Fluid Replacement in AKI

Assessment

1. Rapid restoration of normal fluid volume status is one of the cornerstones of management in AKI
2. Assessment of fluid volume status is often trickier than it looks
3. No single clinical sign is sufficiently sensitive and specific to confirm or exclude hypovolaemia
4. Clinical assessment must be interpreted in context of history and relevant lab results

Are They Dry?

1. Dehydration common in AKI
2. The most useful signs are said to be postural pulse increment >30bt/min, postural fall SBP >20mmHg, dry axillae and response to fluid challenge (asterisked above)
3. Safe to assume dry in AKI if no oedema, no increase JVP, no dyspnoea, no basal creps and normal SpO₂
4. Other clues to dryness include urea:creatinine ratio >100:1 (normal range is 60-80:1)
5. Urine/plasma osmolality >1.1 and urine Na<20 mmol/l also suggest dryness though these tests are rarely necessary
6. CVP measurement does not have a role in routine assessment of volume status

Which Fluid And How Much?

1. Choice and prescription of IV fluid should be guided by daily assessment of water and electrolyte requirements
2. Remember that daily requirement = deficit + ongoing losses + maintenance
3. It can be difficult to quantify fluid defecits and ongoing losses, which means that estimating the volume of replacement fluid required is often no more than an intelligent guess
4. It is also possible to overload a patient with too much fluid after the initial resus phase has been completed
5. Crystalloid solutions are preferred to colloid.
6. Balanced salt solutions eg Hartmann’s should be the standard IV fluids for correction of hypovolaemia.  Saline is alternative but be wary of using large doses (30ml/kg/h) due to risk of hyperchloraemic acidosis.
7. Dextrose 5% should not be used as replacement fluid as only a small amount stays within the intravascular compartment
8. Consider 1 litre over 1 hour, 1 litre over 2 hours, 1 litre over 4 hours if patient dry but halve that rate if patient old and frail or if there is a history of heart failure
9. Alternatively R_x 250-500ml boluses 0.9% saline repeated at 5-10 minute intervals until JVP becomes visible (continuous monitoring mandatory)
10. NB 125ml/hour is never enough to rehydrate a dry patient.  One third of a can of coke per hour never rehydrated anyone!

Are They Wet?

1. Wet usually means pulmonary oedema though could mean massive leg oedema or ascites.
2. Wet less common than dry in AKI and more difficult to manage
3. Causes include LV systolic dysfunction, pericardial effusion, bilateral renovascular disease (an important cause of renal failure, heart failure and hypertension), nephrotic syndrome and hepatorenal failure
4. Investigations should include ECG and urgent echo in most cases, also urinalysis for protein to exclude nephrotic syndrome

What To Do If Wet

1. R_x high flow O₂ to maintain SpO₂ >92%
2. Morphine 2.5-5mg IV boluses if pulmonary oedema. NB – opiate is excreted by kidney
3. R_x trial of frusemide 100mg IV with salt and water restriction if cardiac failure predominates, recognising that worsening kidney function may be the price you have to pay in order to keep lungs free of fluid
4. R_x cautious trial of fluid if renal failure predominates, recognising that peripheral oedema may be an acceptable compromise in the trade off between heart and kidneys.
5. Avoid temptation to transfuse for anaemia if overloaded, until you have established that patients can pass urine
6. Patient more likely to require renal replacement therapy early if wet, therefore early renal referral advised.

Links

• RCP Acute Care Toolkit 12 – AKI & IV FLuid Therapy, Sep 2015
• UK Renal Association Clinical Practice Guideline – AKI, Nov 2011
• NICE Algorithms for IV Fluid Therapy in Adults

Content updated by Chris Isles