- • Insidious nonspecific symptoms include headache
and memory impairment.
- • Elevated Pco2 on
blood gas analysis is a late finding and is necessary for diagnosis.
- • Vital capacity and maximal inspiratory pressure
may help with etiology and severity.
- • Important causes include neuromuscular disease,
chronic obstructive pulmonary disease (COPD), and central hypoventilation.
The term respiratory failure specifies
the failure of the entire integrated respiratory system, including
the lungs, chest wall, and brain, to maintain adequate oxygenation
of arterial blood and proper elimination of carbon dioxide (Figure
26–1). Ultimately, respiratory failure interferes
with respiration at the cellular level. Somewhat arbitrary values
for arterial oxygen tension (Pao2)and
arterial carbon dioxide tension (Paco2)
define respiratory failure, as gas exchange at the tissue level
cannot be easily measured. These values are influenced by altitude
of residence, age, and metabolic processes as well as by breathing
itself. At altitude, oxygenation and Paco2 are
both lower than at sea level. With advanced age, oxygenation worsens
and Pco2 may rise slightly.
Respiratory failure for a patient at rest at sea level is defined
by a Pao2 or Paco2 far outside the normal
range: usually below 60 mm Hg for Pao2 or
above 45–50 mm Hg for Paco2.
Mechanisms of chronic ventilatory failure.
Ventilatory failure is generally
diagnosed by the presence of an arterial Pco2 above
45 mm Hg. This may result from a decreased central nervous system
respiratory drive to breathe or be secondary to impaired function
of the respiratory muscles and/or mechanical abnormalities
of the chest wall. Most often, however, ventilatory failure is caused
by diseases affecting the lung mechanics, such as chronic obstructive
lung disease (Figure 26–1). Hypoxemia in the setting of
ventilatory failure results from low alveolar oxygen tension (Pao2) and worsens with increasing
arterial carbon dioxide tension (Paco2).
Progressive worsening of ventilation (progressive hypoventilation),
even in the absence of intrinsic lung disease, will not only increase
arterial Paco2 but also decrease
alveolar (Pao2) and arterial
(Pao2) oxygen tension.
The alveolar gas equation relates
ventilatory failure to hypoxemia, as an increased Paco2 has a reciprocal relation
Eq. (1)]. A derivative of this equation, the alveolar–arterial oxygen difference [Appendix,
Eq. (2)], should be used at the bedside to distinguish
ventilatory failure from hypoxemic respiratory failure in which
ventilation is preserved but oxygen levels are low. Hypoxemic respiratory failure is usually
acute and is caused by conditions such as pneumonia, acute respiratory
distress syndrome (ARDS), or pulmonary embolism (see Chapter 15: Acute Respiratory Distress Syndrome).
Ventilatory respiratory failure,...