GENERAL ANESTHESIA

General anesthesia allows for anesthesia, analgesia, akinesis, amnesia, and autonomic stability. Traditionally, it has been viewed by some as the “gold standard” for anesthesia for major hip procedures.¹ General anesthesia can be induced and maintained either intravenously or by inhalation with a variety of compounds. In adults, the induction of general anesthesia is most typically performed by the intravenous administration of a barbiturate (e.g., sodium pentothal or methohexital) or a nonbarbiturate (e.g., propofol, etomidate, ketamine). The selection of an induction agent is often determined by the patient’s comorbidities and other parameters such as hemodynamics and intravascular volume status. In pediatric procedures or when adults are without intravenous access, general anesthesia can be induced by inhalation of a volatile anesthetic gas, but this is a slower process compared with intravenous induction. Many adults find this an unpleasant experience, and it may introduce unacceptable anesthetic risk in some patients.

General anesthesia can be maintained either by administration of intravenous agents or by the inhalation of anesthetic gas (e.g., halothane, enflurane, isoflurane, sevoflurane, desflurane). The airway is controlled either by endotracheal intubation or with a supraglottic device (e.g., laryngeal mask airway) or by face mask. Ventilation can be either spontaneous or by positive-pressure mechanical ventilation. When neuromuscular blockade is necessary for muscle relaxation, as is typically needed for major hip procedures, positive-pressure mechanical ventilation is necessary.

Because medication is continuously administered in some form during the course of general anesthesia, the anesthesiologist is able to constantly adjust the depth of anesthesia to changing patient or surgical conditions and to better adapt to shorter or longer than anticipated durations of surgery. The relative risks of general anesthesia and its effect on the incidence of complications are discussed later. However, general anesthesia is unique from the other anesthetic modalities for major hip procedures in its need for definitive airway management and the unique risk of malignant hyperthermia, which is a rare but potentially life-threatening reaction to volatile anesthetic gases and depolarizing neuromuscular blockers. The incidence of malignant hyperthermia susceptibility is believed to be about 1 in 15,000 in pediatric populations and 1 in 50,000 among adults.² Various forms of muscular dystrophy predispose to malignant hyperthermia, and the majority of individuals displaying this abnormal response to anesthesia have inheritable mutations of the skeletal muscle ryanodine type I receptor.³

NEURAXIAL ANESTHESIA

Neuraxial or major conductive anesthesia involves the injection of preservative-free local anesthetic (e.g., lidocaine, bupivacaine, tetracaine) and possibly an opioid into the intrathecal or epidural spaces. With a dense block, either technique can provide anesthesia without unconsciousness and also akinesis and muscle relaxation. Failure to achieve a dense block can occur with either technique, but inadequate block or muscle relaxation occurs more commonly with epidural anesthesia. Neuraxial block is often associated with hypotension, because venous pooling occurs, decreasing cardiac preload. Judicious use of vasopressors and hydration is usually effective in correcting significant hypotensive events. In addition, modest hypotension may be preferred by some surgeons as a method of reducing blood loss. If sedation or amnesia is desired, conscious sedation can be given as well. Each technique can be by either single injection or continuous infusion. Continuous techniques provide the benefit of controlled initial titration followed by continuation into the postoperative period to provide analgesia. Continuous spinal anesthetics are relatively uncommon in the United States because intrathecal catheters were withdrawn from the practice in 1992 by the U.S. Food and Drug Administration (FDA). However, in Europe, continuous spinal techniques have been shown to result in less overall hypotension (including profound hypotension) compared with single-injection spinal anesthetics among patients older than the age of 65 years undergoing operative repair of hip fracture.⁴ This is in part due to differences in dosing requirements between the two techniques. The ED₅₀/ED₉₅ of isobaric bupivacaine for total hip arthroplasty when administered in incremental fashion during continuous spinal anesthesia is substantially lower than the doses typically administered during single-injection techniques.⁵ Most clinicians will administer intravenous crystalloid or colloid before spinal anesthesia to limit the degree of hypotension.

The injection into the intrathecal space required for spinal anesthesia is typically achieved by using a very small bore spinal needle via either a midline or a paramedian approach. Calcification of the ligamentum flavum, which can occur with aging, can make the midline approach technically more difficult. To minimize the risk of direct trauma to the spinal cord, injection is customarily performed below the second lumbar intervertebral space, the usual level of the conus medullaris in adults. Surgical anesthesia can be obtained as high as the T4 dermatome by controlling the quantity of local anesthetic and the baricity of the injectate so that it will rise or settle in the cerebrospinal fluid coupled with corresponding maneuvers in patient positioning. Spinal levels more cephalad than T4 are undesirable because of bradycardia from depression of cardiac accelerator fibers. Adequate analgesia is achieved by having a block of the surgically affected dermatomes, myotomes, and osteotomes. For most procedures on the hip, a T10 sensory level is sufficient.

Dextrose is commonly added to local anesthetic to increase the baricity. Hyperbaric solutions will tend to settle by gravity in the cerebrospinal fluid, hypobaric solutions will tend to rise, whereas isobaric solutions should migrate from the level of injection only minimally. In this manner, the spinal block can be made denser on one side of the patient than the other. A hyperbaric spinal block can be administered with the patient in the lateral decubitus position with the operative hip dependent; conversely, a hypobaric spinal block can be administered for similar effect with the operative hip up. When spinal anesthesia is injected with the patient in the lateral decubitus position with the operative hip up before total hip arthroplasty, the same dose of bupivacaine results in a block of longer duration with a delayed need for analgesics in the postoperative period if the injectate is delivered in a hypobaric, as opposed to an isobaric, suspension.⁶ Small quantities of opioid can be added to improve analgesia, but they must be utilized judiciously because of possible resulting pruritus, nausea, and ventilatory depression. Adding intrathecal morphine to the spinal anesthetic has been shown to reduce the need for patient-controlled analgesia with morphine after the procedure for total hip arthroplasty patients, a benefit that was not observed among total knee arthroplasty patients.⁷ In a series of 60 patients older than the age of 65 years undergoing elective total hip arthroplasty, 50 µg of intrathecal morphine added to hyperbaric bupivacaine failed to improve postoperative analgesia compared with placebo whereas 100 µg and 200 µg improved analgesia equally well, with the 200-µg dose being associated with significantly more postoperative pruritus.⁸

Epidural anesthesia is achieved by the injection of local anesthetic into the epidural space. The epidural space is typically identified by a decrease in tissue of resistance to the injection of air or sterile saline (loss of resistance technique) and can be confirmed under fluoroscopy by the injection of a contrast agent, although this is not commonly performed for operative blocks. As with spinal anesthesia, opioids can be added, if desired. It is generally necessary to deliver significantly higher doses of medication in the epidural space than the intrathecal space to achieve clinical effect. Because the intrathecal space is not violated during a properly performed epidural puncture, epidural anesthesia can be performed at virtually any level along the spine depending on the target dermatomes. Dural puncture from spinal anesthesia can be associated with headache (spinal headache). Because the dura is not intentionally penetrated for epidural injection, the risk of headache is low. If the dural is unintentionally punctured during an attempted epidural injection with the large-bore epidural needle, a resulting spinal headache is quite likely. Unrecognized intrathecal injection of the large doses of medication intended for epidural use can have disastrous consequences.

For major hip procedures, a lumbar epidural anesthetic is sufficient. Unlike spinal anesthesia, baricity of the injectate does not affect distribution. The key determinants of the block include the dose of local anesthetic and the volume in which it is delivered. Distribution of the anesthetic within the epidural space is less predictable than in the intrathecal space, and epidural blocks can be patchy or unilateral. Although technically more difficult with slightly less reliability than spinal anesthetics, epidural anesthesia offers the distinct advantage of an indwelling epidural catheter to deliver postoperative analgesia. Postoperative epidural analgesia, in all its various forms, has been demonstrated in large meta-analyses to supply superior analgesia to either parenteral opioids ⁹ or more specifically parenteral opioids delivered in patient-controlled analgesic fashion.¹⁰

PRE-EMPTIVE ANALGESIA AND POSTOPERATIVE PAIN

A proper anesthetic plan needs to address the postoperative analgesic needs of the patient. Pre-emptive analgesia utilizes analgesics before painful stimuli to prevent central stimulation and thus lessen the subsequent pain experience. Local anesthetics utilized in peripheral nerve blocks or neuraxial anesthesia are an integral part of the multimodal approach to pre-emptive analgesia.¹¹ Total hip arthroplasty patients receiving spinal anesthesia report lower pain scores and require less analgesic medication in the postanesthesia care unit and, as an added benefit, spinal anesthesia can be delivered more cost effectively than general anesthesia for this procedure.¹² Combined spinal and epidural techniques have been used for major hip procedures, taking advantage of spinal anesthesia’s rapid and reliable onset and ability to deliver sustained postoperative analgesia through an epidural catheter. Although efficacious, traditional epidural analgesia introduces additional sources of complications and expense into the postoperative management of surgical patients. Problems with epidural catheters (e.g., kinking, occlusion, or catheter migration) or the infusion pumps (e.g., misprogramming or battery or electrical failure) are common and can lead to periods of disruption of analgesia therapy. Trained personnel need to be available to troubleshoot these problems, and indwelling epidural catheters may increase the risk of neuraxial hematoma formation when postoperative anticoagulation is used.

The duration of effective analgesia provided by preservative-free morphine injected into the epidural space is 24 hours or less. An extended-release epidural morphine (Depodur, Endo Pharmaceuticals, Inc, Chadds Ford, PA) preparation is currently available that delivers 48 hours of postoperative analgesia. Therefore, it is now possible to provide a combined spinal and epidural technique for hip surgery that provides extended postoperative analgesia without an indwelling epidural catheter. Depodur utilizes a multivesicular liposomal delivery system called DepoFoam (SkyPharma, Inc, San Diego, CA). DepoFoam consists of microscopic lipid-based particles with numerous morphine-containing internal vesicles that reorganize after injection into the epidural space causing the sustained release of morphine at a predictable rate. Depodur has been demonstrated to provide analgesia superior to patient-controlled analgesia with fentanyl alone both at rest and with activity to patients undergoing total hip arthroplasty without causing motor block or requiring an epidural catheter.¹³ Some patients receiving Depodur require little or no parenteral opioids postoperatively. A sustained-released formulation of epidural bupivacaine using DepoFoam technology is being developed (Fig. 27-1).