i bc27f85be50b71b1 (257 page)

820 AClITE. CARE HANDBOOK FOR PHYSICAL THERAI'ISTS

Table ill-B.2. Corltj,med

Mode

Characteristics

Airway pressure

Lungs are kept inflated with a preset airway presrelease ventilation

sure, and exhalation occurs during cyclic reduc

(APRV)

tions in pressure.

Advocated for protecting thc lung from high peak

airway pressures, although no benefit ha been

demonstrated over conventional methods.

High-frequency jct

Uses a nozzle and injector to deliver jets of gas directly

ventilation

into the lung at high rates.

Attempts to reduce mean airway pressurc; howcver, no

benefits have been identified.

Partial liquid

Uses perfluorocarbon liquids (docs not mix with

ventilation

surfacranr and has a high solubility for 02 and

CO2). Lungs are filled with the liquid to

approximately functional residual capacity; then,

standard mechanical ventilation is attempted.

Has nor demonstrated benefits above conventional

ventilator modes.

ACV = assist/comrol ventilation; AROS = adult respiratOry distress syndrome; COPO

= chronic obstructive pulmonary disease; I:E rario = inspiratOry lime 10 expIratory time

ratio; PEEP = positive end-expiratory pressure; PSV = pressure supported ventilation;

SIMV = synchronous intcrmittcnr mandatory ventilation; VT = tidal volume.

Sources: Data from P Marino. The ICU Book (2nd cd). Philadelphia: Lea & Fcbiger,

1998; AS Slutsky. Mechanical ventilation. American College of Chest Physicians' Consensus Conference Isee commentsj. Chest 1993; I 04: 1833; and SF Howman. Mechanical ventilarion: a review and updare for clinicians. Hospital Physician I 999;Oecember: 26-36.

as dynamic hyperinf/atioll.2•J The primary consequence of dynamic

hyperinflation is increased air trapping, which results in physiologic

dead space due to pulmonary shunting (perfusion is delivered to alveolar units that are not receiving fresh ventilation), which decreases gas exchange. Ultimately, this leads to an increased work of breathing

owing to higher respiratory demand, as well as altered length-tension

relationships of the inspiratory muscles. Auto PEEP and concomitant

air trapping can occur when the minute ventilation or respiratory rate

is too high, the inspiratory-expiratory ratios are not large enough, or

the endotracheal tube is tOO narrow or kinked, when there is excess

water condensation in the tubing, or in patients with obstructive lung

disease. A combination of the aforementioned factors can increase the

likelihood of auto PEEP. In patients with chronic obstructive pulmo-

Table lli-B.3. Ventilator Serrings

Purpose

Setting

Characteristic

Oxygen

Fraction of

The percentage of inspired air that is oxygen; at normal respiratory rare (RR), tidal volume

ation

inspired oxy

(VT), and flow rates, an FlO! of 21 % (ambient air) yields a normal oxygen partial pressure of

gen (FIO,)

95-100 mm Hg; an increase in the percentage of oxygen delivered to the alveoli results in a

greater Paol and therefore a greater driving force for the diffusion of oxygen 1Oto the blood.

FIOZ of 60% h-as been set as the threshold .... alue ro aVOid toxiclry with prolonged use.

Positive end

The pressure maintained by the mechanical ventilator in (he air""ays at (he end of expiration;

expiratory

normal physiologic PEEP (maintained by sufficient sutfactant levels) is considered to be 5

pressure

em

>

:g

(PEEP)

Semngs

adjusted as needed to maintain functional residual capaclry above closing capacity

�

Z

to avoid closure of alveoli.

"

X

Closure of alveoli can result in shunting of blood past the alveoli without gas exchange, which

results in decreased oxygenation.

..