• Tuberculosis and Respiratory Diseases
• /
• v.44 no.2
• /
• pp.329-337
• /
• 1997

Background : Since endotracheal tube is the most important factor involved in the imposed work of breathing during mechanical ventilation, extubation of endotracheal tube is supposed to reduce respiratory work of patient. However, some patients show labored breathing after extubation despite acceptable blood gases. We investigated the changes of work of breathing before and after extubation and the factors involved in the change of WOB after extubation. Methods : The subjects were 34 patients(M : F = 20 : 14, mean age = $61{\pm}17yre$) who recovered from respiratory failure after ventilatory support and were considered to be ready for extubation. The patients with clinical or radiologic evidences of upper airway obstruction before endotracheal intubation for mechanical ventilation were excluded. Vital sign, physical examination, chest X-ray, work of breathing and other respiratory mechanic indices were measured prior to, immediately, 6, 24 and 48 hours after extubation serially. Definition of weaning failure after extubation was resumption of ventilatory support or reintubation of endotracheal tube within 48 hour after extubation because of respiratory failure. The patients were classified into group 1(decreased work of breathing), group 2(unchanged work of breathing) and group 3(increased work of breathing) depending on the statistical difference in the change of work of breathing before and after extubation. Results : Work of breathing decreased in 33%(11/34, group 1), unchanged in 41%(14/34, group 2) and increased in 26%(9/34, group 3) of patients after extubation compared with before extubation. Weaning failure occurred 9%(1/11) of group, 1, 28.6%(4/14) of group 2 and 44%(4/9) of group 3 after extubation(p = 0.07). The change of work of breathing after extubation was positively correlated with change of mean airway resistance(mRaw). (r = 0.794, p > 0.01). In three cases of group 3 whose respiratory indices could be measured until 48 hr after extubation, the change in work of breathing paralleled with the sequential change of mRaw. The work of breathing was peaked at 6 hr after extubation, which showed a tendency to decrease thereafter. Conclusions : Reversible increase of work of breathing after extubation may occur in the patients who underwent extubation, and the increase in mRaw could be responsible for the increase in work of breathing. In addition, the risk of weaning failure after extubation may increase in the patients who have increased WOB immediately after extubation.

• Tuberculosis and Respiratory Diseases
• /
• v.44 no.3
• /
• pp.592-600
• /
• 1997

Background : The level of imposed work of breathing (WOB) is important for patient-ventilator synchrony and during weaning from mechanical ventilation. Triggering methods and the sensitivity of demand system are important determining factors of the imposed WOB. Flow triggering method is available on several modern ventilator and is believed to impose less work to a patient-triggered breath than pressure triggering method. We intended to compare the level of imposed WOB on two different methods of triggering and also at different levels of sensitivities on each triggering method (0.7 L/min vs 2.0 L/min on flow triggering ; $-1\;cmH_2O$ vs $-2cm\;H_2O$ on pressure triggering). Methods : The subjects were 12 patients ($64.8{\pm}4.2\;yrs$) on mechanical ventilation and were stable in respiratory pattern on CPAP $3\;cmH_2O$. Four different triggering sensitivities were applied at random order. For determination of imposed WOB, tracheal end pressure was measured through the monitoring lumen of Hi-Lo Jet tracheal tube (Mallincrodt, New York, USA) using pneumotachograph/pressure transducer (CP-100 pulmonary monitor, Bicore, Irvine, CA, USA). Other data of respiratory mechanics were also obtained by CP-100 pulmonary monitor. Results : The imposed WOB was decreased by 37.5% during 0.7 L/min on flow triggering compared to $-2\;cmH_2O$ on pressure triggering and also decreased by 14% during $-1\;cmH_2O$ compared to $-2\;cmH_2O$ on pressure triggering (p < 0.05 in each). The PTP(Pressure Time Product) was also decreased significantly during 0.7 L/min on flow triggering and $-1\;cmH_2O$ on pressure triggering compared to $-2\;cmH_2O$ on pressure triggering (p < 0.05 in each). The proportions of imposed WOB in total WOB were ranged from 37% to 85% and no significant changes among different methods and sensitivities. The physiologic WOB showed no significant changes among different triggering methods and sensitivities. Conclusion : To reduce the imposed WOB, flow triggering with sensitivity of 0.7 L/min would be better method than pressure triggering with sensitivity of $-2\;cm\;H_2O$.

• Tuberculosis and Respiratory Diseases
• /
• v.45 no.2
• /
• pp.380-387
• /
• 1998

Background: Minimal pressure support(PSmin) is a level of pressure support which offset the imposed work of breathing(WOBimp) developed by endotracheal tube and ventilator circuits in pressure support ventilation While the lower applied level of pressure support compared to PSmin could induce respiratory muscle fatigue, the higher level than PSmin could keep respiratory muscle rest resulting in prolongation of weaning period during weaning from mechanical ventilation PSmin has been usually applied in the level of 5~10 cm$H_2O$, but the accurate level of PSmin is difficult to be determinated in individual cases. PSmin is known to be calculated by using the equation of "PSmin = peak inspiratory flow rate during spontaneus ventilation$\times$total ventilatory system resistance", but correlation of calculated PSmin and measured PSmin has not been known. The objects of this study were firstly to assess whether customarily applied pressure support level of 5~10 cm$H_2O$ would be appropriate to offset the imposed work of breathing among the patients under weaning process, and secondly to estimate the correlation between the measured PSmin and calculated PSmin. Method : 1) Measurement of PSmin : Intratracheal pressure changes were measured through Hi-Lo jet tracheal tube (8mm in diameter, Mallinckroft, USA) by using pulmonary monitor(CP-100 pulmonary monitor, Bicore, USA), and then pressure support level of mechanical ventilator were increased until WOBimp was reached to 0.01 J/L or less. Measured PSmin was defined as the lowest pressure to make WOBimp 0.01 J/L or less. 2) Calculation of PSmin : Peak airway pressure(Ppeak), plateau airway pressure(Pplat) and mean inspiratory flow rate of the subjects were measured on volume control mode of mechanical ventilation after sedation. Spontaneous peak inspiratory flow rates were measured on CPAP mode(O cm$H_2O$). Thereafter PSmin was calculated by using the equation "PSmin = peak inspiratory flow rate$\times$R, R = (Ppeak-Pplat)/mean inspiratory flow rate during volume control mode on mechanical ventilation". Results: Sixteen patients who were considered as the candidate for weaning from mechanical ventilation were included in the study. Mean age was 64(${\pm}14$) years, and the mean of total ventilation times was 9(${\pm}4$) days. All patients except one were males. The measured PSmin of the subjects ranged 4.0~12.5cm$H_2O$ in 14 patients. The mean level of PSmin was 7.6(${\pm}2.5\;cmH_2O$) in measured PSmin, 8.6 (${\pm}3.25\;cmH_2O$) in calculated PSmin Correlation between the measured PSmin and the calculated PSmin is significantly high(n=9, r=0.88, p=0.002). The calculated PSmin show a tendancy to be higher than the corresponding measured PSmin in 8 out of 9 subjects(p=0.09). The ratio of measured PSmin/calculated PSmin was 0.81(${\pm}0.05$). Conclusion: Minimal pressure support levels were different in individual cases in the range from 4 to 12.5 cm$H_2O$. Because the equation-driven calculated PSmin showed a good correlation with measured PSmin, the application of equation-driven PSmin would be then appropriate compared with conventional application of 5~10 cm$H_2O$ in patients under difficult weaning process with pressure support ventilation.