Ashfaq Hasan

Historical Aspects of Mechanical Ventilation

As early as in the fifth century bc, Hippocrates, described a technique for the prevention of asphyxiation. In his work, “Treatise on Air,” Hippocrates stated, “One should introduce a cannula into the trachea along the jawbone so that air can be drawn into the lungs.” Hippocrates thus provided the first description of endotracheal intubation (ET).4,10

Physiological Considerations in the Mechanically Ventilated Patient

The volume of the upper airway is approximately 72 mL in the adult subject.64 An endotracheal tube of 8 mm internal diameter cuts down this volume by 55-60 mL or by approximately 1 mL/kg body weight.26 By thus reducing the upper airway volume - and the dead-space - this can increase the alveolar ventilation. In health, it appears that the volume of the upper airway can change by as much as 50% by mere changes in head position. Therefore, the diminution in airway volume that occurs when an endotracheal tube is placed may not be greatly beyond the physiological changes that occur in the innate airway.64 In fact, the interposition of a Y-connector adds approximately 75 mL of dead-space to the circuit, and so the impact of the endotracheal tube in reducing the dead-space is largely negated.

Nonconventional Modes and Adjunctive Therapies for Mechanical Ventilation

The science of mechanical ventilation is as yet imperfect. As newer innovations further the frontiers of artificial life support, it is clear that this area will continue to burgeon in the foreseeable future. Several innovations have fallen by the wayside despite showing early promise, but other exciting options have appeared on the horizon. What follows is a brief discussion of the (as yet) unconventional modes of ventilation, some of which have already gained a measure of acceptance.

Ventilator-Associated Pneumonia

The area of the alveolar epithelium of the lung is approximately 70 m2. This area is constantly in contact with the ambient air and is therefore vulnerable to contamination with airborne microbes and particles of respirable size. Due to the configuration of the respiratory tract, airborne particles having diameters in the range of 0.5-2.0 μ can reach and deposit in the terminal part of the tracheobronchial tree - most bacteria are of this size. In reality, very few bacteria cause infections by spreading via the airborne route (e.g., mycobacteria, viruses, and legionella). Most bacteria cause pneumonia by first colonizing the upper respiratory tract and later descending into the tracheobronchial tree.

Noninvasive Ventilation in Acute Respiratory Failure

Noninvasive ventilation (NIV) entails the administration of positive pressure breaths through the patient’s innate airway by means of a close-fitting mask.

The Complications of Mechanical Ventilation

Mechanical ventilation, though potentially lifesaving, is capable of producing complications, some of which may in themselves be life threatening. Several of these complications are related to endotracheal intubation. Critically ill patients are often immunosuppressed and are susceptible to nosocomial pneumonia for a number of reasons. Endotracheal intubation may predispose these patients to ventilator-associated pneumonia (VAP), which is a form of nosocomial pneumonia. A large number of complications are a direct result of generation of positive pressure inside the thorax (e.g., barotrauma and hypotension) and these will be dealt with later in this chapter.

Airway Humidification in the Mechanically Ventilated Patient

Inhaled air is cold and dry. The highly vascularized nasal mucosa warms and humidifies it, and makes it suitable for breathing.

Mechanical Ventilation in Specific Disorders

In myocardial ischemia, the goal of mechanical ventilation is to decrease the work of breathing and thereby the oxygen demands of the respiratory muscles. When the work of breathing is high, as much as 40% of the cardiac output can be diverted to the respiratory muscles69: myocardial ischemia will worsen, and a positive feedback cycle is established (Fig. 9.1). Ventilating patients with strategies that unload the respiratory muscles can be expected to improve myocardial perfusion and break the vicious cycle.

Discontinuation of Mechanical Ventilation

In its truest sense, weaning implies a gradual separation of the patient from the ventilator. During this gradual process, the patient is given increasing responsibility for his breathing, culminating in spontaneous unassisted respiration. In actual fact, abrupt separation of the patient from the ventilator is possible, and so the term weaning has been broadened to include rapid separation from the ventilator - as generally occurs in postoperative circumstances - in addition to the more gradual separation as seen, for example, in patients with chronic obstructive lung disease.37

Negative Pressure Ventilation

In essence, the negative pressure ventilator comprises a rigid shell that partly or completely encloses the patient’s torso, the pressure within which can be dropped by means of an attached pump.27 Air enters the lungs as a result of the fall in pleural pressure produced by expansion of the thoracic cage. Expiration is passive. Negative pressure ventilators were introduced in the middle of the nineteenth century and preceded positive pressure devices by nearly a century (see Chap. 1) (Fig. 14.1).