Page 529 - Clinical Application of Mechanical Ventilation

Critical Care Issues in Mechanical Ventilation 495

(e.g., .50 mm Hg). The tidal volume used in permissive hypercapnia is in the range
In permissive hyper- of 4–7 mL/kg (Feihl et al., 1994). Since the plateau pressure (i.e., end-inspiratory oc-
capnia, the tidal volume
is titrated until the PIP is clusion pressure) is the best estimate of the average peak alveolar pressure, the tidal
near the plateau pressure
measured before the low tidal volume is titrated until the PIP is near the plateau pressure measured before the low
volume procedure. tidal volume procedure. Using the plateau pressure as the target PIP avoids alveolar
overdistention and reduces the likelihood of lung injury (Hall, 1987; Slutsky, 1994).
Since permissive hypercapnia provides minimal ventilation with lower tidal vol-
ume and pressure, it can be a safe mechanism to protect the lungs of patients with

Tromethamine (THAM) ARDS (Feihl et al., 1994; Hickling et al., 1990). (Refer to Chapter 12 for the
is a nonbicarbonate buffer mechanism and physiologic changes of permissive hypercapnia.)
that helps to compensate for
metabolic acidosis by directly The elevated PaCO and acidosis during permissive hypercapnia can lead to CNS
2
decreasing the hydrogen ion dysfunction, increase in intracranial pressure, neuromuscular weakness, cardiovas-
concentration and indirectly
decreasing the carbon dioxide cular impairment, and increased pulmonary vascular resistance. These complica-
level. It is preferable to bicar- tions of permissive hypercapnia may be alleviated by returning the pH to its normal
bonate in patients undergoing
permissive hypercapnia. range, either by renal compensation over time or by neutralizing the acid with bicar-
bonate or tromethamine (a nonbicarbonate buffer) (Marini, 1993).

Decremental Recruitment maneuver to Determine
optimal PEEP

ARDS and PEEP. For patients with ARDS, the potential for lung injury is high be-
cause different lung units have different pressure requirements. Lung units with
low compliance require high opening and sustaining pressures. These high airway
pressures can overstretch and injure the normal compliant lung units.
In mechanical ventilation, PEEP is used to prevent repetitive recruiting and dere-
cruiting of the atelectatic portion of the lung (i.e., lung units with low compliance).
PEEP also enhances alveolar ventilation, improves hypoxemia due to intrapulmo-
nary shunting, and reduces total lung water. When PEEP is used in patients with
ARDS, it carries many detrimental effects. The primary complication of PEEP is
pressure-induced lung injury (due to increased mean airway pressure) and volume-
induced lung injury (due to overdistension of alveoli). Since PEEP raises the peak
inspiratory pressure and the combined pressure is transmitted to the pleural space,
it increases the pulmonary vascular resistance, decreases left-ventricular compliance,
venous return, cardiac output, and systemic oxygen delivery. The reduction in car-
diac output in turn causes renal insufficiency, decreased urine output, and increased
sodium and water retention (Kallet et al., 2007; kznhealth.gov, 2011). Due to these
potential complications, the selection of an optimal PEEP is crucial.
The ARDSNet recommends In 2000, the Acute Respiratory Distress Syndrome Network (ARDSNet) pub-
keeping the P PLAT , 30 cm H 2 O
with low tidal volume and using lished the following initial ventilator settings and combination of F O and PEEP
2
I
a combination of F I O 2 and PEEP for patients with ARDS (Table 15-3). Following initial setup and stabilization,
(See Table 15-3) to maintain O 2
sat >88%. subsequent PEEP levels may be titrated to obtain the optimal PEEP based on the
patient’s requirement.
Titration of Optimal PEEP. The methods to obtain an optimal PEEP have been under-
going changes over the years. Earlier methods used compliance, PaO , SpO , lung
2
2

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