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
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