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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.
..