Page 381 - Clinical Application of Mechanical Ventilation

Ventilator Waveform Analysis 347

pressure (e.g., 210 cm H O intrathoracic versus 22 cm H O at ventilator). Under
2
2
this condition, the metabolic work (O consumption) can be greatly increased.
2
Oxygenation and ventilation are being compromised compared to normal sponta-
neous breathing, especially if sensitivity is set too low.

Dyssynchrony during Constant Flow Ventilation

A lack of ventilator sensitivity to the patient’s inspiratory effort may lead to patient-
(Figure 11-28) Insuf-
ficient flow (letter a) or insuf- ventilator dyssynchrony as presented in examples a and b in Figure 11-28. The
ficient tidal volume (letter b) graphic shows that patient-ventilator dyssynchrony occurs when increased venti-
may cause patient-ventilator
dyssynchrony. latory demands are not met by a sufficient flow or volume. Whether a particular
pattern of dyssynchrony can be proven to be excessive or not is a complicated issue
and beyond the scope of this chapter. Perfect waveforms are not necessary. When
signs of patient-ventilator dyssynchrony occur, decisions about ventilator manage-
ment require a competent clinical assessment. The first sign of increased WOB
is tachypnea (f . 20/min) and a general appearance of agitation. Other signs of
Signs of patient- respiratory muscle stress include use of accessory respiratory muscles, intercostal
ventilator dyssynchrony
include tachypnea, agitation, retractions, and active expiration (use of abdominal muscles). In time, depending
use of accessory respiratory on a patient’s pulmonary reserve and nutritional status, signs of respiratory muscle
muscles, active expiration,
paradoxical breathing, and weakness or fatigue can develop. Paradoxical breathing pattern is the end result.
res piratory alternans. Unfortunately, the last signs of respiratory muscle weakness or fatigue are blood
gas abnormalities showing respiratory failure (Tobin et al., 1986). For this reason,
using arterial blood gas results as the primary assessment tool of competent ventila-
Tachypnea and a general tor management is a serious shortcoming.
appearance of agitation may Tachypnea and a general appearance of agitation may be caused by other factors
be caused by other factors
such as pain and psychologic such as pain and psychologic stress. Patient-ventilator synchrony may not be the
stress.
problem. But if improvement in patient-ventilator synchrony eliminates the signs
of physical and psychologic distress, quality patient care has been enhanced. The
appropriate adjustment on the ventilator to improve synchrony is easily facilitated
through waveform analysis.
The dashed line for the first pressure-time waveform a (Figure 11-28), exemplifies
the ideal waveform for a passive patient being mechanically ventilated; that is, the
ventilator is generating virtually all the pressure necessary to expand the patient’s
lungs. The solid line depicts patient-ventilator dyssynchrony and the corresponding
waveform created during mechanical ventilation. The greater the drop in pressure
and the development of irregular patterns from the ideal pattern (dyssynchrony), the
less the ventilator is assisting the patient with breathing. In fact, the dyssynchrony
can often impose more WOB onto the patient than that incurred by spontaneous
breathing off the ventilator. Physical signs of distress indicate that the patient is
enduring too much WOB. If the patient is not on the ventilator because of respira-
tory failure, some dyssynchrony may not pose a serious problem. Research suggests
that it will compromise oxygenation and ventilation compared to optimal, which
may be uncomfortable for the patient. Morbidity and mortality outcomes, how-
ever, may be the same; the patient is simply doing more of the WOB. But research
shows that if patients are recovering from respiratory muscle failure, dyssynchrony

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