I agree - 0.9% NaCl would not cause hyponatremia. Also, we cannot compare the max fluid rate of human neonate vs an adult cat. Daily maintenance fluids for cats are about 55-60ml/Kg/24hrs (~15ml/hr). So at 60ml/hr I gave 4X the daily maintenance (this condition often causes them to be very dehydrated). The owner could keep the cat only 24hrs in the hospital. No blood work, no antibiotics, nothing else. So, I had to make this cat pee as much as possible. Again - no heart murmur, no pre-existing cardiorespiratory pathology. All the time vitals were WNL. Never had any neuro abnormality. After death not even a drop of fluid came out of his nostrils...
From the Merck Vet Manual
"When crystalloids alone are used for replacing intravascular volume causing perfusion deficits, the volume administered is determined by titrating to effect. A pretreatment blood pressure is obtained. Increments of isotonic crystalloids (10-15 mL/kg) are infused IV. The animal's perfusion parameters are assessed between boluses to determine the effect of the therapy. If the desired endpoints of resuscitation have not been met, another bolus is given (up to 90 mL/kg/hr in dogs, 40-55 mL/kg/hr in cats). Careful monitoring is required to avoid interstitial volume overload."
"Maintenance fluid therapy is meant to replace both ongoing and insensible losses. When there is no vomiting, diarrhea, fever, or loss into the third body fluid space, the average maintenance rate is 40-60 mL/kg/day. During fever, the maintenance rate increases an extra 15-20 mL/kg/day. Fluid losses through vomiting, diarrhea, the third body fluid space, and polyuria must also be estimated and replaced."
40-60 ml/kg/day is 1.7-2.5 ml/kg/hr.
6 kg is 10-15 ml/hr.
It looks like you were giving him 10 ml/kg hypotension resuscitation doses of fluid for 16 hours followed by 2.5-4x maintenance for another 12 hours.
That's seems like a significant fluid overload, and likely caused it's death.
It's maintenance rate for the 28 hours was 280-420ml. You administered 1440ml.
That's 3.4-5.1 times maintenance for 28 hours.
A better approach would be to give a 10-20 cc/kg bolus if it seemed unstable or dehydrated.
Start maintenance at 2 cc/kg/hr.
Calculate the urine output hourly and replace it 1:1 over the next hour (in addition to the 2cc/kg maintenance).
Monitor the HR and BP frequently (Q15 or 30), and if it was looking hypovolemic or unstable based on vital signs, just give another 10cc/kg bolus.
That's what I would have done in a human in the ICU. (and some blood work that you could not do in this case)
I bet you would have needed less than 1/2 the fluid you administered if you used targeted fluid therapy.
Of course it could have been septic, etc. who really knows. I wouldn't lose any sleep over it. You did the best you could, but I'd recommend that you use a more goal directed fluid therapy approach in the future.
