Chapter 7: Mechanical Ventilation

INTRODUCTION

Ventilator management for most thoracic surgery patients involves two distinct phases: (1) intraoperative and (2) postoperative. Intraoperative ventilator management of thoracic surgery patients by the anesthesiologist has been discussed in Chapter 5. Postoperative management of mechanical ventilation by the thoracic surgeon and intensivist is discussed in this chapter.

EFFECTS OF ANESTHESIA AND THORACIC SURGERY ON RESPIRATORY SYSTEM PHYSIOLOGY

It is important to appreciate that general anesthesia and thoracic surgery adversely affect nearly all aspects of respiratory physiology in the postoperative state. These include anesthetic-related alterations in respiratory drive, reductions in lung volume due to loss of chest wall tone, changes in the ventilation-perfusion relationship, and increased airway resistance in the setting of diminished lung volumes. Given the postoperative structural changes in the lung as well as its native state of disease, these alterations may have variable effect on overall function and may be unpredictable in duration. They certainly must be taken into account during initial ventilatory management.

POSTOPERATIVE VENTILATOR STRATEGIES

There are two basic approaches to mechanical ventilation in patients who have undergone thoracic surgery. These are (1) methods used to support postoperative patients who are kept intubated after surgery for a specific indication that is expected to resolve within hours, allowing for rapid discontinuation of ventilator support; and (2) methods used to support patients who develop hypoxic or hypercarbic respiratory failure as a consequence of a primary process that will resolve over a period of days to weeks and may require more gradual weaning.

EXTUBATION OF THE STABLE PATIENT

In most patients successful extubation can be achieved when the patient (1) has normal Pao₂ and Paco₂ blood gas values on minimal ventilator support; (2) has intact mental status; (3) has reasonable assurance that they will have the ability to cough and protect their airway; and (4) has been started on an analgesic protocol that optimizes respiratory mechanics without causing undue respiratory depression.

Although mental status is usually simple to assess, often it is not possible to confirm intact recurrent laryngeal nerve (RLN) function before attempting extubation simply by relying on the patient’s ability to cough and swallow secretions. The risk of injury to the RLN is increased in the thoracic surgery population because many procedures involve anatomic dissection or traction on structures near the left mainstem bronchus where the RLN branches from the vagus.¹ Postextubation evaluation revealing a weak voice and ineffective cough should prompt direct laryngoscopic evaluation of the hypopharynx and vocal cords, followed by vocal cord medialization if indicated.²

Several factors contribute to respiratory muscle dysfunction after thoracic surgery. Pain is a major contributor. Thus, selection of an appropriate analgesic regimen is essential for preventing postoperative respiratory failure.³ Studies of respiratory muscle function also have demonstrated that diaphragmatic contractility is ...