Subarachnoid Hemmorhage And Anesthetic Consideration (PART 3) ~ Medical Review

PREMEDICATION:(1,2,3)

The use of sedative premedication is controversial as pre and postoperative neurologic assessment may be difficult.

Premedicant may also cause respiratory depression leading to hypercarbia,hypertension and increase CBF,CBV and ICP.

Generally grade III-IV patients rarely require an anxiolytic to prevent the hemodynamic fluctuation associated with anxiety. Midazolam iv can be given in the anesthetic room ,it reduces the CMRO2 and hence CBF and CBV without significantly affecting cerebral CO2 reactivity or autoregulation. It's a good idea to have midazolam in hand to give in 0,5 to 1,0 mg increment as dictated by the patient's emotional state as hypotension must be avoided

In fragile or older patients diphenhydramine 12,5 to 25 mg iv may provide very good ,rapid sedation.

Small doses of iv (morphine 1 t0 2 mg) or fentanyl (25 to 50 mcg) may be used by the anesthesiolologist to good grade patients in order to prevent rebleeding from anxiety induces surges in blood pressure.

Poor grade patients who already have depressed levels of conciousness are not candidate for preoperative sedation unless an endotracheal tube is in place and they are mechanically ventilated. In this case they may even muscle relaxant during transportation to the operating room.

The use of anticholenergic which given intravenously in the operating room preferably at least 10 minutes before induction. For non smookers is given o,2 mg of glycopyrolate iv,smookers without a notable history of bronchospasm 0,4 mg and higher doses (up to 10 mg)to bronchospastic patients depending on severity disease
accompanying coronary disease and body habitus.

It is important to note that patients should continue to
receive their calcium channel blocking drugs (nimodipine

or nicardipine),anticonvulsant drugs(phenytoin) and
steroids(dexamethasone) up to the time of operation.

Drugs that reduce gastric acidity(cimetidine,ranitidine)and volume and hasting gastric emptying (metoclopropamide) may also be given before induction of anesthesia.

MONITORING :

Hemodynamic:(1,2,3)

One or two large bore iv catheters should be inserted peripherally,in addition to the CVP or PA catheter.Iv cannulation and monitoring should be accomplished before positioning for surgery which may limit access to arteries and venous,and before intervention that will affect blood pressure,ICP,transmural pressure and CBF.

Once the patient is positioned for the operation,the arterial pressure transducer is secured at the level of the brain to reflex cerebral perfusion and moved accordingly if the patient's position is changed.

Some clinicians prefer to place the arterial catheter after induction to avoid the possibility of aneurysmal rerupture from pain and subsequent hypertension but the other clinicians strongly urge the placement of the canula in awake patient except a sensitive or uncooperative patients.

In addition to the beat to beat monitoring of the patient's blood pressure an arterial catheter will facilitate sampling for measurement arterial blood gases and serum electrolyte ,glucose,Ht and osmolarity.

Potassium may be depleted by the administration of mannitol and furesemide. Futher mannitol will increase serum osmolarity while decreasing brain water.

No further brain relaxation is achieved by additional doses of mannitol however,if the serum osmolarity exceeds 320 mosm. Making this an important parameter to measure if the brain is tight.

Serum glucose should also be maintained in normal range
(less than 200mg/dl) to avoid the hyperglycaemia exacer
bation of ischemic injury to the CNS.

CVP is measured in all patients presenting for aneurysm surgery to guide fluid management as the patients often receive repeated doses of diuretics and/or mannitol and the potential for considerable blood loss.

Central venous canulation may be accomplished after induction of anesthesia and before positioning by way of the internal jugular, subclavian but for the seated position surgery it's recomended by way medial antecubital vein.

Since CVP and left ventricular end diastolic pressure correlate poorly in SAH, pulmonary artery cathetrization (PCWP) may be necessary when patients have severe symptomatic vasospasm and require hypervolemic hypertension or have coronary artery disease,ventricular dysfunction and may need prophylactic hypertension, hypervolemic and hemodilution (Triple H).

What ever the intraoperative scenario the postoperative management of all patients relies heavily on the CVP and PCWP as indicators of iv volume. Analysis of the cycling of arterial pulse pressure waveform is at least as useful as CVP. In brief there's normal so called delta up and delta down of 4 to 5 mmHg of the arterial systolic pressure with mechanical inspiration and expiration respectively.

With volume depletion the delta up is reduced or absent and delta down magnitude is augmented;the opposite occurs with volume overload.This situation is only useful in the paralyzed mechanically ventilated patient who can be made temporarily apneic to establish a pressure base line.

Respiratory monitoring :(2)

The capnograph has become monitor for confirming tracheal intubations and providing continuous information on the state of the respiratory system through out the case.

Once a gradient between arterial and end tidal CO2 has been established the need for further arterial blood gases is diminished. The provision of hypotension to mean blood pressure of 50 to 60 mmHg does not significantly (in clinical sense) increase the gradient.

This a probably because cardiac output and therefore V/Q ratio are not so much altered by hypotension achieved with isoflurane,nitroprusside or other means that produce reductions in blood pressure primarily by decreases in systemic vascular resistance rather than decreases in cardiac output.

To optimize surgical exposure in early aneurysm surgery provide hyperventilation (PaCO2 =24 to 28 range) in spite of theoretical concerns that this might provide a fertile substrate for cerebral ischemia. In the normal brain hyperventilation probably does not produce ischemia in
blood pressure above the lower limits for autoregulation.

The use of hyperventilation is the not so normal brain may carry some risk especially in the presence of vasospasm, but it also provides an essential improvement in operating conditions which appears to justify for its use.

Neurologic monitoring :(2,3)

There's no routine intraoperative of cerebral function for these operation. Different institutions may monitor a variety of evoked potentials or selected electro encephalogram(EEG) by surface or cortical electrode but clinical role of this information is by no means certain to us.In theory these monitors should allows anesthesiologist to use drugs,blood pressure levels,and hyperventilation in certain limit that will not exacerbate ischemia.

EEG has been used determine the lowest blood pressure tolerable during induced hypotension but the results are not consistent.

This is not surprising as significant deterioration in EEG can be compatible with normal neurologic recovery.

In contrast EEG monitoring may be indicated when temporary occlusion is planned either to determine the duration of tolerance or for titration of anesthetic agents when pharmacologic metabolic suppression is desired.

Somato sensory evoked potential(SSEP) monitoring has been investigated for use during procedure on both anterior or posterior circulation aneurysm while brainstem auditory evoked potential(BAEP) monitoring has been primarily investigated for use during procedures on vertebro basillar aneurysm.

Both monitors are probably most useful when temporary or permanent occlusion is planned .Evoked potential can be recorded during barbiturate coma and are therefore the on
ly electrophysiologic monitor available when maximal pharmacologic metabolic suppression is used.

For the aneurysm in the vertebro basillar system BAEP monitoring complement the use of spontaneous breathing as a monitor of brainstem function.

The simultanous use of SSEP and BAEP monitoring during temporary or permanent occlusion provides additional enhancement in sensitivity and specificity.

Jugular bulb oxygen saturation:(3)

Cerebral matabolic rate for oxygen (CMRO2) is the product of CBF and AVDO2 (Arteriovenous oxygen content
difference) derived from blood sample obtained from arterial blood and jugular bulb blood.

This AVDO2 reflects the balance between metabolic demand and blood supply. When oxygen saturation is 100%,the AVDO2 is reflected by (1-SJO2) where SJO2 is the jugular bulb oxygen saturation.

Thus when CBF is higher than what is needed, SJO2 will increase conversely. When CBF is inadequate, SJO2 will decrease.

As hyperventilation is frquently employed during cerebral aneurysm surgery to improve surgical conditions and because excessive hyperventilation can cause cerebral ischemia,intermittent or continuous jugular bulb O2 saturation measurements may be useful in determination of the optimal level of hyperventilation.The most cause of desaturation (<50%) is hyperventilated and this occured in about 60% of the patients studied.

Severe desaturated (SJO2 <45%) was observed in 10% of the patients studied with many of these episodes occuring despite a PaCO2 > 25 mmHg.

In adequate CBF due to a low systolic blood pressue(SBP) can also lead to jugular venous desaturated,although the effects are less dramatic then those seen with hyperven
tilation.The potential utility of continous jugular venous oxymetry for optimal intra operative blood pressure management in patients with cerebral aneurysm shown by Moss and coworkers.

Transcranial doppler ultrasonography:(TCD) (3)

TCD represents the first step toward achieving the goal of non invasive continuos CBF monitoring during anesthesia. Intraoperative use of TCD is generally limited to the middle cerebral artery (MCA) via the transtemporal route.

As the MCA carries about 75 to 80% of the ipsilateralcarotid artery blood flow a change in MCA flow(Vmca) is represen
tative of the charge in hemispheric blood floow.

However,the Vmca is proportional to CBF only when the diameter of vessel remain constant.

As the basal cerebral arteries are conductance vessels, generally they do not dilate or constrict as the vascular resistance change.

CO2 tension,SBP,and inhaled anesthetics have all been shown to have negligible effects on the diameter of the MCA,although the effects of the latter are still in dispute. However the occurence of vasospasm renders this relationship invalid. Indeed the increase in flow velocity with constriction(as in vasospasm) represents one of the most important and established uses of TCD. Both low velocity and waveform changes may be useful during cerebral aneurysm surgery.The former may be useful during induced hypotension to assess the ability of the patient to autoregulate and the later can be used to diagnose or
confirm perioperative aneurysm rupture prior to opening dura. One of the reason for decline in the use of induced hypotension during clipping of cerebral aneurysm is the unpredictable cerebrovascular respons to induced hypo
tension in patients with SAH.

Continuous monitoring with TCD allows the anesthetist to determined the patient's ability to tolerate induced hypotension,as well as the effect of hyperventilation on CBF during this vulnerable period.

The validity of TCD for determination of the lower limit of autoregulation has been confirmed.TCD has been used intraoperatively to confirm and facilitate management in patient whose aneurysm reruptured shortly after induction of anesthesia.

The temperature of the brain is most closely reflected in the tympanic and nasopharyngeal temperatures.After cranial flap is turned, a thermistor may be placed on the surface of the brain to measure temperature of the brain directly.(3)

to be continued