Anesthesia for Pediatric Thoracic Surgery

Anatomy

Pediatric patients have several key differences in anatomy as compared with the adult, of which anesthesiologists must be aware to successfully take care of these patients during thoracic surgery. The most prominent airway characteristics are listed here :

• Large head, short neck, and a prominent occiput.

• Tongue is relatively large.

• Larynx is high and anterior, at the level of C3–C4.

• Epiglottis is long, stiff, and U-shaped.

• Preferential nasal breathing.

• Airway is funnel shaped and narrowest at the level of the cricoid cartilage.

• Airway is small and prone to develop edema resulting in airway obstruction.

Physiology

Pediatric patients have a baseline limited respiratory reserve during normal two-lung ventilation compared with adults because of several reasons listed here :

• Horizontal ribs prevent the “bucket handle” action seen in adult breathing and limit an increase in tidal volume. Ventilation is primarily diaphragmatic.

• The chest wall is significantly more compliant than that of an adult. Subsequently, the functional residual capacity (FRC) is relatively low. FRC decreases with apnea and anesthesia causing lung collapse.

• Minute ventilation is rate dependent because there few ways to increase tidal volume.

• The closing volume is larger than the FRC until 6 to 8 years of age. This causes an increased tendency for airway closure at end expiration. Thus neonates and infants generally need intermittent positive pressure ventilation during anesthesia and would benefit from a higher respiratory rate and the use of positive expiratory end-pressure (PEEP).

• Muscles of ventilation are easily subject to fatigue because of low percentage of type I muscle fibers in the diaphragm. This number increases to the adult level over the first year of life.

• The alveoli are thick walled at birth. There is only 10% of the total number of alveoli found in adults. The alveoli clusters develop over the first 8 years of life.

• Apneas are common postoperatively in premature infants.

• High oxygen (O ₂ ) consumption compared with adult.

In addition to the baseline limited respiratory reserve in two-lung ventilation, the respiratory physiology of pediatric patients is also different from adults in the lateral decubitus OLV. Unlike adults, oxygenation is higher in the nondependent lung rather than the dependent one (healthy lung), especially in neonates and infants. The reason for this is that children have an easily compressible chest wall and FRC becomes equal to the residual volume in the dependent lung, and small airways begin to close even in tidal volume values. Hydrostatic pressure between dependent and nondependent lungs is minimal because of the small size of children. Therefore the advantage of better oxygenation in the nondependent lung is not present in children. As a result, compared with adults, children are at greater risk for developing hypoxemia or airway complications during thoracoscopic surgery or thoracotomy.

Bronchoscopy

Flexible bronchoscopy is helpful in the diagnosis of chronic respiratory ailments. It allows the evaluation of the airway for fixed or dynamic changes occurring with ventilation, such as in vascular rings or tracheomalacia, respectively. Maintaining spontaneous ventilation is necessary to diagnose dynamic changes in airway patency. Intravenous sedation along with topicalization is often the preferred choice of anesthetic for older children. Younger children will often require general anesthesia, which can be facilitated with an airway device such as a laryngeal mask airway, but as mentioned previously, spontaneous ventilation must be insured. If the flexible bronchoscopy is being done for other purposes, such as for biopsy samples or performing an alveolar lavage, general endotracheal anesthesia with or without paralysis is often preferred. It is not uncommon to have to upsize the endotracheal tube (ETT) size to accommodate a flexible fiberoptic scope with a working channel.

• Rigid bronchoscopy (RB) is another diagnostic and therapeutic modality. It is most commonly used in pediatric patients for the treatment of aspirated foreign bodies to be removed through the rigid bronchoscope lumen. Because of the highly stimulating effects from the rigid bronchoscope, it is performed under general anesthesia. The plan for maintaining either spontaneous ventilation or providing controlled ventilation should be discussed and coordinated with the surgeon. The techniques can vary but can be summarized as: (1) insufflating sevoflurane on the airway of a spontaneously breathing patient; (2) intermittent controlled ventilation via the bronchoscope; or (3) intermittent jet ventilation. Each technique has its benefits and downsides. Insufflating sevoflurane on a spontaneously breathing patient, in theory, should minimize dislodgement of a foreign body but can risk hypoventilation if deeply anesthetized, or unwanted motion, cough, retching, laryngospasm and even bronchospasm if the anesthetic depth is not sufficient. Another concern is operating room pollution with volatile anesthetic and aerosol particles spread. Intermittent controlled ventilation via the bronchoscope can work for a deeply anesthetized and perhaps paralyzed patient, but has the downside of air leak through the outer part of the rigid bronchoscope leading to suboptimal tidal volumes. In addition, the positive pressure can dislodge a foreign body distally. During positive pressure ventilation, the viewing port must be occluded, interfering with the procedure, to deliver an effective breath through the ventilation port of the rigid bronchoscope. Jet ventilation offers the advantages of allowing bronchoscopic interventions to be performed without cessations. Complications include barotrauma, inability to predict fraction of inspired oxygen (FiO ₂ ) and monitor end-tidal CO ₂ , foreign body dislocation, and blowing of blood and debris materials to distal airways resulting in inadequate gas exchange. Low-frequency jet ventilation should not be used in patients with tracheobronchial mucosa injury or low respiratory compliance. A total intravenous anesthetic should be considered in cases where controlled ventilation is planned, to minimize operating room air pollution with inhaled anesthetic. It is advised to use spontaneous ventilation for the removal of proximally located foreign bodies and positive pressure ventilation for the removal of distally located foreign bodies. Regardless of the technique used, adequate depth of anesthesia and airway patency need to be ensured. The key to an uncomplicated procedure is the cooperation of the surgeon and the anesthesiologist.

It should be considered that aspirated foreign body retrieval is usually performed emergently. Foreign bodies can swell, progressively occluding the airway, can cause an inflammatory reaction on the airway, or can cause airway perforation leading to pneumomediastinum or subcutaneous emphysema. Fortunately, most patient do not present with many comorbidities because there is minimal time for optimization. Premedication targeting a decrease in the production of saliva and anxiety is beneficial. Albuterol sulfate and budesonide inhalations are reported to be advantageous in reducing perioperative pulmonary complications. The knowledge of the type, place, and aspiration time of the foreign body is important. If it is above the carina, there is a total obstruction risk, which could be precipitated with positive pressure ventilation. Inhalational or intravenous agents or both may be administered. During maintenance, short-acting agents, such as propofol, dexmedetomidine, remifentanil and short-acting muscle relaxants are administered as the procedure is generally quite short. Nitrous oxide is not recommended because many patients undergoing RB have air trapping to some extent.

Severe complications can occur during RB, which are closely related to the patient’s condition, the severity of the pathology, the experience of the surgeon and the anesthesiologist. Failure to retrieve the foreign body might necessitate an emergent thoracotomy or tracheostomy.