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Surgical patients with coexisting pulmonary impairment are at risk for
intraoperative or postoperative pulmonary complications, regardless of
anesthetic technique.1 However, increasing evidence
suggests that regional anesthesia may be associated with improved pulmonary
outcomes compared with those associated with general anesthesia.2–4 A thorough understanding of respiratory physiology and the
implications of regional anesthetic techniques is crucial to the safe and
effective use of regional anesthesia in these patients.
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Epidural & Spinal Anesthesia
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Most of the pulmonary effects of neuraxial anesthesia are due to motor
block of the intercostal and abdominal musculature. If a significant
systemic uptake of local anesthetic occurs, some central and direct
myoneural respiratory depression can also be seen, although this plays a
minor role overall.5 Since neuraxial anesthesia produces a
“differential” blockade of motor, sensory, and autonomic fibers, the degree
to which respiratory function is impaired depends on the relative extent of
segmental motor blockade. Using dilute concentrations of epidural local
anesthetic may provide adequate sensory block as high as the cervical
levels, while sparing the motor function of the respiratory muscles in the
lower somatic segments.6 Achieving diaphragmatic paralysis
via a phrenic nerve (C3 through C5) block in the absence of a total spinal
anesthesia is difficult in practice, since even a sensory block as high as
C3 will only produce a motor block at approximately T1 through
T3.5 However, high neuraxial blocks may precipitate
hypotension sufficient to decrease blood flow to the respiratory center in
the medulla, leading to respiratory arrest.
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With higher levels of epidural or
spinal anesthesia, chest wall musculature, and in
particular the intercostal muscles, become segmentally weakened and
contribute less to the respiratory effort. This in turn, may eventually lead
to altered chest wall motion during spontaneous respiration. For instance,
some studies have suggested that during high neuraxial anesthesia, the more
compliant chest wall is retracted during inspiration and may actually
display paradoxical rib cage motion.7,8 Others, however,
have found that epidural blockade to sensory levels of T6 or even T1 do not
lead to rib cage constriction with inspiration and may in fact increase the
contribution that chest wall expansion makes to tidal
volume.9,10 This may be explained by an incomplete motor
block of high, intercostal muscles or the compensatory role played by the
“accessory” muscles of respiration such as the anterior and middle scalene
muscles.11
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Lumbar epidural anesthesia does not impair
resting minute ventilation, tidal volume, or respiratory rate.12–14 Furthermore, functional residual capacity (FRC) and closing
capacity appear to be relatively unchanged during lumbar epidural
anesthesia.15–17 Effort-dependent tests of respiratory
function, such as forced expiratory volume in one second
(FEV1), forced vital capacity, and peak expiratory flow
rate, do exhibit modest decreases in the setting of lumbar epidural
blockade, reflecting the reliance of these indices on intercostal and
abdominal musculature.18 In contrast, the effect of
thoracic epidural anesthesia on pulmonary mechanics is less clear, with
studies showing both a decrease14,19 and
increase20 in minute ventilation and tidal volumes. One
volunteer study found that high thoracic epidural anesthesia (T1 sensory
level) led to an increase in FRC of approximately 15% with no change in
tidal volume or respiratory rate.10 This somewhat
surprising finding may be explained by two mechanisms offered by the
investigators. First, most volunteers exhibited a decrease in their
intrathoracic blood volume, a physiologic occurrence confirmed by Arndt and
colleagues.21 Second, the study also found that the
end-expiratory position of the diaphragm was shifted caudally, which is
possibly related to a relative increase in diaphragmatic tonic activity or a
reduction in intraabdominal pressure. Thoracic epidural anesthesia results
in a modest decrease in vital capacity (VC), FEV1, total
lung capacity, and maximal midexpiratory flow rate.15
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The ventilatory response to hypercarbia and hypoxia is preserved with
neuraxial anesthesia.9,14 Partial pressures of both oxygen
(Po2) and carbon dioxide
(Pco) are essentially unchanged during epidural or
spinal anesthesia.9,10 In addition, bronchomotor tone is
not altered to any significant degree, despite theoretical concerns of
bronchoconstriction secondary to sympatholysis.21 Indeed,
epidural anesthesia has been used successfully for high-risk patients with
chronic obstructive pulmonary disease and asthma undergoing abdominal
operations.22,23
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Neuraxial anesthesia has been shown in a number
of settings to lead to reduced postoperative pulmonary complications
compared with general anesthesia. The reasons behind this are probably
multifactorial, owing in part to superior analgesia, reduced diaphragmatic
impairment, altered stress response, and a decreased incidence of
postoperative hypoxemia.24,25 Epidural anesthesia provides
better pain control than general anesthesia for abdominal and thoracic
surgery, which leads to reduced splinting, a more effective cough mechanism,
and preserved postoperative lung volumes, including FRC and
VC.26 One study directly comparing epidural and general
anesthesia in high-risk patients concluded that overall outcomes, including
the need for prolonged postoperative ventilation, were improved with the
regional technique.27 Another trial in patients undergoing
lower limb vascular surgery reported a greater than 50% reduction in the
incidence of respiratory failure in the group randomized to epidural
anesthesia.28 A more recent meta-analysis of 141
randomized trials (including over 9000 patients) comparing regional and
general anesthesia for hip surgery showed a risk reduction for pulmonary
embolism, pneumonia, and respiratory depression of 55%, 39%, and
59%, respectively, with the regional anesthesia.25
Interestingly, these outcomes were unchanged regardless of whether neuraxial
anesthesia was continued into the postoperative period, illustrating that
the beneficial effect of epidural and spinal anesthesia on pulmonary
physiology occurs, at least in part, at the time of surgical insult.
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Brachial Plexus Block
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In the absence of inadvertent complications such as pneumothorax,
alterations in respiratory mechanics seen with brachial plexus block are due
primarily to phrenic nerve blockade and hemidiaphragmatic paralysis. This
has been shown to occur in 100% of patients receiving interscalene
blockade,29 and leads to a reduction by 27% in both FVC
and FEV1.30 While the clinical
significance of this reduction in healthy patients is not entirely clear, it may be useful to
risk-stratify patients about to undergo interscalene blocks as one would a
patient undergoing lung resection. In other words, ask the question: “Will
this patient tolerate a perioperative FEV1 reduction of
27%?” Some investigators have attempted to reduce the incidence of
phrenic nerve palsy by decreasing the volume of local anesthetic; however,
volumes as little as 10–20 mL still result in diaphragmatic
paralysis.31,32 In fact, one case report illustrated
clinically significant respiratory compromise requiring tracheal intubation
following an interscalene block using a volume of 3 mL of 2% mepivacaine.33
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The risk of phrenic nerve blockade decreases as one moves more distally
along the plexus. Axillary nerve block has no effect on diaphragm function
and presents a good choice for those patients with marginal pulmonary
reserve (ie, cannot tolerate a 27% reduction in lung function). On the
other hand, the supraclavicular block is associated with a 50–67%
incidence of hemidiaphragm paralysis.34–36 The
infraclavicular approach is probably sufficiently distant from the course of
the phrenic nerve so as to spare the diaphragm,37,38
although there are case reports of phrenic nerve
involvement.39 These discrepancies probably relate to the
different approaches to the infraclavicular block—for instance the
“coracoid block” is performed with a relatively lateral or distal puncture
site, whereas the vertical infraclavicular block begins at a more medial
location. Although the infraclavicular or axillary blocks may be desirable
for their relative pulmonary-sparing profiles, they carry the disadvantage
of providing incomplete anesthesia for the upper arm and shoulder. However,
creative solutions have been employed to get around this issue. Martinez and
coworkers combined an infraclavicular block with a suprascapular nerve block
for emergent humeral head surgery in a patient who was acutely asthmatic and
had a baseline FEV1 of 1.13 L (32% predicted).
Therefore, a knowledgeable combination of peripheral nerve blocks can
provide complete anesthesia of the upper limb while avoiding respiratory
complications in patient with a pulmonary disease.40
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Continuous brachial plexus blocks with perineural catheters are an
attractive method of maintaining the advantages of plexus blockade into the
postoperative period and have been shown to reduce postoperative pain, oral
opioid requirements and their side effects, and sleep disturbances after
shoulder surgery.41 However, there have been reports of
complications attributed to the prolonged phrenic nerve paresis that
invariably occurs with this technique. These have included chest pain,
atelectasis, pleural effusion, and dyspnea.42,43 This is
of particular concern because many patients are being discharged home
with catheters and may not have access to timely intervention should these
complications arise. On the other hand, the degree of clinically significant
respiratory impairment with continuous interscalene blockade varies among
patients and, in fact, may be well tolerated, especially if using relatively
dilute concentrations of local anesthetic that only provide a partial
phrenic paresis.44
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Maurer and associates reported a case of a patient with no preexisting
pulmonary disease who underwent bilateral shoulder arthroplasty under
combined bilateral continuous interscalene blockade and general
anesthesia.45 Postoperative analgesia was maintained in
the hospital for 72 h via the catheters using infusions of 7 mL/h of 0.2% ropivacaine for each side (total 14 mL/h). Despite a marked postoperative
reduction in FVC (60%) from baseline as well as sonographic evidence of
diaphragmatic impairment, the patient had an uneventful postoperative course
(with excellent analgesia) and good recovery. This anecdotal example
illustrates that the clinical significance of phrenic paresis in patients
with good respiratory function is questionable. Regardless the use of continuous
brachial plexus techniques should be carefully considered in patients with
preexisting pulmonary disease, especially if they are to be discharged home
with the catheters in situ.
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Paravertebral & Intercostal Nerve Blocks
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Several studies investigated the effects of paravertebral and
intercostal blocks on pulmonary function in patients with rib fractures or
those undergoing thoracotomy. Intercostal blockade has been shown to improve
arterial oxygen saturation (Sao2) and
peak expiratory flow rate (PEFR) in patients with traumatic rib fractures
associated with severe pain.46 Likewise, Karmakar and
investigators found that continuous paravertebral blockade over a period of
4 days in patients with multiple fractured ribs led to significant
improvement in respiratory rate, FVC, PEFR,
Sao2, and the
Pao2 fraction of inspired oxygen
ratio.47 These findings are probably related to the
favorable effect of analgesia on respiratory efforts by the patient and
improved respiratory mechanics.
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Paravertebral blocks are very effective for management of pain following
thoracotomy and can significantly improve postoperative spirometry. One
review of 55 randomized, controlled trials of analgesic techniques following
posterolateral thoracotomy revealed that paravertebral blockade was the
method that best preserved pulmonary function compared with either
intercostal or epidural analgesia.48 The combined results
showed an average preservation of approximately 75% of preoperative
pulmonary function when paravertebral analgesia was used versus 55% for
both intercostal and epidural analgesia. It is unclear why paravertebral
blockade might result in improved PEFR and
Sao2 compared with epidural analgesia
in this and other studies, but it may be related to increased utilization of
opioids, higher incidence of nausea and vomiting, and the presence of
bilateral intercostal muscle blockade (and therefore
diminished chest wall mobility) in the epidural cohorts.49
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Pulmonary Complications Not Related to Conduction Blockade
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Pulmonary complications related to the use of regional anesthetic
techniques fall into two categories. The first is those related directly or
indirectly to the physiologic changes that occur with the blockade itself.
Examples include atelectasis and pneumonia resulting from an inability to
mobilize secretions. The second category comprises those that are
independent of the effect of blockade, and although there are sporadic
reports of rare complications such as pulmonary
hemorrhage50 and chylothorax,51 the most
common of these is pneumothorax. Not surprisingly, pneumothoraces occur most
frequently when the puncture site overlies the pleura, and especially when
performing supraclavicular and intercostal blocks. The overall incidence is
low, however,52–55 and refinements of previously
published techniques based on MRI studies and ultrasound guidance can
decrease the incidence further.56,57 Nevertheless,
techniques with a risk of pneumothorax should be carefully considered or avoided in patients with
borderline pulmonary function.