Category Archives: Pulmonary Function : Part 9

The Control of Breathing during Weaning from Mechanical Ventilation (12)

Thus, during mechanical ventilation with a volume preset mode, minute ventilation and PaCOz are as much determined by machine settings as they are dependent on the state of the respiratory controller. When the level of ventilatory support is progressively reduced, as is the case during weaning with IMV and/or PS modes, Ve and PaC02 become increasingly dependent on mechanical loads and the performance capacity of the respiratory pump. In accordance with this reasoning, patients whose lungs were ventilated in the assist control mode (subjects 4, 5, 9, 10, and 11) had C02MV tensions that were significantly lower than either C02RT or C02SB (4±3 and 8±3 mm Hg, respectively, p<0.05). Four patients in group 2    (subjects 6, 12, 13, and 14) whose lungs were ventilated in an IMV or PS mode had C02MV tensions that were between 4 and 8 mm Hg greater than at the end of a symptom-limited weaning trial. The latter observation is consistent with reports suggesting that mechanical ventilation with low IMV backup rates may mask the presence of respiratory pump failure by blood gas criteria.
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The Control of Breathing during Weaning from Mechanical Ventilation (11)

The Control of Breathing during Weaning from Mechanical Ventilation (11)C02 retention above CO£BT can be viewed as evidence for incomplete load compensation, but it need not imply overt pump failure. We have shown that patients without weaning-induced respiratory distress usually maintain an alveolar ventilation predicted by C02RT. Only one of five patients in group 1 retained C02 during weaning relative to C02RT. The 2 mm Hg difference between COsSB and C02RT in this patient falls within the variability of the recruitment threshold measurement. Wfe have also demonstrated that most patients with weaning-induced dyspnea and/or tachypnea already retain C02 compared with C02RT 5 min after the appearance of these signs of respiratory distress.

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The Control of Breathing during Weaning from Mechanical Ventilation (10)

Table 2 shows the individual and group mean data for respiratory system compliance (Crs), VP6, Pimax, P0.i, spontaneous respiratory rate (f), tidal volume (Vt), IVTr, dyspnea score, C02RT, and weaning duration. There was considerable overlap of these variables between groups. At the end of the weaning trial, patients of group 2 had higher respiratory rates (caused by group selection criteria) and smaller tidal volumes (8.7 vs 5.0 ml/kg, p<0.05). Subjects in group 2 tended to be weaker (mean Pimax of group 2=27 cm H20 vs 37 cm H20 in group 1, p = 0.11), while the compliance of the relaxed respiratory system, the severity of airways obstruction (VP6), and indices of pressure output of the respiratory muscles during spontaneous breathing (P01) were similar. Furthermore, average C02RT was the same, ie, 40 mm Hg, in both groups. Whereas all subjects in group 1 tolerated the full 1-h weaning period, the mean duration of the weaning period for group 2 was 13 ± 11 min.
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The Control of Breathing during Weaning from Mechanical Ventilation (9)

The Control of Breathing during Weaning from Mechanical Ventilation (9)Four of nine patients in group 2 had signs and symptoms of weaning-induced respiratory distress at a C02SB, 4 to 8 mm Hg lower than C02MV. These patients had been mechanically ventilated using either an intermittent mandatory ventilation (IMV) mode or a pressure support (PS) mode with low backup rates and remained ventilator dependent 24 hours after our study.
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The Control of Breathing during Weaning from Mechanical Ventilation (8)

Figure 2 shows the differences between C02RT and C02SB in each patient. Subjects 1 through 5 tolerated the full hour of weaning (group 1, dark shaded bars). In three of them, C02SB was ^2 mm Hg lower than C02RT. In group 1, the mean difference between C02RT and C02SB of 1.2 mm Hg did not achieve statistical significance (p^O.l). Patients 6 through 14 had symptom-limited weaning failure (group 2, light shaded bars). In these patients, the mean duration of the weaning trial was 13 min. Five of them developed asynchronous chest wall movements (detected by inductance plethysmography) near the end of the weaning trial, but only patient 8 became frankly acidemic (pH<7.35). In group 2, seven of nine patients retained C02 during unassisted breathing relative to C02RT (group mean difference C02SB — C02RT=5 ± 5 mm Hg, <0.01). C02RT and C02SB were equal in the remaining two patients.
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The Control of Breathing during Weaning from Mechanical Ventilation (7)

The Control of Breathing during Weaning from Mechanical Ventilation (7)Results
The ages of patients ranged from 56 to 87 years (mean age, 70 years) (Table 1). Various cardiopulmonary disorders were present. The majority of patients (9 of 14) had recently undergone major thoracic or abdominal surgery with secondary complications leading to prolonged respiratory failure. The most common complicating illnesses were obstructive lung disease (eight patients) and sepsis (four patients) combined with varying degrees of cardiac dysfunction. These complications had led to ventilator dependency for more than 1 week in 12 of 14 subjects.
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The Control of Breathing during Weaning from Mechanical Ventilation (6)

Parameters Recorded during Spontaneous Breathing (‘Weaning): All patients were observed by an investigator during a T-piece weaning trial of 1 h maximal duration. The Flo, was adjusted to assure a Sa02 of 90 percent to 93 percent. Respiratory rate, tidal volume, and breathing patterns were determined from the respiratory inductive plethysmograph recordings. At 5 min intervals, the perception of dyspnea was scored on a modified Borg scale with values ranging between 6 (indicating no discomfort) and 20 (indicating extreme shortness of breath). The airway pressure 100 ms following the occlusion of the inspiratory port of a unidirectional balloon occlusion valve (P0,, Hans Rudolph 2400) was also recorded every 5 min. Symptom-limited failure to wean was defined prospectively as an increase in dyspnea score to 19 or greater and/or as a sustained increase in respiratory rate over 30 breaths per minute. At the end of 1 h or within 5 min of weaning failure, arterial blood gas tensions were measured, from which COzSB was determined.
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The Control of Breathing during Weaning from Mechanical Ventilation (5)

The Control of Breathing during Weaning from Mechanical Ventilation (5)The Flo, was adjusted with an oxygen blender (Bird 960, Palm Springs, CA) to maintain an arterial oxygen saturation between 90 percent and 93 percent. The ventilator assist rate was then increased slowly until spontaneous respiratory activity ceased, as assessed from inspection of the Pao and flow tracings. Absence of phasic respiratory muscle activity was assumed when mechanical inflation with a constant flow caused a step-like increase in airway pressure followed by a linear rise to a peak value and a quasiexponential decline of expiratory flow. After 15 minutes of mechanical ventilation at these settings, the inspired CO, concentration (FIco,) was raised by 1 percent to 3 percent in stepwise increments of 3 to 5 min until spontaneous inspiratory muscle activity reappeared. Respiratory muscle recruitment was defined by the absence of relaxation criteria on ten or more consecutive breaths (Fig 1). An arterial blood gas sample was obtained within 30 s of recruitment, the CO, tension of which defined CO,RT. In seven patients, the CO,RT measurement was repeated immediately following the weaning trial.
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The Control of Breathing during Weaning from Mechanical Ventilation (4)

Patients were mechanically ventilated (with a Siemens Servo 900B or 900C ventilator). Initially, the ventilator rate was increased to abolish triggering efforts in order to assess the respiratory system mechanics using the interrupter technique. Pao, V, and V were recorded during stepwise deflations (Hans Rudolph 7200 interrupter valve) of the relaxed respiratory system between end inspiration and relaxation volume. Pressure-volume and pressure-flow plots were constructed, from which the elastic and flow resistive properties of the relaxed respiratory system were derived. The severity of airways obstruction was estimated from the expiratory flow at a system recoil pressure of 6 cm H,0 (VP6). Inspiratory muscle strength was assessed from airway occlusion pressure recordings during maximal voluntary efforts (Pimax); the best of three maneuvers was chosen.

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The Control of Breathing during Weaning from Mechanical Ventilation (3)

The Control of Breathing during Weaning from Mechanical Ventilation (3)Instrumentation
Gas flow (V) through the endotracheal tube was measured with a pneumotachograph (Hans Rudolph, Kansas City, MO) and differential pressure transducer (Honeywell, Freeport, IL). Volume (V) was derived by integration of the flow signal. Airway opening pressure (Pao) was measured 1 cm from the oral end of the endotracheal tube. Inspired and expired CO* concentrations were continuously monitored between pneumotachograph and endotracheal tube with an infrared capnograph (Puritan Bennett, Los Angeles, CA). The arterial oxygen saturation (SaOJ was monitored with a fingertip pulse oximeter (Nellcor Inc, Hayward, CA). Arterial blood samples were obtained from an indwelling radial artery catheter, and gas tensions were measured with a blood gas instrument (Instrument Laboratories, Lexington, MA, IL 813). Rib cage and abdominal motion were continuously monitored with a respiratory inductive plethysmograph (Respigraph, NIMS, Miami Beach, FL) during the weaning period. All data were displayed on an eight-channel hot pen recorder (Astro-Med, West Warwick, RI).
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