Page 788 - Clinical Small Animal Internal Medicine

756 Section 8 Neurologic Disease

Oxygen Therapy and Management of Ventilation Diuretics
VetBooks.ir following head trauma. Control of PaO 2 and PaCO 2 is with the administration of osmotic diuretics. Osmotic
Intracranial pressure can be aggressively addressed
Oxygen supplementation is recommended in all patients
diuretics such as mannitol should not be given to any
mandatory and will affect both cerebral hemodynam-
ics and ICP. Permissive hypercapnia should be avoided patient without being certain that the patient has been
because of its cerebral vasodilatory effect that increases volume resuscitated. If not, their use can precipitate
ICP. Hypocapnia can produce cerebral vasoconstriction acute renal failure. For this reason, they are reserved as
through serum and CSF alkalosis. Reduction in CBF tier 2 therapies.
and ICP is almost immediate although peak ICP reduc- Mannitol improves CBF and reduces ICP by decreasing
tion may take up to 30 minutes after PCO 2 has been edema. After administration, mannitol expands the plasma
changed. volume and reduces blood viscosity, which improves CBF
The amount of oxygen within the blood can be assessed and delivery of oxygen to the brain. Additionally, mannitol
by measuring oxyhemoglobin saturation with a pulse assists in scavenging free radicals, which contribute to sec-
oximeter (SpO 2 ), measuring the PaO 2 with blood gas ondary injury processes. Vasoconstriction occurs as a
analysis in conjunction with measurement of circulating sequela to the increased partial pressure of oxygen, leading
haemoglobin concentration. Calculation of oxygen deliv- to an immediate decrease in ICP. Additionally, the osmotic
ery to the tissues requires measurement of both arterial effect of mannitol reduces extracellular fluid volume within
oxygen content and cardiac output. Measurement of the brain.
mixed venous oxygen can provide an indirect measure of Mannitol (0.5–2.0 g/kg) should be given as a bolus over
adequacy of oxygen supply to the tissues. The amount of 15 minutes to optimize the plasma‐expanding effect.
carbon dioxide within the blood can also be assessed by Continuous infusions of mannitol increase the permea-
arterial blood gas analysis as well as via capnography. bility of the blood–brain barrier, exacerbating edema.
Capnography provides breath‐by‐breath assessment of Lower doses of mannitol are as effective at decreasing
adequacy of ventilation assuming normal cardiovascular ICP as higher doses, but may not last as long. Mannitol
function. This technique measures CO 2 in the expired reduces brain edema over about 15–30 minutes after
patient gases (P’ETCO 2 ), which approximates the CO 2 administration and has an effect for approximately 2–5
tension in the alveoli. As alveolar gases should be in hours. Repeated dosing of mannitol can cause diuresis,
equilibrium with arterial blood, P’ETCO 2 can be used to leading to reduced plasma volume, increased osmolarity,
approximate PaCO 2 unless severe pulmonary dysfunc- intracellular dehydration, hypotension, and ischemia.
tion is present. Therefore, adequate isotonic crystalloid and colloid
The goal of oxygen therapy and management of ven- therapy is critical to maintain hydration. Additionally,
tilation is to maintain the partial pressure of oxygen in administration of mannitol should be reserved for the
the arterial blood supply (PaO 2 ) greater than or equal to critical patients (MGCS of <8), a deteriorating patient,
90 mmHg and the PaCO 2 below 35–40 mmHg. If the or a patient failing to respond to other treatment.
patient is able to ventilate spontaneously and effectively, Administration of furosemide (0.7 mg/kg) prior to
supplemental oxygen should be delivered via “flow‐by”; administration of mannitol has a synergistic effect at
confinement within an oxygen cage prevents frequent lowering ICP. Currently, there is no evidence to indicate
monitoring. Facemasks and nasal catheters should be that mannitol is contraindicated in the presence of
avoided if possible as they can cause anxiety which may intracranial hemorrhage, as has been suggested.
contribute to elevations of intracranial pressure.
Patients with severe head injury require mechanical Seizure Therapy
ventilation to maintain these arterial blood gas concen- Seizures should be aggressively treated to prevent
trations at their optimal levels. The absolute indications worsening of the secondary effects in the brain paren-
for mechanical ventilation include loss of consciousness, chyma due to associated brain hypoxia and subsequent
rising PaCO 2 of >50 mmHg and falling SPO 2 despite development of edema. Seizure activity may occur
appropriate treatment. immediately following trauma or may be delayed in
There are no contraindications to the use of positive onset. The need for antiseizure prophylaxis after severe
end‐expiratory pressure (PEEP) in hypoxemic patients. brain trauma remains controversial in human medi-
With adequate volume resuscitation, PEEP does not cine. Human patients treated in the first seven days
increase ICP nor does it lower CPP, and it may actually after head trauma with anticonvulsants have a signifi-
decrease ICP as a result of improved cerebral oxygena- cantly lower risk of posttraumatic seizures within this
tion. Assessment of adequacy of ventilation can be made time period than if not treated. Beyond seven days
by measurement of arterial CO 2 or alternatively using from injury, there appears to be no benefit to prophy-
capnography. lactic treatment.