NEURAXIAL BLOCKADE

Although general anesthesia is employed for millions of surgical procedures each year in the United

States, many operations can be performed safely using neuraxial blockade. The two primary neuraxial

techniques, a “single-shot” spinal and continuous epidural catheter, can be used for lower extremity and

lower abdominal procedures. In both techniques, a small dose of local anesthetic is administered near

spinal nerve roots in order to temporarily ablate sensory input from the peripheral somatic and visceral

structures. In the case of a spinal anesthetic – also known as a subarachnoid block – the intrathecal sac

surrounding the cauda equina at vertebral interspace L2-L3 or below is located using a sterile, smallcaliber needle (25 gauge typically). Once cerebrospinal fluid is observed in the hub of a needle, 1 to 2

mL of preservative-free local anesthetic (typically bupivacaine or lidocaine) is injected into the

intrathecal space. The needle is then completely withdrawn. This local anesthetic serves to directly

inactivate efferent and afferent transmission at the nerve roots it comes in contact with. Because local

anesthetics are not specific to specific nerve fiber types, blockade of sensory, motor, and sympathetic

nerves occurs. The spread of local anesthetic within the subarachnoid space is primarily determined by

three factors: (a) the vertebral interspace accessed, (b) the density of the local anesthetic in relation to

the density of cerebrospinal fluid (a concept known as baricity), and (c) the position of the patient

during injection and immediately thereafter. In order to eliminate the risk of needle puncture of the

spinal cord, subarachnoid blocks are only performed below L2-L3 in adults and L3-L4 in children. The

local anesthetic solution may be combined with vasoconstrictors such as epinephrine or opioids such as

fentanyl or morphine in order to increase the density or duration of the sensory blockade. Surgical

anesthesia ranging from 1 to 2 hours can be achieved using a subarachnoid block. Because of concerns

regarding permanent nerve damage, intrathecal catheters are typically not used.6,7 As a result, most

subarachnoid blocks are “single-shot” techniques that cannot be redosed.

In the case of epidural techniques, the nerve roots are blocked outside the thecal sac in potential

space between the ligamentum flavum and dura mater. This space is accessed sterilely using a 19-gauge

introducer needle and a loss of resistance technique. Once the space is identified, a 21-gauge catheter is

inserted into the space via the introducer needle and the needle is removed. After testing to reduce the

likelihood of inadvertent intravascular or intrathecal placement of the catheter, the epidural catheter

can be taped in place. Because the epidural catheter can be left in place for several days, redosing is

possible. Dilute local anesthetics combined with vasoconstrictors or opioids are the mainstay of epidural

therapy. Epidural neuraxial techniques can be used for surgical anesthesia, as an adjunct to general

anesthesia, or for postoperative pain relief. Epidural catheters can be placed in the thoracic or lumbar

regions because the intrathecal sac is not being accessed; associated dermatomal spread and analgesia is

observed. Epidural techniques often fail to result in a dense sacral nerve root blockade, so this may be a

poor choice for surgical anesthesia at or below the knee.

PERIPHERAL NERVE BLOCKADE

7 Peripheral nerve blockade (PNB) has been used for surgical anesthesia of the extremities since the

days of intravenous regional anesthesia described by Bier in 1908. PNB differs from neuraxial

techniques in that it targets peripheral nerves after they have formed from the combinations of nerve

roots. Upper extremity, lower extremity, and visceral peripheral nerves are targets of these peripheral

nerve blocks. Much like neuraxial blockade, PNB can be used to achieve intraoperative surgical

anesthesia, intraoperative analgesia as an adjunct to general anesthesia, or for postoperative analgesia.

Use of long-acting local anesthetics or placement of an indwelling continuous catheter provides longterm (16 hours to several days) pain relief. The effective use of PNB requires excellent communication

between the anesthesiologist and surgeon to ensure that the planned surgical procedure site(s) and any

other sources of procedural stimulation (e.g., tourniquet) are adequately addressed by the block. A

specific preoperative planning discussion to match the planned surgical procedure to a feasible

dermatomal distribution of blockade is essential. Even when blockade of a dermatomal distribution

specific to a surgical procedure is feasible, a PNB may be contraindicated if a neurologic examination

performed at or near the surgical site in the early postoperative period is necessary.

Because of the limited distribution of sympathetic blockade, PNB has a much smaller hemodynamic

effect than general or neuraxial anesthesia. In addition, if general or neuraxial anesthesia can be

supplanted by a PNB, the residual unwanted side effects of these anesthetics can be avoided, resulting in

improved patient satisfaction, improved quality and speed of recovery, and expedited

419

http://surgerybook.net/

hospital/procedural center discharge. However, PNB is not without side effects; the impact of phrenic

nerve motor blockade associated with specific upper extremity blocks (common with interscalene and

rare with supraclavicular) may have significant consequences for patients with underlying pulmonary

disease.

Historically, anatomic landmark-based identification of peripheral nerves was complemented by use

of electrical stimulator needles with the hope of eliciting specific motor responses confirming correct

needle placement. The technical challenges of establishing precise anatomic location percutaneously by

assessing patient symptoms in response to electric stimulation of specific muscle groups are significant.

Concerns regarding possible intraneural injection or vascular injury persisted for many years. However,

modern anesthesia techniques now employ real-time ultrasound guidance of a PNB needle under direct

visualization. Vascular structures and nerves are visualized in relation to a PNB needle in order to

decrease the likelihood of intravascular or intraneural injection, increase the likelihood of an efficacious

block, and minimize the dose of local anesthetic required to achieve an efficacious block. Despite direct

visualization using ultrasound, PNB in adults are typically performed in the awake state to minimize the

risk of intraneural injection, which may be detected via patient complaint of significant pain upon

injection. PNB can be extremely difficult or contraindicated in patients with challenging body habitus,

local superficial infection at the site of needle entry, significant coagulopathy, or implants near the area

to be visualized or injected.

While PNB targets named major peripheral nerves resulting in both motor and sensory blockade

sufficient for surgical anesthesia, field blocks target small cutaneous sensory nerve fibers, used more

commonly to achieve moderate sensory blockade for postoperative analgesia. These blocks typically do

not achieve sensory blockade sufficient for surgical anesthesia and must be augmented by general

anesthesia or deep sedation. Procedures such as transversus abdominis plane (TAP), adductor canal,

intercostal nerve, and local infiltration enable postoperative analgesia.

SEDATION ANALGESIA FOR MINOR SURGICAL PROCEDURES

There are a variety of minor surgical procedures that can be accomplished safely and comfortably with

anesthesia provided by infiltration of local anesthetics (most commonly 1% lidocaine or 0.25%

bupivacaine) and mild sedation/anxiolysis provided by IV agents. All IV benzodiazepines, narcotics, and

other IV anesthetics produce apnea if given in a high enough dose. Because there is a substantial patientto-patient variability in response to a given dose, IV anxiolytics must be given in small incremental

doses slowly to achieve a safe sedated state. When attempting to provide appropriate sedation analgesia

for a procedure, it is worth noting that inadequate infiltration of local anesthetic cannot be compensated

by increased doses of IV sedatives. Such doses of sedatives as well as narcotics cannot overcome the

pain associated with a surgical incision; furthermore, if large doses of narcotics are given for this

purpose, a patient may quickly become apneic once the surgical stimulus ends. This is due to the fact

that duration of the respiratory depression for even short-acting narcotics is much longer than the

painful stimulus of the incision. Because of the potentially serious consequences of an apneic episode,

the Joint Commission has required that all patients receiving sedation for minor surgical or medical

procedures undergo the following8:

Figure 13-1. Classification of the patient’s upper airway based on the size of the tongue and the pharyngeal structures visible on

420

http://surgerybook.net/

mouth opening. Class I, soft palate and anterior/posterior tonsillar pillars, and uvula visible; Class II, tonsillar pillars and part of

uvula hidden by base of tongue; Class III, soft and hard palate visible; Class IV, soft palate not visible, only hard palate visible.

(Redrawn from Stoelting RK, Miller RD. Basics of Anesthesia, 5th ed. New York: Churchill Livingston; 2007:146.)

Table 13-7 Sedation Scale

1. A preprocedure evaluation including an airway examination (Fig. 13-1)

2. Appropriate monitoring: pulse oximetry as a minimum

3. Documentation of the patient’s vital signs and arterial saturation as well as the dose and timing of

sedatives provided during the procedure

4. Documentation of a recovery period and a return to a safe recovered state

The preprocedure evaluation should include current medications, coexisting disease, and a brief

physical examination including an evaluation of the airway. The most common drug used to provide

sedation is midazolam. This is a fast-onset, relatively short-acting benzodiazepine that can be easily

titrated to produce a sedated yet cooperative arousable state. It is usually given to adults in incremental

doses of 1 mg (0.01 mg/kg in children). Narcotics such as fentanyl even in small doses act

synergistically with benzodiazepines to cause a more sedated state with a much higher incidence of

apnea. To assess the effect of the drug, a validated sedation scale can be of value. The scale used at the

University of Michigan is presented in Table 13-7.9

AIRWAY EVALUATION FOR THE NONANESTHESIOLOGIST

8 9 An essential skill for all clinicians is the assessment of a patient’s airway. It is important to

determine how difficult it may be to obtain control of the airway if a patient requires ventilatory

support. The concept of airway management should be focused on not just endotracheal intubation, but

also mask ventilation. Until the airway can be secured via intubation, the patient must be supported

through mask ventilation.10 Supraglottic airway devices (laryngeal mask airways) are of additional

consideration, used for both definitive airway management as well as a rescue technique for failed mask

ventilation or failed intubation. Limitations to supraglottic airway devices, however, include a relative

lack of protection from aspiration, as well as a limited ability to provide adequate positive pressure

ventilation in patients prone to airway obstruction (e.g. morbidly obese, sleep apnea). The key elements

of an airway examination are an assessment of obesity, mouth opening, neck flexion and extension,

Mallampati oropharyngeal classification, presence of beard, and mandibular protrusion ability (Table

13-8).11 Despite decades of research, there is no perfect combination of clinical tests to predict difficult

intubation. However, the presence of abnormalities in three or more of the aforementioned elements

increases the likelihood of a difficult mask ventilation and/or difficult intubation by more than eightfold.11 In patients with multiple (three or more) airway abnormalities, the presence of a beard is an

easily corrected characteristic: the patient should be asked to shave the beard in order to improve the

ability to manage the airway. Assessment of oropharyngeal classification is performed by having

patients open their mouth and maximally protrude the tongue without phonation. This Mallampati

oropharyngeal assessment can be classified depending on whether the uvula can be completely seen

(class 1), only partly seen (class 2), or not seen, with the hard and soft palate visible (class 3), or only

the hard palate visible (class 4) (Fig. 13-1).12

Table 13-8 Standard Airway Examination for Nonanesthesiologists

421

http://surgerybook.net/

RISKS ASSOCIATED WITH ANESTHESIA

Because the entire purpose of surgical anesthesia is to obtund or completely block physiologic

protective mechanisms, there is an underlying baseline anesthetic risk even without a surgical

procedure. Fortunately, with the advent of newer agents and monitoring techniques, it is estimated that

the mortality rate directly attributable to anesthesia alone has decreased from about 1 in 10,000

patients in the 1950s to as low as 1 in 200,000 or less for healthy patients today.13 Although a 1 in

200,000 risk of death or serious neurologic impairment may appear small, when dire consequences

occur in a young patient undergoing a purely elective procedure, the consequences are devastating for

everyone involved. When patients are placed in a condition in which they cannot breathe, there is

always the possibility of a technical or judgmental error resulting in hypoxia and brain damage or

death. It has been estimated that between 50% and 75% of anesthetic-caused deaths are due to human

error and are preventable. Because the consequences of an anesthetic mishap are usually severe, the

emotional and financial costs are high.

Historically, the most common problems associated with adverse outcomes were related to the airway

and included inadequate ventilation, unrecognized esophageal intubation, unrecognized extubation, and

unrecognized disconnection from the ventilator. The incidence of these problems has been significantly

reduced by including capnometry and pulse oximetry in addition to other noninvasive monitors,

although a cause-and-effect relation has been difficult to prove. Efforts to improve outcome can be

approached at three levels: (a) reduction of the incidence of rare but catastrophic anesthetic-related

problems, (b) improvement of the care and experience of every patient undergoing anesthesia and

surgery, and (c) improvement of the preparation and management of patients with pre-existing medical

conditions who have higher morbidity and mortality rates. The first goal has been addressed in part

with improved monitoring techniques, standardized anesthesia machine checklists, and anesthesiology

training. Others have been advanced by the addition of comprehensive pain management, as discussed

later in this chapter. Issues of pre-existing medical disease and how they affect the anesthetic plan are

also briefly discussed later in this chapter.

Cardiovascular Diseases

Hypertension

Hypertension is the most common pre-existing medical disease in patients presenting for surgery and is

a major risk factor for renal, cerebrovascular, peripheral vascular, and coronary artery diseases, as well

as congestive heart failure (CHF). It is particularly associated with lipid disorders, diabetes, and obesity.

It is these associated comorbidities that are most likely to lead to morbidity and mortality in the

perioperative period, and therefore, the presence of hypertension should prompt the surgeon to review

the history and physical examination for them. Hypertensive patients should be treated medically to

render them normotensive before elective surgery. For elective surgical procedures, a sufficient period

of time preoperatively should be allocated for antihypertensive management, as rapid correction of

hypertension immediately prior to surgery is not without risk of comorbidities, including stroke and

other end-organ malperfusion. In general, antihypertension medications should be continued throughout

the perioperative period. However, patients treated with angiotensin receptor blockers (ARBs), such as

valsartan, candesartan, losartan, or angiotensin-converting enzyme inhibitors (ACE-Is), such as

lisinopril, captopril, or ramipril, who are exposed to general anesthesia, are at risk for developing

profound, refractory intraoperative hypotension. This ACE-I/ARB hypotension has been treated

422

http://surgerybook.net/

successfully with terlipressin, vasopressin, and methylene blue.14–16 As a result, most medical centers

now recommend withholding ACE-Is/ARBs the morning of surgery.17,18 Patients on concomitant diuretic

therapy are at greatest risk for intraoperative hypotension requiring treatment.19,20

The incidence of hypotension and myocardial ischemia intraoperatively is higher in untreated

hypertensive patients than in adequately treated hypertensive patients if the preoperative diastolic

pressure is 110 mm Hg or higher.21 Inadequately treated hypertensive patients undergoing carotid

endarterectomies have an increased incidence of neurologic deficits, and those with a history of prior

myocardial infarctions have an increased incidence of reinfarction. Patients commonly have an elevated

blood pressure on admission to the hospital. Hypertensive patients can have exaggerated responses to

painful stimuli and have a higher incidence of perioperative ischemia.

Coronary Artery Disease

Much of the anesthetic preoperative evaluation has historically been focused on the detection and

treatment of coronary artery disease. Coronary artery disease or its risk factors are present in about

30% of patients who undergo major surgery each year.22 It is the leading cause of death in the United

States and continues to be a major cause of postoperative morbidity and mortality.23 The goal of the

preoperative cardiac evaluation is to identify patients who are at increased risk of perioperative cardiac

morbidity and ensure that their chronic conditions are optimized. Although perioperative cardiac events

are the leading cause of death following anesthesia and surgery, it has been difficult to define patient

characteristics that accurately predict a high risk of adverse outcome.24 It has been even more difficult

to modify that risk effectively.25 Preoperative CHF is clearly a significant risk factor, as is recent

myocardial infarction or unstable angina (Table 13-9). Diabetes mellitus (DM), atherosclerotic vascular

disease, and hypertension also appear to confer risk, although less than with CHF or unstable angina.

Perioperative risk in patients with valvular heart disease varies with the severity of the disease as

represented by CHF, pulmonary hypertension, and dysrhythmias. Dysrhythmias are also a concern in

the presence of coronary artery disease. Age and stable angina remain controversial as predictors of

perioperative risk, with equal numbers of supporting and refuting studies. The value of

revascularization remains controversial as well. In patients without significant pulmonary disease, the

ability to climb two flights of stairs without stopping or experiencing symptoms of angina or shortness

of breath is considered a good practical test of cardiac reserve. Unfortunately, many patients with

ischemic heart disease have concomitant pulmonary disease or other medical problems that limit their

activity. A history of myocardial infarction is important information. Large retrospective studies have

found that the incidence of reinfarction is related to the time elapsed since the previous myocardial

infarction.26–28 The incidence of reinfarction appears to stabilize at about 6% (50-fold higher than

patient without myocardial infarction) after 6 months. The highest rate of reinfarction occurs in the 0-

to 3-month period. Mortality from reinfarction, for patients undergoing noncardiac surgery (Table 13-

10), has been reported to be between 20% and 50% and usually occurs within the first 48 hours after

surgery.

Table 13-9 Clinical Predictors of Increased Perioperative Cardiovascular Risk

(Myocardial Infarction, Heart Failure, Death)

423

http://surgerybook.net/