Systemic Diseases

         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

1. 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.
2. Hemodynamic goals in the patient with aortic insufficiency include augmenting preload, supporting heart rate, maintaining contractility, and afterload reduction.
3. 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.
4. 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

1. Atherosclerosis at the bifurcation of the common carotid artery is the source of most cerebral ischemic events.
2. 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.
3. 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.
4. 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.
5. No particular anesthetic technique for CEA has been shown to improve outcome.
6. 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.
7. 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

1. Acute onset.
2. PaO2/Fio2 ratio = 300 for ALI.
3. PaO2/Fio2 ratio = 200 for ARDS.
4. Chest radiograph with diffuse infiltrates.
5. Pulmonary capillary wedge pressure = 18 mmHg.

• CAUSES AND TREATMENT OF ARDS

1. Historically, sepsis has been identified as the most common risk factor for ARDS. Now Pneumonia ??
2. 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.

3. 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.
4. PEEP should be adjusted to prevent end-expiratory collapse.
5. 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

1. 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.
2. Severe hypertension and tachycardia are risks during airway management unless the patient is deeply anesthetized.
3. 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

1. Hyperthermia and cardiovascular collapse with ecstasy.
2. Dissociative state, severe behavior disturbances, and enhanced sympathomimetic effects with PCP.

·         DIABETES MELLITUS

1. 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.
2. The goal for insulin management during surgery is to maintain glucose between 120 and 200 mg/dL.
3. Intraoperative glucose control in all but the shortest cases is best achieved by using a glucose-insulin intravenous infusion.
4. 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.
5. 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

1. Perioperatively, mild to moderate hypothyroidism is of little concern even for elective surgery. Patients with severe, symptomatic hypothyroidism should be treated preoperatively.
2. Minimum alveolar concentration of volatile anesthetics is unchanged in both hypothyroid and hyperthyroid states.
3. 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.
4. Perioperative glucocorticoid supplementation should be considered for patients receiving exogenous steroids.
5. Chronic exogenous glucocorticoid therapy should not be abruptly discontinued. Doing so may precipitate acute adrenocortical insufficiency.

·         OBESITY

1. 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.
2. 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.
3. Because of the increased mass and decreased oxygen reserves, obese patients also desaturate quickly postanesthetic induction, complicating airway management.
4. 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.
5. 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.