Frank P. Primiano, Jr., PhD
Amethyst Research, LLC
fpp@amethyst-research.com
San Diego, CA

Introduction

Mechanical ventilatory support should be initiated whenever a patient is unable to maintain gas exchange such that death would occur were support not provided (1). Respiratory failure can be arbitrarily defined to exist, for sea level, when two or more of the following four conditions exist (2):

i) Acute dyspnea
ii) PaO2 < 50 mmHg in room air
iii) A PaCO2 > 50 mmHg
iv) Significant respiratory acidemia.

General Indications for Mechanical Ventilation (3, 4, 5)

• Acute or Impending Ventilatory Failure (elevated PaCO2 [> 50 mmHg] with pH < 7.30)
• Severe Oxygenation Deficit in Spite of Administration of Enriched Oxygen Mixtures (PaO2 < 60 mmHg on FiO2 > 0.6)
• Secretion/Airway Control
• Apnea, Respiratory Arrest (especially in neonates)

Conditions that could Necessitate Mechanical Ventilation (1, 2, 3, 6, 7)

Diseases

• Acute Obstructive Disease (e.g., acute severe asthma, airway mucosal edema)
• Altered Ventilatory Drive (e.g., hypothyroidism, idiopathic central alveolar hypoventilation, dyspnea-related anxiety, apnea of prematurity, intracranial hemorrhage)
• Cardiopulmonary Problems (e.g., congestive heart failure; in neonates: persistent bradycardia, massive pulmonary hemorrhage)
• Chest Wall Deformities (e.g., kyphoscoliosis, severe obesity, rheumatoid spondylitis; in neonates: hypercompliant rib cage [prematurity], large diaphragmatic hernia)
• Chronic Obstructive Pulmonary Disease (e.g., emphysema, chronic bronchitis, asthma, bronchiectasis, cystic fibrosis)
• Chronic Restrictive Pulmonary Disease (e.g., pulmonary fibrosis)
• Neuromuscular Disease (e.g., polio myelitis, Duchenne muscular dystrophy, amyotrophic lateral sclerosis, Guillain-Barre syndrome, peripheral neuropathies, malnutrition, cancer, infections)
• Atelectatic Disease (e.g., ARDS, neonatal RDS, hyaline membrane disease, pneumonia)

External Interventions

• Burns and Smoke Inhalation (e.g., surface burns, inhalation injury)
• Chest Trauma (e.g., blunt chest injury, penetrating injuries, flail chest, rib fractures, thoracotomy)
• Fatigue/Atrophy (muscle overuse, disuse)
• Head/Spinal Cord Injury (e.g., neurogenic pulmonary edema, Cheyne-Stokes breathing, apnea from severe insult, medullary brainstem injury)
• Postoperative Conditions (e.g., thoracic and cardiac surgeries, apnea from unreversed anesthesia)
• Pharmocological Agents/Drug Overdose (e.g., long-term adrenocorticosteroids, aminoglycoside antibiotics, Ca+ channel blockers, muscle relaxants, barbiturates)

Complications of Mechanical Ventilation (1, 3, 4, 6)

Positive Pressure Ventilation

Because of the positive pressure it produces, positive pressure ventilation causes some degree of hemodynamic compromise (e.g., hypotension, decreased cardiac output). This can be controlled usually by administration of fluids, or, in severe cases, vasoactive drugs. Other complications of positive pressure ventilation include: pulmonary barotrauma (pneumothorax, subcutaneous emphysema, interstitial pulmonary emphysema, pneumomediastinum, pneumopericardium, pneumoperitoneum [transdiaphragmatic], and air embolus), localized pulmonary hyperinflation, nosocomial infections (pneumonia), and increased intracranial pressure (cerebral edema). In addition to these conditions, non-invasive positive pressure ventilation can also produce its own unique complications, such as skin breakdown and gastric distension. However, these do not occur often and, when they do, are generally not severe.

Negative Pressure Ventilation

It is possible that negative pressure ventilation could cause localized pulmonary hyperinflation. It can be uncomfortable and cumbersome, can elicit upper airway obstruction, and can fail to suppress inspiratory muscle activity. Depending on how the negative pressure ventilation is administered, e.g., full body capsule (iron lung), venous pooling in the gut may occur.

High Frequency Ventilation

Air trapping is a potential problem when high frequency ventilation is used in obstructive lung disease because of the short expiratory times at the frequencies employed. High Frequency Oscillatory Ventilation (HFOV) can decrease cardiac output compared to conventional ventilation if, as often happens, its administration is begun at higher mean airway pressures than those used for conventional ventilation. Mucus can build up in the airways during HFOV. High frequency jet ventilation can cause airway injury if humidity is insufficient.

References

1. Hess DR and Kacmarek RM: Essentials of Mechanical Ventilation. McGraw-Hill, NY, 1996.

2. Burton GG, Hodgkin JE and Ward JJ: Respiratory Care, 3rd Edition. JB Lippincott Co., Philadelphia, PA, 1991.

3. Tobin MJ (Ed.): Principles and Practice of Mechanical Ventilation. McGraw-Hill, NY, 1994.

4. Lipschik G: Introduction to Mechanical Ventilation. Lecture notes, VA Hospital, Philadelphia, PA, 2000.

5. Dantzker DR, MacIntyre NR and Bakow ED: Comprehensive Respiratory Care. WB Saunders Co., Philadelphia, PA, 1995.

6. Sinha SK and Donn SM: Manual of Neonatal Respiratory Care. Futura Publishing Co., Inc., Armonk, NY, 2000.

7. JP Goldsmith and EH Karotkin: Assisted Ventilation of the Neonate. W.B. Saunders Co., Philadelphia, PA, 1988