The resting CBF
averages about 225 mL/min, which is 4-5% of the total
cardiac output in normal adults. The CBF increases three- to
fourfold to supply the extra nutrients needed by the heart at maximum exercise
level. The CBF is determined by the pressure gradient between the aorta and the
ventricles. There are phasic changes in CBF during
systole and diastole in the left ventricle.
·
Myocardial
ischemia occurs when coronary blood flow is inadequate to meet the
needs of the myocardium. The main coronary artery epicardial
branches have lumens that are 2-4 mm
in diameter. In the absence of collaterals, exertional
angina occurs when the lumen area is reduced to 1 mm2 (50-60%
reduction in diameter or 75% reduction in cross-sectional area) and angina at rest occurs when the lumen area is reduced
to 0.65 mm2 (75% reduction in diameter or 90% reduction in
cross-sectional area). Most of the sclerotic lesions are eccentrically located
so the remainder of the arterial wall is responsive to vasoactive
stimuli and is capable of contraction. Therefore, the severity of the stenosis is dynamic and influenced by the vasomotor
activity of the free arterial wall.
Nonstenotic
causes of myocardial ischemia include aortic valve disease, left
ventricular hypertrophy, ostial occlusion, coronary
embolism, coronary arteritis, and vasospasm.
·
The ability to climb two to three flights of
stairs without significant symptoms (angina, dyspnea,
syncope) is usually an indication of adequate cardiac reserve > 4 metabolic
equivalents (METs) exercise capacity (1 MET
equals 3.6 mL/kg/min oxygen consumption at rest).
·
Clinical
predictors for perioperative myocardial infarction
include the following:
Major predictors include
severe or unstable coronary syndromes, decompensated
CHF, significant arrhythmias, and severe valvular
disease.
Intermediate clinical
predictors include mild angina pectoris, prior MI, compensated or prior
CHF, and diabetes mellitus.
Minor clinical predictors
include advanced age, abnormal ECG (LVH, LBBB, nonspecific ST-T abnormalities),
rhythm other than sinus, low functional capacity, history of stroke, and
uncontrolled systemic hypertension.
·
A pressure-volume
loop plots left ventricular pressure against volume through one
complete cardiac cycle. Each valvular lesion has a
unique profile that suggests compensatory physiologic changes by the left
ventricle.
·
VALVULAR HEART
DISEASE
- Hemodynamic goals in the patient with aortic stenosis include maintaining intravascular volume, contractility, peripheral vascular resistance, and sinus rhythm while avoiding extremes in heart rate. Arrhythmias associated with hypotension require emergent cardioversion.
- Hemodynamic goals in the patient with aortic insufficiency include augmenting preload, supporting heart rate, maintaining contractility, and afterload reduction.
- Hemodynamic goals in the patient with mitral stenosis include maintaining intravascular volume, sinus rhythm, a slower heart rate, and afterload. Avoid hypoxemia, hypercarbia, and acidosis because they may increase pulmonary vascular resistance. Sedative medications should be given with great care.
- Hemodynamic goals in the patient with mitral regurgitation include maintaining intravascular volume, contractility, and an elevated heart rate while reducing afterload. As in mitral stenosis, avoid situations that will increase pulmonary vascular resistance.
·
INTRACRANIAL AND
CEREBROVASCULAR DISEASE
- Atherosclerosis at the bifurcation of the common carotid artery is the source of most cerebral ischemic events.
- Cerebral autoregulation usually maintains cerebral blood flow relatively constant over a wide range of arterial pressures. It is critically important to maintain the blood pressure of the carotid endarterectomy patient because they have minimal or no autoregulatory reserve to counter anesthetic-induced reductions in blood pressure.
- Normal cerebral vessels are highly sensitive to arterial carbon dioxide partial pressure, dilating in response to hypercapnia and constricting in response to hypocapnia. However, in ischemic and already maximally vasodilated areas of the brain, this relationship breaks down, and responses to hypercapnia and hypocapnia may be paradoxic.
- In the normal brain, cerebral blood flow varies directly with the cerebral metabolic rate. Inhalational agents are said to "uncouple" this relationship in that they decrease the cerebral metabolic rate while concurrently dilating cerebral blood vessels and increasing cerebral blood flow.
- No particular anesthetic technique for CEA has been shown to improve outcome.
- None of the methods of monitoring cerebral blood flow during CEA has been demonstrated to improve outcome, and none has gained widespread acceptance as the monitor of choice.
- Postoperative complications of CEA include blood pressure instability, potential for airway obstruction, cerebral hyperperfusion, and stroke.
·
Traditionally,
vasospasm of SAH has been treated with hypertensive hypervolemic hemodilution (HHH).
·
The phenomenon of cerebral edema is also
called "autoregulation breakthrough." It is commonly seen
following AVM resection or embolization.
With large AVMs, the high-flow, low-resistance shunt can lead to underperfusion of adjacent brain tissue so the vessels
supplying the underperfused region of brain lose the
ability to autoregulate. Once the shunt is excised,
all of the blood flow is diverted to the previously marginally perfused tissues and the maximally dilated vessels are
unable to vasoconstrict. This leads to the potential
of cerebral edema, hyperperfusion, and hemorrhage
into surrounding areas. The precise mechanism of how and why this occurs is not
clear. Neurologic dysfunction following such episodes is a major cause of
morbidity and mortality following AVM surgery. Treatment modalities of hyperperfusion include hyperventilation, osmotic diuresis (mannitol), head-up
positioning, cautious use of deliberate hypotension,
barbiturate coma, and moderate hypothermia.
·
New therapies for
treating asthmatic patients in bronchospasm:
Magnesium
sulfate: has been administered to patients in status asthmaticus. Hypothetically magnesium interferes with
calcium-mediated smooth muscle contraction and decreases acetylcholine release
at the neuromuscular junction. Magnesium reduces histamine- and methacholine-induced bronchospasm
in controlled studies, but so far clinical studies have failed to show a
significant response.
Heliox: a blend of helium and oxygen
that decreases airway resistance, peak airway pressures, and PaCO2 levels when administered
to spontaneously and mechanically ventilated patients. The mixture contains
60-80% helium and 20-40% oxygen and is less dense than air. The decrease in
density allows less turbulent flow and significant declines in resistance to
flow. The device for heliox administration in intubated patients is cumbersome unless the anesthesia
machine is already equipped.
The
Lita-Tube endotracheal
tube allows intraoperative instillation of lidocaine at and below the cords of the intubated
patient. This technique decreases airway stimulation from the endotracheal tube and may prevent reflex bronchospasm.
·
ASPIRATION
1. For
elective procedures, the most current fasting guidelines are as follows:
Clear liquids (water,
clear juices): 2 hours
Nonclear liquids (Jell-O, breast milk): 4 hours
Light meal or snack
(crackers, toast, liquid): 6 hours
Full meal (fat
containing, meat): 8 hours
2. Numerous
patient subgroups are at increased risk of aspiration, including patients
presenting for emergency surgery, those having had a recent meal, those with
bowel obstruction or delayed gastric emptying, the obese, trauma or pregnant
patients, those having pain or being treated with opioids,
and those who cannot protect the airway, such as patients with a depressed
level of consciousness or neuromuscular disease.
3. Such
patients may require prophylaxis to decrease the severity of aspiration, should
it occur, and medications valuable for decreasing the acidity of gastric
secretions include nonparticulate antacids, H2
blockers, and proton pump inhibitors, given at an interval prior to surgery
appropriate to their onset of action. Patients with bowel obstruction should
receive gastric decompression prior to anesthetic induction.
4. Regional
anesthetics are ideal for patients at risk for aspiration if appropriate. A
rapid sequence induction with cricoid pressure is the
technique of choice when general anesthesia is required in patients with
manageable airways. Awake intubation may
be necessary in patients with difficult airways.
5. Should
aspiration occur, the treatment is mostly supportive.
Antibiotics should be given if aspiration with gram-negative or anaerobic
organisms is suspected (e.g., bowel obstruction).
·
SMOKING
CESSATION:
Cessation for 48 hours prior to surgery decreases carboxyhemoglobin levels. The oxyhemoglobin
dissociation curve shifts to the right, allowing increased tissue oxygen
availability.
Cessation for 4-6 weeks before surgery has been shown to
decrease the incidence of postoperative pulmonary complications.
Cessation for 2-3 months before surgery results in all the
above benefits plus improved ciliary function,
improved pulmonary mechanics, and reduced sputum production.
·
Air trapping is
known as auto-PEEP (positive end-expiratory pressure) and results
from "stacking" of breaths when full exhalation is not allowed to
occur. Auto-PEEP results in impairment of oxygenation and ventilation as well
as hemodynamic compromise by decreasing preload and increasing pulmonary
vascular resistance. Increasing expiratory time reduces the likelihood of
auto-PEEP. This can be accomplished by increasing the expiratory phase of
ventilation and decreasing the respiratory rate.
·
NECESSARY
CRITERIA FOR ALI/ARDS
- Acute onset.
- PaO2/Fio2 ratio = 300 for ALI.
- PaO2/Fio2 ratio = 200 for ARDS.
- Chest radiograph with diffuse infiltrates.
- Pulmonary capillary wedge pressure = 18 mmHg.
- CAUSES AND TREATMENT OF ARDS
- Historically, sepsis has been identified as the most common risk factor for ARDS. Now Pneumonia ??
- VALI is thought to be caused by two mechanisms:
Overdistention
of normal aerated lung by using high tidal volumes.
Lung collapse
that occurs as a result of ventilating the lungs with low end-expiratory
volumes and pressures.
- Mechanical ventilation settings for patients with ARDS or ALI include tidal volume at 6-8 mL/kg of ideal body weight and limiting plateau pressures to < 30 cm H2O.
- PEEP should be adjusted to prevent end-expiratory collapse.
- FiO2 should be adjusted to maintain oxygen saturations between 88% and 92%.
·
In general, the potential of an inhalational
anesthetic agent to induce immune complexe hepatitis is related to the extent of
metabolism. Generally, the degree of metabolism of agents is halothane > sevoflurane >
enflurane > isoflurane
> desflurane.
·
There is now
considerable evidence that dopamine is not renoprotective
(that is, improves renal perfusion), nor does it improve splanchnic
perfusion. Two recent meta-analyses determined that low-dose
dopamine did not prevent mortality or acute renal failure, result in
improvement in serum creatinine, or decrease the need
for dialysis. Dopamine redistributes renal blood flow to the renal cortex,
putting the renal medulla at risk for hypoperfusion
and acute renal failure. While dopamine is natriuretic
and can increase urine output, this may be deceiving because overall renal
function may be deteriorating, especially (but not exclusively) in hypovolemic patients. Dopamine also suppresses anterior
pituitary hormonal function and blunts both hypercarbic
and hypoxic ventilatory drive, increasing the risk of
ventilator dependency.
·
AGENTS TO AVOID
IN THE SETTING OF ELEVATED ICP
1.
Ketamine
2.
Etomidate
3.
Nitrous oxide
4.
Hypotonic or glucose-containing intravascular
fluid
·
Masseter muscle rigidity (MMR) is defined as jaw
muscle tightness with limb muscle flaccidity following a dose of succinylcholine. There is a spectrum of masseter
response, from a tight jaw to a rigid jaw to severe spasticity, or trismus, otherwise described as "jaws of steel." Of concern, the mouth
cannot be opened sufficiently to intubate the
patient. If jaws of steel are present the incidence of MH susceptibility is
increased. There is some controversy as to the management of patients
experiencing MMR. Most pediatric anesthesiologists agree that if trismus occurs the triggering agent should be halted along
with the surgical procedure if feasible. The patient should be admitted to the
hospital for 24 hours of close observation. Creatine kinase levels should be followed every 6 hours. Creatine kinase
levels greater than 20,000 have a 95% predictive value that the patient is MH
susceptible.
·
Dantrolene pretreatment is no longer indicated providing
a nontriggering agent and appropriate monitoring are used and an adequate supply of dantrolene
is available. Dantrolene pretreatment may cause mild
weakness in normal patients and significant weakness in patients with muscle
disorders. MH-susceptible patients with an uncomplicated intraoperative
course should be monitored for at least 4 hours postoperatively.
·
Only two
disorders are clearly associated with a risk of MH:
Central
core disease is a channelopathy that
presents in infancy and is characterized by generalized muscle weakness. It is
generally not debilitating and is autosomal dominant
in inheritance.
King-Denborough syndrome is a very rare disorder
characterized by myopathy, short stature, pigeon
breasts, high forehead, and low-set ears.
Disorders with less
convincing evidence of an association with MH include
hypokalemic and hyperkalemic
periodic paralysis, Charcot-Marie-Tooth disease, Smith-Lemli-Opitz syndrome, strabismus, sudden infant death syndrome,
Hurler's syndrome, familial hereditary fever, Angelman's
syndrome, and dermatomyositis.
·
Succinylcholine must be avoided in children with MD and
should be avoided except in airway emergencies in young males.
·
Patients with MG
are resistant to succinylcholine. However,
the degree of resistance does not appear to be of great clinical significance,
and increasing the dose of succinylcholine to 2 mg/kg
results in satisfactory intubating conditions.
Myasthenic
patients are more sensitive than nonmyasthenic
persons to nondepolarizing relaxants.
Dosing nondepolarizing relaxants should start at
about one-tenth the usual recommended doses. Recovery time for these reduced
doses is quite variable but may be quite prolonged. Relaxation should be
reversed at case conclusion and the patient carefully evaluated for return of
strength.
·
Regional
anesthesia may be beneficial in the patient with MS because of a
decreased stress response to surgery. Epidural
block may be safer than spinal block because the local anesthetic
concentration at the spinal cord is lower than following spinal block.
In patients with multiple
sclerosis spinal anesthesia should be used with caution and only in situations
where the benefits of spinal anesthesia over general anesthesia are clear.
·
CONCERNS IN
PATIENTS TAKING COCAINE
- Myocardial ischemia is not uncommon in cocaine-abusing patients, and selective beta2 blockade should be avoided because it may cause vasoconstriction and worsen the ischemia.
- Severe hypertension and tachycardia are risks during airway management unless the patient is deeply anesthetized.
- Cocaine sensitizes the cardiovascular system to the effects of endogenous catecholamines. Ketamine and pancuronium potentiate the cardiovascular toxicity of cocaine and should be avoided.
·
CONCERNS IN
PATIENTS TAKING ECSTASY AND PCP
- Hyperthermia and cardiovascular collapse with ecstasy.
- Dissociative state, severe behavior disturbances, and enhanced sympathomimetic effects with PCP.
·
DIABETES MELLITUS
- Careful attention to glucose control prior to, during, and after surgery is important to reduce risk of infection, promote more rapid healing, avoid metabolic complications, and shorten hospital stay.
- The goal for insulin management during surgery is to maintain glucose between 120 and 200 mg/dL.
- Intraoperative glucose control in all but the shortest cases is best achieved by using a glucose-insulin intravenous infusion.
- Diabetic patients have a high incidence of coronary artery disease with an atypical or silent presentation. Maintaining perfusion pressure, controlling heart rate, continuous ECG observation, and a high index of suspicion during periods of refractory hypotension are key considerations.
- The inability to touch the palmar aspects of index fingers when palms touch (the prayer sign) can indicate a difficult oral intubation in diabetic patients.
·
THYROID AND
ADRENAL DISEASE
- Perioperatively, mild to moderate hypothyroidism is of little concern even for elective surgery. Patients with severe, symptomatic hypothyroidism should be treated preoperatively.
- Minimum alveolar concentration of volatile anesthetics is unchanged in both hypothyroid and hyperthyroid states.
- Thyroid storm may mimic malignant hyperthermia. It is detected by an increased serum T4 level, and treated initially with beta blockade followed by antithyroid therapy.
- Perioperative glucocorticoid supplementation should be considered for patients receiving exogenous steroids.
- Chronic exogenous glucocorticoid therapy should not be abruptly discontinued. Doing so may precipitate acute adrenocortical insufficiency.
·
OBESITY
- Morbidly obese patients have numerous systemic disorders, including restrictive lung disease, obstructive sleep apnea, coronary artery disease, diabetes mellitus, hypertension, cardiomegaly, pulmonary hypertension, and delayed gastric emptying, all of which increase the likelihood of anesthetic difficulties and postoperative complications.
- Obese patients may be difficult to ventilate and difficult to intubate and backup strategies should always be considered and readily available before airway management begins.
- Because of the increased mass and decreased oxygen reserves, obese patients also desaturate quickly postanesthetic induction, complicating airway management.
- Because obese patients have altered volumes of distribution of anesthetic agents, these drugs should be titrated to affect and not dosed based on body weight.
- Respiratory complications are particularly common in obese patients.
·
While the symptoms are indistinguishable from
anaphylaxis, an anaphylactoid reaction is nonimmune
mediated. Release of inflammatory mediators from mast cells and basophils results in activation of the complement cascade.
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