Page 686 - Clinical Application of Mechanical Ventilation

652 Chapter 19

Patient Monitoring

In addition to blood gases and vital signs, other cardiopulmonary monitoring in-
With cardiogenic cluded chest radiography and hemodynamic measurements. Her chest radiograph
pulmonary edema, the PCWP is
usually elevated (.18 mm Hg). at that time revealed initial findings consistent with ARDS including a uniform
reticulogranular pattern or ground glass appearance throughout both lung fields.
Arterial and pulmonary artery catheters were used to continuously monitor blood
pressure, cardiac output, and hemodynamic status. Cardiac output was stable at
Pulmonary edema with
normal PCWP is likely caused 4.2 L. The pulmonary capillary wedge pressure (PCWP) was around 11 mm Hg
by lung parenchymal changes and thus ruled out cardiogenic pulmonary edema. The development of refractory
(e.g., ARDS).
hypoxemia in ARDS may be summarized as follows:
Hypoxic vasoconstriction in ARDS increases the vascular resistance in the
lungs and can diminish cardiac output. The lack of oxygen in the pulmo-
nary circulation also causes necrosis of the tissue lining of the alveolar-
capillary membrane which, in turn, induces pulmonary capillary leak into
the interstitium (third spacing) and hence, impairs gaseous exchange with
the blood. This eventually causes a severe V/Q mismatch and further causes
hypoxemia. As a result of these factors, perfusion (CO) must be maintained
and oxygenation (PaO , SaO , and CaO ) restored for effective management
2
2
2
of ARDS.
Patient Management

Management strategies for ARDS include correcting hypoxemia and acid-base dis-
turbance, restoring cardiac function, and treating the underlying disease or other
precipitating factors. This is generally accomplished with mechanical ventilation
at 6 to 8 mL/kg of body weight and the application of PEEP to correct refractory
hypoxemia. Patient tidal volume is based on an ideal body weight calculation.
This is calculated from the following equation:

Male: 50 1 2.3 (Height in Inches 2 60) 3 6 mL
Hypoxemia caused by Female: 45.5 1 2.3 (Height in Inches 2 60) 3 6 mL
intrapulmonary shunting is
usually managed by mechani-
cal ventilation with PEEP. A high level of positive pressure may be required to produce adequate ventila-
tion and oxygenation, but its adverse effect on cardiac function must be monitored
carefully. Some patients may benefit from pressure-controlled ventilation (PCV) to
limit the mean airway and alveolar pressures. In PCV mode, the inspiratory time
is increased and pressures are generally reduced, but the mean airway pressure may
remain nearly the same or slightly increased. This ultimately shortens the expira-
tory time, inverts the I:E ratio, and may potentially increase alveolar ventilation. As
inspiratory time (T ) increases and the elastic limit of the lung is reached, intrinsic
I
PEEP increases the risk of air trapping and thus, may also increase the PaCO while
2
concomitantly increasing the occurrence of pneumothorax in an already stiff lung.
These important adjuncts must be kept in fine balance, however, with each factor
receiving equal consideration. To counter these trends, high frequency oscillation

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