Neural control of mechanical ventilation in respiratory failure

Prolonged neural expiratory time induced by mechanical ventilation in infants

Mechanical ventilation may interfere with the spontaneous breathing pattern in infants because they have strong reflexes that play a large role in the control of breathing. This study aimed to answer the following questions: does a ventilator-assisted breath 1) reduce neural inspiratory time, 2) reduce the amplitude of the diaphragm electrical activity, and 3) prolong neural expiration, within the delivered breath? In 14 infants recovering from acute respiratory failure (mean age and weight were 2.3 +/- 1.3 mo and 3.95 +/- 0.82 kg, respectively), we measured 1) the electrical activity of the diaphragm with a multiple-array esophageal electrode, and 2) airway opening pressure, while patients breathed on synchronized intermittent mandatory ventilation (mean rate, 11.2 +/- 6.5 breaths/min). We compared neural inspiratory and expiratory times for the mandatory breaths and for the spontaneous breaths immediately preceding and following the mandatory breath. Although neural inspiratory time was not different between mandatory and spontaneous breaths, neural expiratory time was significantly increased (p < 0.001) for the mandatory breaths (953 +/- 449 ms) compared with the premandatory and postmandatory spontaneous breaths (607 +/- 268 ms and 560 +/- 227 ms, respectively). Delivery of the mandatory breath resulted in a reduction in neural respiratory frequency by 28.6 +/- 6.4% from the spontaneous premandatory frequency. The magnitude of inspiratory electrical activity of the diaphragm was similar for all three breath conditions. For the mandatory breaths, ventilatory assist persisted for 507 +/- 169 ms after the end of neural inspiratory time. Infant-ventilator asynchrony (both inspiratory and expiratory asynchrony) was present in every mandatory breath and constituted 53.4 +/- 26.2% of the total breath duration.

Understanding and implementing advances in ventilator capabilities

PURPOSE OF REVIEW

To review the changes in mechanical ventilation technology over the past year and identify areas that provide a benefit.

RECENT FINDINGS

The literature demonstrates a continued effort to improve patient ventilator synchrony though the development of new triggering and cycling methods. These techniques include using new signals and using closed loop techniques to respond to changes in patient breathing pattern. New modes of ventilation continue to be introduced, often without proof of efficacy. Fortunately, clinicians have developed alterations to new modes that improve utility and they continue to study these techniques clinically to determine appropriate use. Monitoring the patient remains an important area of investigation, with a flurry of activity surrounding pressure volume curves of the respiratory system. Finally, new ventilators have been introduced that combine high-end performance with small size and weight, while providing an on-board source of air.

SUMMARY

Mechanical ventilation is ubiquitous to intensive care. Advances in ventilator technology are rapid, and clinicians must keep abreast of changes in ventilator performance and application.

Measurement of diaphragm loading during pressure support ventilation

OBJECTIVE

The diaphragmatic pressure-time product (PTPdi) has been used to quantify the loading and unloading of the diaphragm. The validity of the relationship between PTPdi and diaphragm electrical activity (EMGdi) during pressure-support ventilation (PSV) is unclear. We examined this relationship.

DESIGN AND SETTING

Physiological study in a physiology laboratory.

SUBJECTS

Six healthy adults.

INTERVENTIONS

Spontaneous breathing (SB) and two levels of PSV (6 and 12 cmH(2)O), breathing room air and incremental concentrations of carbon dioxide, sufficient to achieve an EMGdi signal of approximately 200% of baseline value.

MEASUREMENTS AND RESULTS

We measured the electrical (EMGdi) and mechanical (PTPdi) activity of the diaphragm using oesophageal electrode and oesophageal and gastric balloon catheters. The relationship between EMGdi and PTPdi during SB was linear in five subjects and curvilinear in one. However, with PSV 12 cmH(2)O we observed that the relationship between EMGdi and PTPdi was 'left shifted'; specifically, for any given level of EMGdi the PTPdi was smaller with PSV 12 cmH(2)O than during SB. However, when PTPdi was converted to power (the product of pressure and flow) the tendency to left shift was largely reversed.

CONCLUSIONS

We conclude that when assessing of diaphragm unloading during PSV flow measurements are required. Where flow is constant, PTPdi is a valid measure of diaphragm unloading, but if not these data may be used to make an appropriate correction.

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.

New modes of mechanical ventilation: proportional assist ventilation, neurally adjusted ventilatory assist, and fractal ventilation

Increased knowledge of the mechanisms that determine respiratory failure has led to the development of new technologies aimed at improving ventilatory treatment. Proportional assist ventilation and neurally adjusted ventilatory assist have been designed with the goal of improving patient-ventilator interaction by matching the ventilator support with the neural output of the respiratory centers. With proportional assist ventilation, the support is continuously readjusted in proportion to the predicted inspiratory effort. Neurally adjusted ventilatory assist is an experimental mode in which the assistance is delivered in proportion to the electrical activity of the diaphragm, assessed by means of an esophageal electrode. Biologically variable (or fractal) ventilation is a new, volume-targeted, controlled ventilation mode aimed at improving oxygenation; it incorporates the breath-to-breath variability that characterizes a natural breathing pattern.

Effect of ventilator mode on sleep quality in critically ill patients

To determine whether sleep quality is influenced by the mode of mechanical ventilation, we performed polysomnography on 11 critically ill patients. Because pressure support predisposes to central apneas in healthy subjects, we examined whether the presence of a backup rate on assist-control ventilation would decrease apnea-related arousals and improve sleep quality. Sleep fragmentation, measured as the number of arousals and awakenings, was greater during pressure support than during assist-control ventilation: 79 +/- 7 versus 54 +/- 7 events per hour (p = 0.02). Central apneas occurred during pressure support in six patients; heart failure was more common in these six patients than in the five patients without apneas: 83 versus 20% (p = 0.04). Among patients with central apneas, adding dead space decreased sleep fragmentation: 44 +/- 6 versus 83 +/- 12 arousals and awakenings per hour (p = 0.02). Changes in sleep-wakefulness state caused greater changes in breath components and end-tidal CO2 during pressure support than during assist-control ventilation. In conclusion, inspiratory assistance from pressure support causes hypocapnia, which combined with the lack of a backup rate and wakefulness drive can lead to central apneas and sleep fragmentation, especially in patients with heart failure.

Non-invasive detection of respiratory muscles activity during assisted ventilation

The instantaneous pressure applied by the respiratory muscles [Pmus(t)] of a patient under ventilatory support may be continuously assessed with the help of a model of the passive respiratory system updated cycle by cycle. Inspiratory activity (IA) is considered present when Pmus goes below a given threshold. In six patients, we compared IA with (i) inspiratory activity (IAref) obtained from esophageal pressure and diaphragmatic EMG and (ii) that (IAvent) detected by the ventilator. In any case, a ventilator support onset coincides with an IA onset but the opposite is not true. IA onset is always later than IAref beginning ((0.21 +/- 0.10 s) and IA end always precedes IAref end (0.46 +/- 0.16 s). These results clearly deteriorate when the model is not updated.

Progress in mechanical ventilation

Mechanical ventilation is a life-supporting process employed in the management of respiratory failure. Over the years, our understanding of the pathophysiology of lung injury has greatly improved, and has aided the technological development of ventilatory modes that are more patient 'sensitive' and less traumatizing to the lungs. This review will discuss the fundamental modes of mechanical ventilation, and present current concepts regarding patient-ventilator interaction that either promote lung healing and weaning from positive pressure ventilation or delay recovery because of the injudicious use of ventilatory modalities that are incapable of meeting the ventilatory demands of the patient on a breath-by-breath basis. In addition, the current strategy for mechanical ventilation in acute lung injury and acute respiratory distress syndrome will be summarized.

Patient-ventilator asynchrony

The basic mechanism of patient-ventilator asynchrony is the mismatching between neural inspiratory and mechanical inspiratory time. Alterations in respiratory drive, timing, respiratory muscle pressure, and respiratory system mechanics influence the interaction between the patient and the ventilator. None of the currently available partial ventilatory support modes are exempt from problems with patient-ventilator asynchrony. Ventilator triggering design in the trigger phase and the set variables in the post-trigger phase contribute to patient-ventilator interaction. The set inspiratory flow rate in the post-trigger phase for assist-control volume cycled ventilation affects patient-ventilator asynchrony. Likewise, the initial pressure rise time, the pressure support level, and the flow-threshold for cycling off inspiration for pressure support ventilation are important factors affecting patient-ventilator asynchrony. Current investigations have advanced our understanding in this area; however, its prevalence and the extent to which patient-ventilator asynchrony affect the duration of mechanical ventilation remain unclear.

Electrical activity of the diaphragm during pressure support ventilation in acute respiratory failure

We compared crural diaphragm electrical activity (EAdi) with transdiaphragmatic pressure (Pdi) during varying levels of pressure support ventilation (PS) in 13 intubated patients. With changing PS, we found no evidence for changes in neuromechanical coupling of the diaphragm. From lowest to highest PS (2 cm H(2)O +/- 4 to 20 cm H(2)O +/- 7), tidal volume increased from 430 ml +/- 180 to 527 ml +/- 180 (p < 0.001). The inspiratory volume calculated during the period when EAdi increased to its peak did not change from 276 +/- 147 to 277 +/- 162 ml, p = 0.976. Respiratory rate decreased from 23.9 (+/- 7) to 21.3 (+/- 7) breaths/min (p = 0.015). EAdi and Pdi decreased proportionally by adding PS (r = 0.84 and r = 0.90, for mean and peak values, respectively). Mean and peak EAdi decreased (p < 0.001) by 33 +/- 21% (mean +/- SD) and 37 +/- 23% with the addition of 10 cm H(2)O of PS, similar to the decrease in the mean and peak Pdi (p < 0.001) observed (34 +/- 36 and 35 +/- 23%). We also found that ventilator assist continued during the diaphragm deactivation period, a phenomenon that was further exaggerated at higher PS levels. We conclude that EAdi is a valid measurement of neural drive to the diaphragm in acute respiratory failure.

Diaphragm activation during exercise in chronic obstructive pulmonary disease

Although it has been postulated that central inhibition of respiratory drive may prevent development of diaphragm fatigue in patients with chronic obstructive pulmonary disease (COPD) during exercise, this premise has not been validated. We evaluated diaphragm electrical activation (EAdi) relative to maximum in 10 patients with moderately severe COPD at rest and during incremental exhaustive bicycle exercise. Flow was measured with a pneumotachograph and volume by integration of flow. EAdi and transdiaphragmatic pressures (Pdi) were measured using an esophageal catheter. End-expiratory lung volume (EELV) was assessed by inspiratory capacity (IC) maneuvers, and maximal voluntary EAdi was obtained during these maneuvers. Minute ventilation (V E) was 12.2 +/- 1.9 L/min (mean +/- SD) at rest, and increased progressively (p < 0.001) to 31.0 +/- 7.8 L/min at end-exercise. EELV increased during exercise (p < 0.001) causing end-inspiratory lung volume to attain 97 +/- 3% of TLC at end-exercise. Pdi at rest was 9.4 +/- 3.2 cm H(2)O and increased during the first two thirds of exercise (p < 0.001) to plateau at about 13 cm H(2)O. EAdi was 24 +/- 6% of voluntary maximal at rest and increased progressively during exercise (p < 0.001) to reach 81 +/- 7% at end-exercise. In conclusion, dynamic hyperinflation during exhaustive exercise in patients with COPD reduces diaphragm pressure-generating capacity, promoting high levels of diaphragm activation.