Anesthetic consideration for anterior mediastinal mass resection

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