Chest
Volume 103, Issue 4, April 1993, Pages 1215-1219
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Clinical Investigations in Critical Care
Prediction of Minimal Pressure Support During Weaning From Mechanical Ventilation

https://doi.org/10.1378/chest.103.4.1215Get rights and content

Our goal was to evaluate the accuracy of a prediction equation that estimates the minimal level of pressure support (PS) required to overcome the resistance of the ventilator circuit and the endotracheal tube during mechanical ventilation. We calculated the minimal PS (PSmin) by means of the relationship between airway resistance and peak inspiratory flow rate during mechanical ventilation. Measurements of breathing pattern, flow rates, work of breathing (WOB), pressure time product (PTP), and PEEPi were made during several modes of ventilation (PSmin, PSmin + 25 percent, PSmin-25 percent, flow by, CPAP 0 cm H2O) and while breathing through an endotracheal tube (ETT) and spontaneous breathing (EXT). The WOB was significantly higher during EXT than PSmin, PSmin-25 percent, and ETT (1.04 vs 0.45, 0.54, and 0.74 J/L, respectively, p<0.05). An unexpected finding was a higher WOB and PTP during EXT as compared with ETT in six of seven of our patients (1.04 vs 0.74 J/L). Examination of breathing pattern and flow volume loops in these two breathing modes raises the possibility that the post-EXT pathology increases in WOB is related to upper airway abnormality. Because of this, our predicted PSmin underestimated the WOB required for spontaneous breathing immediately post EXT.

Section snippets

MATERIAL AND METHODS

Seven patients were studied at the time that they were considered weanable by their primary physicians. There were five male and two female subjects. The mean age of the patients was 71 years (range, 35 to 84 years) and the mean time of intubation was 7.5 days (range, 2 to 12 days). Three of the patients had pneumonia; one each had COPD, interstitial lung disease, and congestive heart failure, respectively, and one of the patients had taken a drug overdose. All patients were ventilated by means

RESULTS

The mean PSmin was 7.0 cm H2O with a range of 4 to 10 cm H2O. The mean VT, RR, WOBpat, PTP, and PEEPi are shown in Table 2. As the level of pressure support was decreased, the WOBpat increased (Fig 1). Progressive increases in the WOBpat were noted in the FB and CPAP modes of ventilation. With the ETT in position, but with the patients off the ventilator, WOBpat was less than CPAP. Immediately post-EXT, the WOBpat increased as compared with breathing through the ETT. This change was evident in

DISCUSSION

We found that a predictor of PSmin overestimates the PS required to simulate spontaneous breathing in our patients immediately post-EXT. Moreover, contrary to expectations, we found that the WOBpat during spontaneous breathing during EXT was higher than spontaneous breathing through the ETT. Abnormalities in the flow volume loops of these patients post-EXT suggest that upper airway abnormalities are a possible source of the increased WOB post-EXT.

The formula PIFR × R gives an estimate of the

ACKNOWLEDGMENTS:

The authors acknowledge the support of Bicore Monitoring Systems who provided the monitor and the esophageal catheters. We also acknowledge the secretarial support of Linda L. Randolph.

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Manuscript received May 6; revision accepted August 25

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