Chapter 2 - ANESTHETIC CONSIDERATIONS
TYPES OF ANESTHESIA
This type causes the least systemic effects and is generally accepted as the lowest risk modality. Monitored anesthesia care (MAC) involves the combination of local anesthesia, IV sedation and appropriate monitoring, with or without premedication.
This type includes spinal, epidural, and peripheral nerve blocks. Peripheral nerve blocks are also considered a relatively low risk modality. There is controversy regarding whether or not spinal or epidural anesthesia is in fact a safer modality than general anesthesia. A number of studies have shown no difference in major morbidity or mortality rates between patients receiving general vs. spinal/epidural anesthesia for the same surgery performed.
The choice of anesthetic should be individualized based on the patient's underlying medical conditions. Regional anesthesia may be deemed safer in some patients; general anesthesia, in others. Patient comfort is an additional issue of concern. *The final decision regarding the choice of anesthetic is ultimately left to the anesthesiologist, this in consultation with the patient, the surgeon, and-if necessary-the medicine physician involved.
This type involves loss of consciousness sufficient enough to provide loss of sensation. The anesthetic agents are either inhalational or intravenous, but it is common to utilize a combination of the two. Neuromuscular blocking agents may also be needed for skeletal muscle relaxation/paralysis. Airway is managed with a mask or an endotracheal tube.
EFFECTS OF ANESTHESIA
General anesthesia causes a decrease in functional residual capacity and an increase in ventilation/perfusion mismatching. There is also suppression of CNS chemoreceptors which control respiratory drive. Intubation may also trigger a reflex bronchospasm in patients with reactive airways.
Spinal/epidural anesthesia has the potential advantage of fewer adverse pulmonary effects in patients with severe lung disease, but this may be the case only for surgeries requiring a low dermatome level (such as lower extremity or prostate procedures). Higher levels may have an equally or more deleterious effect on respiration. Protection of the airway becomes a prominent concern as well.
All inhalational anesthetics cause myocardial depression to a variable degree. Halothane, for example, decreases cardiac output by 15% in healthy subjects; nitrous oxide has about half of that effect or less. Typically the depression of myocardial contractility is dose-dependent. Heart rate responses are variable.
Opioids are commonly used and can cause a decrease in heart rate and, to a lesser extent, in blood pressure. Succinylcholine may produce either tachycardia or bradycardia, but the bradycardia responses can be much more profound (severe sinus bradycardia, nodal rhythms, even asystole).
Spinal/epidural anesthesia does not directly depress myocardial contractility; however, hypotension and bradycardia are the most frequent physiologic changes associated with spinal/epidural anesthesia. The higher the level of the block, the more evident the hypotension.
General anesthesia causes a decrease in renal blood flow and glomerular filtration rate, contributing to the commonly encountered oliguria intraoperatively and immediately postop. The relationship of this intraoperative oliguria to postoperative acute renal failure is unclear, however. (Intraoperative hypotension, on the other hand-particularly if prolonged and severe-is clearly associated with risk of postoperative acute renal failure, regardless of the type of anesthesia.)
All inhalational anesthetics decrease portal blood flow and, to a variable extent, hepatic arterial flow. Intravenous and regional anesthetics affect liver function only by the degree to which they alter systemic hemodynamics. (Surgical stress is likely to be an even more important factor in determining postoperative liver dysfunction than is the specific anesthetic agent.)