulas in and you have lovely nursing staff, you can switch between 10% dextrose with 0.15% KCL with a continuous VRIII if the capillary blood glucose is 14 mmol/L or less and 0.9% saline with 0.15% KCl if the capillary blood glucose is 15 mmol/L or more.  This does mean the nursing staff will have to change the bags each time the glucose level crosses 15, and fluid balance charts will be harder to accurately complete.

Fluid	Drug added	Dose	Volume	Route	Rate
0.9% Sodium Chlorideif capillary glucose 15 mmol/L or greater	Potassium Chloride	20 mmol	1 litre	IV	10 hours
10% Dextroseif capillary glucose 14 mmol/L or less	Potassium Chloride	20 mmol	1 litre	IV	10 hours

If you cannot get two cannulas in and you do not think switching the fluid bag is practical, at least give 0.18% saline with 4% glucose with 0.15% potassium chloride rather than 5% dextrose with a VRIII. This is however still associated with hyponatremia and not recommended for paediatric VRIIIs.

Fluid	Drug added	Dose	Volume	Route	Rate
0.18% Sodium Chloride with 4% Dextrose	Potassium Chloride	20mmol	1 litre	IV	10 hours

When exactly do surgical patients go on a VRIII? 

In general, when diabetic patients who have insulin requirements miss more than one meal they will go on a VRIII. The threshold for starting a VRIII is reduced when the patient is dependent on insulin (Type I and severe Type II) and when the patient has poorly controlled blood glucose.

Should we stop all other insulins before starting a VRIII?

No. Stop all insulins apart from any long acting insulin analogues e.g. Lantus.

What do you make of the sudden improvement in his kidney function?

How do we know there is actually an improvement in his kidney function? eGFR is calculated using the modified MDRD equation. The MDRD equation takes into account age, sex and whether or not the patient is black. These variables are used because they relate to total muscle mass, which in turns determines serum creatinine. If you visit renal.org’s guidance on the eGFR, you need to be cautious about interpreting it in patients of extreme body types. This is because the total muscle mass of these people may be different to what you would expect for their age/sex/race. Renal.org specifically cautions about using eGFR in amputees.

This patient could very well be going into AKI despite a reduction in his creatinine.

Why was he hyponatremic?

The causes of hyponatremia can be split into hypovolemic, euvolemic and hypervolemic. There’s another blog post on hyponatremia.

The release of ADH is either appropriate or inappropriate. It is appropriate when the serum osmolality is high, and when the effective circulating volume is low. If these conditions are not met, then ADH secretion is inappropriate.

Any cause of a decreased effective circulating volume eventually stimulates ADH. This is appropriate from the point of view of the circulation, but a bit dumb from the point of view of the overall total body fluid and its distribution. However, the stretch receptors in arteries and the heart don’t know the body’s total fluid status. This is why fluid overload states, like heart failure, cirrhosis and renal failure which have a reduced effective circulating volume but increased total body fluid, will tend to become hyponatremic.

It makes sense then to treat this with fluid restriction. However, there was a tailing urine output. Would you really fluid restrict someone with a gradually decreasing urine output?

I did an ABG to get a quick recent Na level, as well as look at his pH and potassium in case this really was AKI.

pH 7.44

pCO2 5.4 kPa

pO2 12.1 kPa on room air

BE +3.4

Na 114 mmol/L

K 4.5 mmol/L

Lactate 1.3 mmol/L

There were no neurological signs of hyponatremia. I discussed this case with the renal registrar, who said:

“It’s probably dilutional hyponatremia. Give him the second unit of blood. What fluid were you using on the sliding scale? Watch carefully for signs of overload. Give 40mg furosemide. Treat any low blood glucose with non fluids if possible. I know the patient is hyponatremic, but furosemide will probably raise the sodium in this case. I know the urine output is dropping, but post operatively this can happen without being in AKI. Don’t use the urine output to guide fluid input at the moment.”