Frequency of mechanical ventilation and respiratory activity after double lung transplantation

Ventilatory failure, ventilator support, and ventilator weaning

The development of acute ventilatory failure represents an inability of the respiratory control system to maintain a level of respiratory motor output to cope with the metabolic demands of the body. The level of respiratory motor output is also the main determinant of the degree of respiratory distress experienced by such patients. As ventilatory failure progresses and patient distress increases, mechanical ventilation is instituted to help the respiratory muscles cope with the heightened workload. While a patient is connected to a ventilator, a physician's ability to align the rhythm of the machine with the rhythm of the patient's respiratory centers becomes the primary determinant of the level of rest accorded to the respiratory muscles. Problems of alignment are manifested as failure to trigger, double triggering, an inflationary gas-flow that fails to match inspiratory demands, and an inflation phase that persists after a patient's respiratory centers have switched to expiration. With recovery from disorders that precipitated the initial bout of acute ventilatory failure, attempts are made to discontinue the ventilator (weaning). About 20% of weaning attempts fail, ultimately, because the respiratory controller is unable to sustain ventilation and this failure is signaled by development of rapid shallow breathing. Substantial advances in the medical management of acute ventilatory failure that requires ventilator assistance are most likely to result from research yielding novel insights into the operation of the respiratory control system.

Transdiaphragmatic pressure control of airway pressure support in healthy subjects

We designed a new servoventilator that proportionally adjusts airway pressure to transdiaphragmatic pressure (Pdi) generated by the subject during inspiration. Each cycle is triggered by either a preset Pdi increase or a preset inspiratory flow value (whichever is reached first), whereas cycling-off is flow-dependent. We evaluated the servoventilator in seven healthy subjects at normocapnia and three levels of hypercapnia (normocapnia + 3, + 6, and + 9 mm Hg) comparatively with spontaneous breathing. Triggering was by Pdi in six subjects and flow in one. At all end-tidal carbon dioxide pressure levels, time from onset of diaphragm electromyographic activity to inspiratory flow was similar with and without the servoventilator. Airway pressure increased proportionally to Pdi variation during servoventilator breathing. Flow, tidal volume, respiratory rate, intrinsic positive end-expiratory pressure, and esophageal and transdiaphragmatic pressure-time products increased significantly with hypercapnia with and without the servoventilator. Breathing pattern parameters were similar in the two breathing modes, and no differences were found for intrinsic positive end-expiratory pressure or gastric pressure variation during exhalation. Esophageal and transdiaphragmatic pressure-time products were lower with than without the servoventilator. The Pdi-driven servoventilator was well synchronized to the subjects effort, delivering a pressure proportional to Pdi and reducing respiratory effort at normocapnia and hypercapnia.

Nonchemical Elimination of Inspiratory Motor Output via Mechanical Ventilation in Sleep

In six dogs studied in nonrapid eye movement (NREM) sleep, we found that the frequency, volume, and timing of application of mechanical ventilator breaths had marked and sustained inhibitory effects on diaphragm electromyogram (EMG(di)). Single ventilator breaths of tidal volume (VT) 75-200% of control caused apnea (up to three times eupneic expiratory time [TE]) when applied during the initial 25-65% of expiratory time. When continuous controlled mechanical ventilation (CMV) was applied with ventilator frequency increased as little as 1 cycle/min > eupnea and Pa(CO(2)) and VT maintained at near eupneic control levels, EMG(di) was silenced and triangularis sterni EMG (EMG(ts)) became tonic within 2 to 5 ventilator cycles. On cessation of normocapnic CMV, apnea ensued with TE ranging from 1.2 to five times eupneic TE. The spontaneous VT and EMG(di) determined immediately after these prolonged apneas were also markedly reduced in amplitude. The larger the VT applied during the isocapnic CMV (120-200% of eupnea) and the longer the duration of the CMV (3-90 s), the longer the duration of the postventilator apnea. Significant postventilator apneas and postapneic hypoventilation also occurred even when end-tidal CO(2) pressure (PET(CO(2))) was raised 3-5 mm Hg > eupnea (and 7-10 mm Hg > normal apneic threshold) throughout CMV trials at raised frequency and VT. Our findings demonstrate that the increased frequency of CMV was critical to the elimination of inspiratory motor output and the onset of tonic expiratory muscle activity; furthermore, once EMG(di) was silenced, the tidal volume and duration of the passive mechanical ventilation determined the magnitude of the short-term inhibition of inspiratory motor output after cessation of CMV.

Diaphragm inhibition with positive pressure ventilation: quantification of mechanical effects

To quantify any mechanical inhibitory effect of nasal intermittent positive pressure ventilation (IPPV) on inspiratory activity of the diaphragm we ventilated five conscious relaxed subjects on two occasions at respiratory rates similar to quiet breathing (QB) and at three levels of applied pressure (Pappl)--6, 9 and 12 cmH2O, each during hypocapnia (P(CO2) allowed to decrease) and eucapnia (CO2 added to inspired gas). Diaphragm activity was assessed from transdiaphragmatic pressure (esophageal and gastric balloons) and diaphragm EMG (surface electrodes) both integrated with time (integral(Pdi x dt) and integral(EMGdi x dt), respectively). Neural inspiratory time (Tin) was measured as onset to peak of the integral(EMGdi x dt) signal. Relative to QB, integral(Pdi x dt) was 50-69% less during eucapnic IPPV 6-12 cmH2O (P < 0.005) and 67-85% less during hypocapnic IPPV (P < 0.005). Tin decreased (P < 0.05) with IPPV and, on ceasing IPPV, there was apnoea (prolonged expiratory time) on 23 of 27 occasions; these changes were independent of P(CO2). Integral(EMGdi x dt) decreased (P < 0.05) at Pappl 12 cmH2O during eucapnia and at all Pappl during hypocapnia. The repeatability of integral(EMGdi x dt) was substantially less than integral(Pdi x dt) (F = 42, P << 0.01). We conclude that, during non-invasive IPPV in awake healthy subjects mechanical factors are of major importance in inhibiting inspiratory activity of the diaphragm.

Nonchemical influence of inspiratory pressure support on inspiratory activity in humans

To determine whether nonchemical inhibition of respiratory activity occurs during inspiratory pressure support (IPS) ventilation (IPSV), respiratory motor output (in 9 subjects), obtained by calculating transdiaphragmatic pressure-time products, and central respiratory output (in 5 subjects), obtained by integrating the electromyographic activity of the diaphragm (EMGdi) during mechanical inspiratory time, EMGdi per minute, and electrical inspiratory time, as determined from onset to peak EMGdi, were compared during spontaneous ventilation (control) and IPSV with (IPS+CO2) and without (IPS) correction of hypocapnia. Both IPS and IPS+CO2 induced significant decreases in transdiaphragmatic pressure-time products (46 +/- 31 and 53 +/- 23%, respectively), EMGdi during mechanical inspiratory time (49 +/- 12 and 57 +/- 14%, respectively), EMGdi per minute (65 +/- 22 and 69 +/- 15%, respectively), and electrical inspiratory time (73 +/- 8 and 65 +/- 6%, respectively). Because correction of hypocapnia failed to eliminate the marked inhibition of both respiratory and central motor output seen with IPS, we conclude that nonchemical inhibition of respiratory activity occurs during IPSV.

Apneic threshold for CO2 in the anesthetized rat: fundamental properties under steady-state conditions

Experiments were performed to measure the apneic threshold for CO2 and its fundamental properties in anesthetized rats under steady-state conditions. Breathing was detected from diaphragmatic electromyogram activity. Mechanical hyperventilation resulted in apnea once arterial PCO2 (PaCO2) had fallen far enough. Apnea was not a reflex response to lung inflation because it did not occur immediately, was not prevented by vagotomy, and was reversed by raising PaCO2 without changing mechanical hyperventilation. The apneic threshold was measured by hyperventilating rats mechanically with O2 until apnea had occurred and then raising PaCO2 at constant hyperventilation until breathing reappeared. The mean PaCO2 level of the apneic threshold in 42 rats was 32.8 +/- 0.4 Torr. The level of the threshold did not depend on the volume at which the lungs were inflated. The level of the threshold, under steady-state conditions, was the same when approached from hypocapnia as from eupnea. The level of the threshold could be raised by 9 Torr by chronic elevation of the eupneic PaCO2 level by 18 Torr.

Fibroblast growth factor-2 expression is up-regulated after denervation in rat lung tissue

To understand the role of fibroblast growth factor-2 (FGF-2) during the denervation-reinnervation processes which occur after lung transplantation, we studied FGF-2 gene expression in a rat lung denervation model. The temporal profile of FGF-2 mRNA in denervated rat lungs was quantitatively assessed by competitive reverse transcription polymerase chain reaction (RT-PCR) method. The level of FGF-2 mRNA was consistently higher in denervated lungs, showing a peak value on the 5th post-operative day. Immunohistochemical analysis with an anti-FGF-2 monoclonal antibody disclosed immunoreactivity in Schwann cells at the distal severed end of the nerve fascicle located at the lung hilus, 1 week post-surgery. This study indicates that FGF-2 gene expression is up-regulated following denervation and suggests possible roles of FGF-2 in the reinnervation process of lung tissue.

The effect of the level of ventilatory assist on the level of respiratory drive in decerebrate cats

The present study was undertaken to investigate whether, independent of changes in PaCO2, ventilatory assist influences not only the pattern but also the level of the respiratory drive. The experiments were performed on decerebrate and paralyzed cats ventilated by a phrenic-driven servo respirator at three different FICO2 levels (0, 0.30, 0.05). The level of ventilatory assist was altered within the range where PaCO2 did not exceed 80 Torr. A higher FICO2 accompanied a higher level of ventilatory assist. The relationship between the minute phrenic activity and log10 PaCO2 at a given FICO2 was linear. No significant difference was found in the regression lines at different levels of FICO2. We conclude that ventilatory assist has little effect on the respiratory drive at a constant level of chemical feedback during hypercapnia.

Carbon dioxide respiratory response during positive inspiratory pressure in COPD patients

The aim of this study was to compare the effect of inspiratory pressure support (IPS) on the respiratory CO2 response in 13 stable COPD patients and in 13 normal subjects. Without IPS, the slopes of the ventilatory response to CO2 were lower in the patients than in the normal subjects (mean +/- SEM, 0.82 +/- 0.19 vs. 1.69 +/- 0.31 l.min-1.mmHg-1). When IPS was applied, both groups showed, at any level of end-tidal CO2 pressure, an increase in ventilation due to an increase in tidal volume (VT) associated with a decrease in occlusion pressure (P0.1). In addition, respiratory parameters (VE, VT, P0.1, inspiratory flow) were insensitive to CO2 as long as PETCO2 remained below a threshold which was slightly above the eupneic value. However, above this CO2-threshold, no differences in slopes were observed between the IPS and control conditions in either group, except for a decrease in the P0.1 slope during IPS in the COPD patients. In conclusion, IPS induced similar respiratory changes during CO2 response in stable COPD patients and in normal subjects. Above the eupneic value, IPS did not change the slope of the ventilatory response to CO2.