MOST of us will be comfortable
with choosing a particular fluid type
for a particular situation and opting
for a rate and route of
administration which we have been
using for many years.
Advances in the field of fluid
therapy have been less concerned with
new and exciting treatment options
(although the availability
of fluid pumps and
syringe drivers has
allowed for increased
safety during
administration and the
advent of pet blood
banks has made fluid
therapy with blood
products less daunting),
but rather focus on
moving away from the
use of a “recipe” and
towards adopting a less
prescriptive and more
flexible approach.
The choice of
fluids, rates and volumes
is the subject of much debate and there
is no such thing as an “ideal” fluid type
for all situations.
Amanda Boag, in her lecture at
London Vet Show in November, urged
vets to be more thoughtful in their
approach to fluid therapy, carefully
considering why the fluid is being
administered, what outcome they are
looking for and then looking at the pros
and cons of each fluid type before
choosing one to try to achieve the
desired outcome.
Not every RTA will need fluids;
colloids aren’t always the best first
choice for hypovolaemic shock. Every
patient should be individually assessed
and the benefits v. risks weighed up for
each situation.
Vital work
The two main indications for fluid
therapy are hypovolaemic shock and
dehydration. The former represents
acute, rapid loss of fluid from the
circulating blood volume. Since the
intravascular volume represents only
approximately 12% of total body water
and, in essence, it is this portion which
is doing the most vital work, loss of
fluid from this compartment has a
much more profound effect than the
gradual, “global” loss of fluid seen in
dehydration.
In the latter, fluids are lost from the
intravascular, interstitial and intracellular
spaces and there has been time for
compensation between compartments.
Hypovolaemic shock can result from
direct loss of blood through
haemorrhage (internal or external),
severe acute GI loss (fluid is first drawn from the intravascular space), loss
through the kidneys or into a third
space such as the peritoneal cavity.
Careful clinical assessment will help
distinguish between hypovolaemic shock
and dehydration. Shock can be evaluated
by looking at the perfusion parameters
listed in Table 1, and it is recommended
that these should be reassessed every 10-15 minutes in
patients undergoing
fluid resuscitation.
Other diagnostics
can lend support such
as blood pressure
measurement
(although this
decreases only in the
later stages of shock)
or blood lactate
measurement, which
gives an objective
measurement of
shock and is now a
tool which can be
used in general practice with the
availability of in-house lactate machines.
There is no one ideal fluid choice
for treatment of hypovolaemic shock.
The various fluid types should be
assessed with respect to pros and cons
and the best fit chosen.
In her lecture, Ms Boag
recommended that treatment be given
by way of “bolus” doses (i.e. a ml/kg
dose given over a specified time frame).
Assessment should be made after each
dose to determine whether the patient is
becoming more stable and the treatment
plan re-evaluated each time.
If cardiovascular stability is achieved
then it may be possible to switch the
patient to a slower, longer-term fluid
rate. Clinical judgement should be used
in every case and consideration given to
concurrent disease; the suggested “full
shock” bolus doses of isotonic
crystalloid solution (e.g. 0.9% NaCl;
Hartmann’s) of 60-90ml/kg for a dog
and 40-60ml/kg for a cat are rarely
needed.
A dog presenting with GDV in
severe shock is one such example where
it could be indicated, however, and a
bolus of 90ml/kg could be considered
and administered as quickly as possible.
The dose for other cases can be tailored
according to the clinical need, e.g. a dog
with mild-moderate hypovolaemia may
require 25% of the full shock dose (i.e.
20ml/kg) over 30 minutes.
If using hypertonic saline (7.2-7.5%
NaCl), a much lower dose is suggested
(4-7ml/kg in the dog and 2-4ml/kg in
the cat given over 5-10 minutes),
followed by other fluids such as isotonic
crystalloids over a longer period. The
smaller dose volumes used here can be advantageous, e.g. where a large patient
with severe shock is presented: using an
isotonic crystalloid solution on its own
would require a very large dose to be
needed rapidly, and this would be
difficult.
Hypertonic saline is also a preferred
fluid for the treatment of hypovolaemic
shock with concomitant traumatic brain
injury. However, care needs to be
exercised in when to use it as a fluid
choice – for example, it is
contraindicated where dehydration is
present.
Starch-based products
Colloids can also be used for volume
expansion, with suggested shock bolus
doses of 10-20ml/kg in the dog. The
starch-based products such as Voluven
and pentastarch tend to persist longer in
the intravascular space. Because of
potential side-effects though, such as
coagulopathies or peripheral oedema in
patients with SIRS (systemic
inflammatory response syndrome), they
are rarely recommended as first choice
fluid.
Blood or blood products are now
available in the UK through pet blood
banks, made possible by recent changes
in legislation. In many critical cases,
these are used following initial
resuscitation with other fluids, and the
choice of whether to use whole blood
or products such as packed red cells or
fresh frozen plasma is indicated by the
individual clinical situation.
Dehydration is assessed through
various methods, and attention should
be paid to estimating on-going losses.
Correction of dehydration is
recommended through the use of
isotonic crystalloids given over a longer
period of time – typically 24 hours,
allowing time for the fluid to re-
distribute back to all the body
compartments.
Circulatory overload is a very serious
complication of intravenous fluid
therapy and can be a result of adopting
a more prescriptive approach and not regularly re-
evaluating the
need for on-
going therapy.
Aggressive
fluid therapy should be
avoided in
patients with
heart or lung
disease, including those suspected of
pulmonary contusions, brain disease or
anuric renal failure.
A final word of caution regarding
fluids administered peri-operatively.
The CEPSAF (Confidential Enquiry
into Perioperative Small Animal
Fatalities, Brodbelt et al., 2007) found
an increased risk (four-fold)
associated with fluid administration in
cats within 48 hours of
sedation/anaesthesia.
The association is not yet fully
elucidated but it is thought to be
possibly due to excessive fluid
administration, leading to increased
cardiac preload and pulmonary
overload.
The use of fluid pumps and
syringe drivers in general practice may
help to decrease this risk, although
the study showed that few practices
used them. Dial-up giving sets are
also available at lesser cost and allow
an infusion rate per hour to be set up.
Further reading
- British Small Animal Veterinary
Association Congress 2009: Scientific
Proceedings: Shock and blood volume
replacement, pp281-283. - Scientific proceedings of London Vet
Show 2009: The truth about fluid
therapy – when the choice of product
really matters, pp93- 96. - ESFM Feline Symposium Pre-BSAVA
Congress 2009: Peri-operative morbidity
and mortality in cats. - Brodbelt, D. C. (2006) The Confidential
Enquiry into Perioperative Small Animal
Fatalities, PhD thesis, Royal Veterinary
College.