Page 379 - Clinical Application of Mechanical Ventilation

Ventilator Waveform Analysis 345

in the V delivered. Compared to example A, less volume is delivered as shown by
T
During pressure- the area under the second inspiratory flow waveform in example B. Peak flow is
controlled ventilation, a reduc-
maintained, but the slope or descent in flow rate from peak level is greater. Thus, the
tion in C LT will reduce the V T.
average flow and V are reduced (V 5 average Flow 3 T ). Letter b shows that T is
E
T
I
T
reduced since less volume is expired. The peak expiratory flow rate is about the same
since the resistance to flow is the same, and the driving pressure for expiratory flow,
the PC level set, and end-inspiratory lung pressure, was held relatively constant.
During pressure-controlled ventilation, the pressure is held relatively constant while
the flow and V delivered are altered by changing lung/airway characteristics.
T
USING WAVEFORMS FOR PATIENT-VENTILATOR
SYSTEM ASSESSMENT

Positive pressure in the lungs is not natural. Too much volume under pressure can
Patient-ventilator dys- cause barotraumas or volutrauma. Too little volume or not enough positive end-
synchrony increases the work
of breathing and prolongs expiratory pressure (PEEP) to keep alveoli from collapsing (derecruitment) and
weaning from mechanical then reopening (recruitment) during tidal ventilation can cause lung injuries. Too
ventilation.
little respiratory muscle use (disuse atrophy) leads to respiratory muscle weakness,
and excessive work of breathing (WOB) leads to respiratory muscle weakness or
fatigue. Either way, weaning from mechanical ventilation is delayed.
No research to date suggests that patients triggering all the volume- or pressure-
controlled breaths in synchrony with the ventilator will develop respiratory muscle
weakness from either too little or too much WOB. Conversely, volumes of re-
search suggest that dyssynchrony during ventilator management may impose ex-
cessive WOB and prolong mechanical ventilation (Dick et al., 1996). Research
also suggests lack of assisted ventilation (patient-triggered breaths) from prolonged
paralysis or sedation as a reason for prolonged mechanical ventilation. The best
oxygenation and ventilation should be derived at the lowest volume and least lung
pressure if patients are breathing in synchrony with the ventilator, which obviates
excess WOB and mitigates the potential for excessive lung pressures. Auto-PEEP
can be easily observed and corrected using graphics, which reduces the potential
for volume trauma. Also, patient-ventilator synchrony can be easily verified and
controlled through waveform analysis.

Patient-Ventilator Dyssynchrony

Dyssynchrony during mechanical ventilation increases the WOB, oxygen consump-
Dyssynchrony during me- tion, minute ventilation, and myocardial work. Dyssynchrony can occur with the
chanical ventilation increases
the WOB, oxygen consump- assisted (patient-triggered) or controlled (time-triggered) breaths. The first pressure
tion, minute ventilation, and waveform in Figure 11-27 depicts a detailed analysis of an assist (patient-triggered)
myocardial work.
breath. Both pressure-time waveforms depict the step ascending ramp pattern.
Thus, a constant (square) flow pattern must have been set to produce these pressure
waveforms. The 22 cm H O value below baseline (zero) represents the sensitivity
2
threshold level setting. In the first waveform, the patient’s effort to trigger the breath

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