Anesthetic consideration for anterior
mediastinal mass resection
Mediastinal anatomy
The mediastinum is the space in the center of the chest bounded by
the plurae on either side, the sternum anteriorly and the thoracic vertebral
column posteriorly. The upper boundary is the thoracic inlet, and the lower
boundary is the diaphragm. It is divided into superior and inferior portions by
a line joining the sternal angle to the fourth thoracic vertebra. The inferior
portion is further divided into anterior, middle and posterior parts
Table 10.1 Contents of the mediastinum.
Superior
Inferior
Aortic arch and branches Anterior
Innominate vein Lymph nodes
Superior vena cava Middle
Trachea Heart
Esophagus
Ascending aorta
Thoracic duct
Pulmonary vessels
Thymus Superior vena cava
Recurrent laryngeal nerves Phrenic and vagus nerves
Lymph nodes Lymph nodes
Posterior
Retrosternal extension ofthe thyroid
Main bronchi
Esophagus
Descending aorta
Azygos veins
Thoracic duct
Lymph nodes
The signs and symptoms of mediastinal pathology range from trivial
to life-threatening, and include airway compression, superior vena cava
syndrome, compression of the right heart and pulmonary arteries, and dysphagia
from esophageal compression. Venous obstruction is a particular problem,
because the smaller veins expand to allow collateral flow, increasing the risk
of bleeding during anesthetic or surgical instrumentation. Neural compression
is also common, and may produce severe pain, vocal cord palsy (recurrent
laryngeal nerve) or Horner’s syndrome.
Anesthesia for resection of anterior
mediastinal tumors
Evaluation of risk factors
Patients who have marked symptoms of airway compromise such as
dyspnea at rest, postural dyspnea, orthopnea, or even stridor are
at high risk of intraoperative airway problems. Compression of the heart, SVC, and
pulmonary arteries also can cause syncope, arrhythmias, head and neck edema,
and even a degree of cyanosis, particularly in children.
Lung function tests, including arterial blood gas analysis, usually
are performed as a baseline.
Careful evaluation of the airway is necessary during pre-operative
assessment for surgery within the mediastinum. Chest X-ray, including thoracic
inlet views when necessary, and thoracic computerized tomographs should be
studied for evidence of airway compression or displacement. erect and supine
spirometry may be helpful.l
The patient presenting for mediastinal surgery may also have systemic
disease as a result of chemotherapy.
Systemic effects of the tumor
Patients who have an intrathoracic goiter may have abnormalities of
thyroid function that require monitoring and treatment before surgery. Up to 30%
of patients who have a thymoma have symptomatic myasthenia gravis, which
obviously has significant anesthetic implications.
Management of airway obstruction
Treatment modalities that can (or have been) used to minimize risk
are:
Posture
Ø _ Induce in sitting position
Ø _ Change supine position to lateral or
prone position (access ?)
Maintain spontaneous respiration
Ø _ Awake fiberoptic intubation
Ø _ Inhalational induction
Ø _ Intravenous induction (ketamine?)
Airway stenting: conventional intravenous
induction
Ø _ Long endotracheal tube
Ø _ Double-lumen endobronchial tube
Ø _ Rigid bronchoscope
Ø _ Insertion of tracheobronchial stents
Cardiopulmonary bypass
Ø _ Commenced under local anesthesia before
induction
Ø _ Vessels prepared under local
anesthesia, then general anesthesia
Airway stenting /conventional intravenous
induction
a rigid bronchoscope can be advanced to stent the airway. After
initial assessment of the anatomy/pathology, it may be possible to stent the
airway for resection surgery with an endotracheal tube or a double-lumen
endobronchial tube placed under direct vision into the most patent main
bronchus. If this technique is not possible, ventilation can be maintained down
the rigid bronchoscope via a Venturi injector, in the usual way, and anesthesia
can be maintained intravenously.
Cardiopulmonary bypass
The use of cardiopulmonary bypass to overcome the problems of
intraoperative gas exchange in patients who have severe airway narrowing and
pulmonary artery involvement is used in some centers.
Vascular involvement/intraoperative bleeding
mediastinal tumors may invade or compress many of the intrathoracic
vascular structures. SVC obstruction is a common presenting sign of large
anterior mediastinal tumors, and bleeding from the SVC is a relatively common
problem intraoperatively.
Effects of chemotherapy
Patients presenting for surgery and anesthesia may have undergone
chemotherapy previously.
The pulmonary toxicity of bleomycin has been well documented, and
it is important to have baseline pulmonary function tests in these patients.
Nerve section
Anterior mediastinal tumors may advance to surround or invade the
phrenic and/or recurrent laryngeal nerves. Surgical division of
these nerves
may affect the postoperative course and in the case of the phrenic
nerve is an
indication for postoperative ventilation.
Anesthesia for thymectomy in
myasthenia
Thymectomy is a major undertaking in a myasthenic patient,
and comprehensive pre-operative preparation of the patient, together with
communication between surgeon and anesthetist, are important
to success. Surgery is best performed while the disease
is in remission, but early thymectomy is usually the treatment of choice and
excessive delaymay
result in worsening of myasthenic symptoms.
Optimization of anticholinesterase therapy improves muscle function, and
plasmapheresis to reduce the concentration of circulating auto-antibodies may be
useful in some cases, producing improvement in post-operative respiratory
function. Psychological preparation of the patient is important, firstly because
stress may precipitate a myasthenic crisis and secondly because the improvement
in symptoms following thymectomy is not always immediate, and patients may be
disappointed with the result in the early post-operative period.
Pre-operative assessment should include baseline respiratory
function tests, and appropriate thoracic imaging as described above for
mediastinoscopy.
Patients with bulbar muscle involvement may have an impaired
cough reflex, leading to tracheobronchial soiling which predisposes to chest infection.
Thyroid function should be checked, as there may occasionally be associated
thyroid abnormalities. There may also be some myocardial degenerative change
associated with myasthenia, and consideration should be given to pre-operative echocardiography.Opinions
vary as to the best way to manage anticholinesterase therapy, but on balance it
is probably best to continue it on the day of surgery. Patients will be
reluctant to omit their pyridostigmine
altogether, and post-operative respiratory function
will probably be better if it is continued.
However, omitting the anticholinesterase may allow
avoidance of neuromuscular blocking drugs during surgery.
Thymectomy is usually performed via a median sternotomy,
although a limited upper hemisternotomy or a trans-cervical approach (similar to
the incision for mediastinoscopy) may also be used. There is little difference
in functional outcome between these approaches, although there is less
disruption of chest mechanics with the transcervical approach, which may make
it easier to
avoid prolonged post-operative mechanical ventilation.
Median sternotomy is easier for the surgeon and allows
for more radical surgery, which is better for large masses or suspected
thymoma.
An endobronchial tube is rarely required with any
surgical approach for thymectomy. There is little to choose between anesthetic
agents, provided
the problems associated with myasthenic patients are
appreciated. Following induction of anesthesia immediate assisted ventilation
may be required, even before neuromuscular blocking drugs are given. In severe
myasthenia neuromuscular blocking drugs may be avoided completely, as the
muscle-relaxing effect of volatile anesthetics is enhanced. Competitive
neuromuscular blocking drugs are not contraindicated in myasthenic patients if
used in small doses with adequate monitoring.
Suxamethonium is best avoided, though it rarely causes
problems, as myasthenic patients are resistant to it and a prolonged phase II
block
may develop (myasthenic patients do not fasciculate following
depolarizing neuromuscular blocking drugs).
Median sternotomy is the least painful approach to
major thoracic surgery, but adequate analgesia is vital for effective chest
physiotherapy postoperatively.
A thoracic epidural is effective in the early
post-operative period, but concerns about respiratory depression from opioid
analgesia in the presence of neuromuscular disease should not prevent adequate
post-operative analgesia. Tracheostomy was routine when thymectomy was first
introduced, but in modern practice extubation at the
end of the procedure should be the aim.
However, this is not always possible and around 50% of
patients who have a trans-sternal thymectomy require prolonged mechanical
ventilation.
Scoring systems have been devised to predict the need
for post-operative ventilation, but there is debate about the reliability of
these and patients
should be considered individually. Severity of disease (Osserman
groups III and IV), low forced vital capacity (<15 ml/kg), surgery
viamedian sternotomy,
a history of respiratory failure secondary to
myasthenia, and pre-operative steroid therapy are all associated with prolonged
post-operative ventilation.
Osserman classification:
ossooogravis.
Group I Ocular symptoms only
Group IIA Mild generalized weakness
Group IIB Moderate bulbar and skeletal symptoms
Group III Acute severe disease (with respiratory compromise)
Group IV Chronic severe diseas
