Page 147 - Clinical Application of Mechanical Ventilation

Operating Modes of Mechanical Ventilation 113

Airway Pressure-Release Ventilation
Volume Pressure

P high and T high P low and T low T high :T low = 4:1 Spontaneous breaths occur
at any point without altering
the ventilator-delivered
breaths
Biphasic Positive Airway Pressure
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P high and T high P low and T low T high :T low = 1:1 2 4
Figure 4-12 APRV and BiphasicPAP waveforms. In biphasicPAP, the patient spends more time
at the low pressure level. (Reference: Mireles-Cabodevila, 2009)
INVERSE RATIO VENTILATION (IRV)

The ratio of inspiratory time (I time) to expiratory time (E time) is known as the I:E
ratio. In conventional mechanical ventilation, the I time is traditionally lower than
the E time so that the I:E ratio ranges from about 1:1.5 to 1:3. This resembles the
normal I:E ratio during spontaneous breathing, and it is considered physiologically
beneficial to normal cardiopulmonary function.
Since the mid-1980s, investigators have been extending the inspiratory time
during mechanical ventilation to promote oxygenation in patients with ARDS
(Gurevitch et al., 1986; Marcy et al., 1991). The inverse I:E ratio in use is between
2:1 and 4:1 and often it is used in conjunction with pressure-controlled ventilation
(Lain et al., 1989; Tharratt et al., 1988).

Physiology of IRV

Inverse ratio ventilation (IRV) improves oxygenation by (1) reduction of intrapul-
Inverse ratio ventilation monary shunting, (2) improvement of V/Q matching, and (3) decrease of deadspace
(IRV) improves oxygenation ventilation. From the review of available literature, Shanholtz et al. et al. (1994) con-
by (1) intrapulmonary
➞
shunting, (2) ➞ V/Q match- cluded that these mechanisms were also achievable by use of conventional ventilation
ing, and (3) deadspace
➞
ventilation. with PEEP. However, two notable changes are observed during IRV. They are (1) in-
crease of mean airway pressure and (2) presence of auto-PEEP. These two changes are
likely the reason for the improvement of shunting and hypoxemia in ARDS patients.
Increase of Mean Airway Pressure. To achieve the same degree of ventilation and oxy-
The increase in mPaw genation, IRV requires a lower peak airway pressure and PEEP, but a higher mean
during IRV helps to reduce
shunting and improve oxy- airway pressure (mPaw) than conventional mechanical ventilation. The increase in
genation in ARDS patients. mPaw during IRV helps to reduce shunting and improve oxygenation in ARDS
patients (Shanholtz et al., 1994).

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