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Authors: Unknown

770 AClITE CARE HANDBOOK FOR PIIYSICAL TIIERAI'ISTS

Oxygen Therapy

The general indication for 0, therapy is hypoxemia. Hypoxemia is

considered to be present when the arterial oxyhemoglobin saturation

(Sao,) is less than 90%, corresponding to an arterial blood 0, partial

pressure (Pao,) of less than 60 mm Hg.' Refer to Table 2-4 for the relation between 0, saturation as measured by pulse oximetry (Spo,) and Pao, and to Figure 2-7 for the oxyhemoglobin dissociation curve. The

goal of 0, therapy is to treat and prevent hypoxemia, excessive work

of breathing, and excessive myocardial work by increasing the Pao,.3

0, moves across the alveolar-capillary membrane by diffus-ioll,

the physiologic mechanism by which gas moves across a membrane from a region of higher to lower pressure and is driven by the partial pressure gradient of 0, between alveolar air (Pao,) and

pulmonary capillary blood. To improve diffusion, a rise in I'ao,

can be attained by increasing the fraction of inspired 0, IFio,)

with supplemental 0,'

Supplemental 0, is delivered by variable performance ITable lIl

A.l) or fixed performance (Table IlI-A.2) systems. Each cannula or

mask is designed to provide a range of Fio,. A variable performance

system should not be used if a specific Fio, is required. Variable performance systems are nOt intended to meet the total inspiratory requirements of the patient. The actual Fioz for a given flow rate in a

variable system is dependent on a patient's tidal volume and respiratory rate, and the type, fit, and placement of the cannula or mask. If a specific Fio, is required, then a fixed performance system is indicated.

Fixed performance systems deliver a specific Fio, despite the patient's

respiratory rate and pattern.4

0, delivery devices with masks or reservoirs allow 0, to collect

about the nose and mouth during exhalation, which increases the availability of 0, during inhalation. As the storage capacity of the mask or reservoir is increased, the Fioz for a given flow rate is also increased.s

The supplemental 02 requirements of a patient may fluctuate with

activity. Monitoring Sao, with pulse oximetry (identified as Spo,) and

subsequent titration (weaning) of 0, may be indicated during exercise. The physician may order parameters for resting and exercise Spo, if a patient has a low activity tolerance or an abnormally low

Spo, at baseline.

A patient with chronic obstructive pulmonary disease who has

chronic carbon dioxide retention may become desensitized to the

respiratory stimulant effects of carbon dioxide. In these patients,

ventilation is driven by means of a reflex ventilatory response ro a