Relationship between mouth occlusion pressure and electrical activity of the diaphragm: effects of flow-resistive loading

Neurally Adjusted Ventilatory Assist Versus Pressure Support Ventilation: A Comprehensive Review

Mechanical ventilation serves as crucial life support for critically ill patients. Although it is life-saving prolonged ventilation carries risks and complications like barotrauma, Ventilator-associated pneumonia, sepsis, and many others. Optimizing patient-ventilator interactions and facilitating early weaning is necessary for improved intensive care unit (ICU) outcomes. Traditionally Pressure support ventilation (PSV) mode is widely used for weaning patients who are intubated and mechanically ventilated. Neurally adjusted ventilatory assist (NAVA) mode of the ventilator is an emerging ventilator mode that delivers pressure depending on the patient's respiratory drive, which in turn prevents over-inflation and improves the patient's ventilator interactions. Our article revises and compares the effectiveness of NAVA compared to PSV ventilation under different contexts. Overall we conclude that NAVA level of ventilation can be safely administered in a patient with acute respiratory failure, provided diaphragmatic paralysis is not considered. NAVA improves asynchrony index, wean-off time, and sleep quality and is associated with increased ventilator-free days. These results are based on small-scale studies with low power, and further studies are warranted in large-scale cohorts with more diverse populations to confirm these results.

Do Adolescents Use Substances to Relieve Uncomfortable Sensations? A Preliminary Examination of Negative Reinforcement among Adolescent Cannabis and Alcohol Users

Alcohol and cannabis use are highly prevalent among adolescents and associated with negative consequences. Understanding motivations behind substance use in youth is important for informing prevention and intervention efforts. The present study aims to examine negative reinforcement principles of substance use among adolescent cannabis and alcohol users by pairing a cue reactivity paradigm with an aversive interoceptive stimulus. Adolescents (ages 15–17), classified as controls (CTL; n = 18), cannabis and/or alcohol experimenters (CAN+ALC-EXP; n = 16), or individuals meeting clinical criteria for cannabis and/or alcohol use disorder (CAN+ALC-SUD; n = 13) underwent functional magnetic resonance imaging during which they experienced an aversive interoceptive probe delivered via breathing load while simultaneously performing a cue reactivity paradigm. Participants also provided self-report ratings of how their substance use is positively or negatively reinforced. While experiencing the breathing load, CAN+ALC-SUD exhibited greater (p < 0.05) deactivation in the right amygdala, the left inferior frontal gyrus, and the left parahippocampal gyrus than CAN+ALC-EXP and CTL, who did not differ. Across all substance users, activation during the breathing load within the left parahippocampal gyrus negatively correlated with cannabis and alcohol lifetime use episodes and the left inferior frontal gyrus activity negatively correlated with lifetime alcohol use episodes. CAN+ALC-SUD reported experiencing more positive and negative reinforcement of using their substance of choice than CAN+ALC-EXP; both user groups reported higher levels of positive than negative reinforcement. Adolescents with a cannabis/alcohol use disorder demonstrate an altered response to interoceptive perturbations. However, adolescent cannabis/alcohol use does not appear to be driven by negative reinforcement, as viewing substance images did not dampen this response. Based on self-report data, the experience of positive reinforcement may be stronger for adolescents. Future studies should examine whether positive reinforcement contributes to adolescent substance use.

Information conveyed by electrical diaphragmatic activity during unstressed, stressed and assisted spontaneous breathing: a physiological study

Background

The electrical activity of the crural diaphragm (Eadi), a surrogate of respiratory drive, can now be measured at the bedside in mechanically ventilated patients with a specific catheter. The expected range of Eadi values under stressed or assisted spontaneous breathing is unknown. This study explored Eadi values in healthy subjects during unstressed (baseline), stressed (with a resistance) and assisted spontaneous breathing. The relation between Eadi and inspiratory effort was analyzed.

Methods

Thirteen healthy male volunteers were included in this randomized crossover study. Eadi and esophageal pressure (Peso) were recorded during unstressed and stressed spontaneous breathing and under assisted ventilation delivered in pressure support (PS) at low and high assist levels and in neurally adjusted ventilatory assist (NAVA). Overall eight different situations were assessed in each participant (randomized order). Peak, mean and integral of Eadi, breathing pattern, esophageal pressure–time product (PTPeso) and work of breathing (WOB) were calculated offline.

Results

Median [interquartile range] peak Eadi at baseline was 17 [13–22] μV and was above 10 μV in 92% of the cases. Eadi_max defined as Eadi measured at maximal inspiratory capacity reached 90 [63 to 99] μV. Median peak Eadi/Eadi_max ratio was 16.8 [15.6–27.9]%. Compared to baseline, respiratory rate and minute ventilation were decreased during stressed non-assisted breathing, whereas peak Eadi and PTPeso were increased. During unstressed assisted breathing, peak Eadi decreased during high-level PS compared to unstressed non-assisted breathing and to NAVA (p = 0.047). During stressed breathing, peak Eadi was lower during all assisted ventilation modalities compared to stressed non-assisted breathing. During assisted ventilation, across the different conditions, peak Eadi changed significantly, whereas PTPeso and WOB/min were not significantly modified. Finally, Eadi signal was still present even when Peso signal was suppressed due to high assist levels.

Conclusion

Eadi analysis provides complementary information compared to respiratory pattern and to Peso monitoring, particularly in the presence of high assist levels.

Trial registration The study was registered as NCT01818219 in clinicaltrial.gov. Registered 28 February 2013

Electronic supplementary material

The online version of this article (10.1186/s13613-019-0564-1) contains supplementary material, which is available to authorized users.

Implementation of a respiratory drive monitor on a Servo Ventilator

OBJECTIVE

To design and evaluate a clinical monitor of respiratory drive (P0.1) and other respiratory variables in a simple way, using a commercial ventilator.

METHODS

Nine healthy males were studied as they were breathing spontaneously in a Servo 900C Ventilator, at rest and during light exercise (50 W). The ventilator was slightly modified to improve its mechanical performance during spontaneous breathing, and was used as a measuring instrument. All the relevant information was retrieved, calculated and monitored by a PC. Respiratory drive was assessed as occlusion pressures from the inspiratory airway pressure signal. The equipment was compared with a two-way non-rebreathing laboratory system. Furthermore, negative and positive inspiratory pressures were applied from the ventilator, to study respiratory responses to mechanical loads.

RESULTS

At rest, the ventilator introduced a minor influence on inspiratory time and P0.1, but not in ventilation, tidal volume, expiratory duration and respiratory frequency. During exercise, the influence was more evident. This effect could also be noticed in the coefficients of variation. The responses to mechanical loads were easily recorded and can be used as a simple test of central load-compensating mechanisms. CONCLUSIONS. The ventilator, with limitations, may be an alternative to conventional techniques, especially in clinical studies of the central inspiratory activity with and without respiratory loading.

Voluntary activation of the human diaphragm in health and disease

Intersubject comparison of the crural diaphragm electromyogram, as measured by an esophageal electrode, requires a reliable means for normalizing the signal. The present study set out 1) to evaluate which voluntary respiratory maneuvers provide high and reproducible diaphragm electromyogram root-mean-square (RMS) values and 2) to determine the relative diaphragm activation and mechanical and ventilatory outputs during breathing at rest in healthy subjects (n = 5), in patients with severe chronic obstructive pulmonary disease (COPD, n = 5), and in restrictive patients with prior polio infection (PPI, n = 6). In all groups, mean voluntary maximal RMS values were higher during inspiration to total lung capacity than during sniff inhalation through the nose (P = 0.035, ANOVA). The RMS (percentage of voluntary maximal RMS) during quiet breathing was 8% in healthy subjects, 43% in COPD patients, and 45% in PPI patients. Despite the large difference in relative RMS (P = 0.012), there were no differences in mean transdiaphragmatic pressure (P = 0.977) and tidal volumes (P = 0.426). We conclude that voluntary maximal RMS is reliably obtained during an inspiration to total lung capacity but a sniff inhalation could be a useful complementary maneuver. Severe COPD and PPI patients breathing at rest are characterized by increased diaphragm activation with no change in diaphragm pressure generation.

Immediate response to inspiratory resistive loading in anesthetized patients with kyphoscoliosis: spirometric and neural effects

In kyphoscoliosis (KS), lung volumes are reduced, respiratory elastance and resistance are increased, and breathing pattern is rapid and shallow, attributes that may contribute to defense of tidal volume (VT) in the face of inspiratory resistive loading. The control of ventilation of 12 anesthetized patients about to undergo corrective spinal surgery was compared to that of 11 anesthetized patients free of cardiothoracic disease during quiet breathing and the first breath through one of three linear resistors. Mean forced vital capacity (FVC) of the KS group was 48% that of the controls (C). Passive elastance (Ers) and active elastance and resistance (E'rs and R'rs, respectively) were computed according to previously described techniques (Behrakis PK, Higgs BD, Baydur A, Zin WA, Milic-Emili J (1983) Active inspiratory impedance in halothane-anesthetized humans. J Appl Physiol 54: 1477-1481). Baseline tidal volume VT, inspiratory duration Tl, expiratory duration TE, duration of total breathing cycle TT, and inspiratory duty cycle TI/TT were significantly reduced, while VE was slightly decreased in the KS. Ers, E'rs, and R'rs, were, respectively, 72, 69, and 89% greater in the KS. Driving pressure (Pmus) was derived from the equation of motion, using active values of respiratory elastance. With resistive loading, there was greater prolongation of TI in the C, while percent reduction in VT and minute ventilation VE was less in KS. Compensation in both groups was achieved through three changes in the Pmus waveform. (1) Peak amplitude increased. (2) The duration of the rising phase increased. (3) The rising Pmus curve became more concave to the time axis. These changes were most marked with application of the highest resistance in both groups. Peak driving pressure and mean rate of rise of Pmus were greater in the KS. Increased intrinsic impedance, Pmus, and differences in changes in neural timing in anesthetized kyphoscoliotics contribute to modestly greater VT defense, compared to that of anesthetized subjects free of cardiorespiratory disease.

Influence of non-REM sleep on inspiratory muscle activity and lung volume in asthmatic patients

To determine the effect of non-REM (NREM) sleep on inspiratory muscle electromyographic (EMG) activity in asthmatic patients and the subsequent effect of such changes on FRC, 12 asthmatic patients with nocturnal worsening were monitored overnight in a horizontal volume-displacement body plethysmograph. During studies FRC was monitored using the single inspiratory occlusion technique, whereas EMG activities of the diaphragm (DI), intercostal (IC), and sternocleidomastoid (SCM) muscles were monitored using surface electrodes. FRC decreased progressively, from 3.63 +/- 0.27 L while awake prior to "lights out" to 3.15 +/- 0.27 L after 60 min of NREM sleep (p < 0.005). With awakening at the end of the study FRC increased to 4.69 +/- 0.48 L (p < 0.01). Onset of NREM sleep was also associated with significant (p < 0.05) reductions in EMG tonic activities of all three inspiratory muscles. These levels of tonic activity returned and were further augmented (p < 0.001) with awakening at the conclusion of the study. Linear relationships were demonstrated between sleep-state dependent changes in FRC and EMG tonic activity for each of the three inspiratory muscles (r = 0.98, 0.93, and 0.97 for DI, IC, and SCM, respectively, p < 0.01). NREM sleep did not significantly alter mean inspiratory phasic activities of these muscles, although there was considerable heterogeneity between individual subjects in the effects of NREM sleep.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of hypoxia and hypercapnia on respiratory drive in patients with COPD

The objective of this study was to compare the response of respiratory drive to progressive hypoxia under eucapnic and hypercapnic conditions in patients with severe COPD. Twenty-five patients with severe COPD and 13 nonsmoking young men were studied. The pressure in the occluded airway measured 0.1 second after the onset of inspiration was used as an index of respiratory drive. The occlusion pressure was measured at levels of SaO2 between 97 and 85 percent while eucapnic. The PETCO2 was then increased 10 mm Hg and the study repeated. The response of respiratory drive to hypoxia as measured by the slope of the regression line relating occlusion pressure to SaO2 was weak and variable in eucapnic hypoxia, and some subjects had no demonstrable response. When mild respiratory acidosis was created by increasing the PETCO2, the response to hypoxia was much greater and occurred in all subjects studied. Respiratory acidosis resulting from acute elevation of the PaCO2 greatly potentiates the increase in respiratory drive in response to hypoxia in normal subjects and in patients with severe COPD. Increase in occlusion pressure may occur with slight degrees of hypoxia when acute hypercapnia is present. These observations suggest that patients with acute respiratory failure complicating COPD, treated with controlled oxygen administration with only partial correction of hypoxia and continued respiratory acidosis, will have high respiratory drive.

Respiratory drive and pattern during inertially-loaded CO2 rebreathing: implications for models of respiratory mechanics in obesity

Chest wall inertance (Iw) constitutes a mechanical load which is small in normal individuals, but increases in the morbidly obese. The effect of increased Iw on respiratory drive and pattern has not previously been investigated. We studied this effect by measuring rebreathing CO2 response in 10 normal subjects with and without a 45.5 kg inertial load. Changes in inspiratory occlusion pressure (P0.1), ventilation, respiratory rate, mean inspiratory flow (VT/TI) duty cycle (TI/TT), inspiratory and expiratory times, and [P0.1/(VT/TI)] were assessed. The P0.1 vs. PETCO2 response shifted to the left during inertial loading, while the slope remained unchanged (x-intercept = 36.7 +/- 4.9 mm Hg unloaded vs. 32.7 +/- 7.5 mm Hg loaded, P less than 0.05), increasing P0.1 from 18.5 +/- 8.0 to 21.0 +/- 7.6 cm H2O at PETCO2 = 6.5 mm Hg (P less than 0.005). Respiratory pattern was unchanged with the inertial load except for a slight decrease in tidal volume. The inspiratory transfer characteristic [P0.1/(VT/TI)] at PETCO2 = 65 mm Hg increased significantly (8.3 +/- 2.0 to 10.5 +/- 2.9 cm H2O.(L.sec-1)-1, P less than 0.025) illustrating the strategy of maintaining similar ventilation by increasing inspiratory force against the load.

Ventilatory adjustments during sustained resistive unloading in exercising humans

Effect of He-O2-breathing (79.1%:20.9%) compared to air-breathing on inspiratory ventilation (VI) and its different components [tidal volume (VT), the duration of the phases of each respiratory cycle (tI, tTOT)] as well as on inspiratory mouth occlusion pressure (P0.1) were studied in six normal men at rest and during 72 constant-load exercises (90 W) over a much longer period than in previous studies. Results showed that, irrespective of the order of administration of the two gases (7 min air----7 min He-O2 or vice versa): at rest, P0.1 decreased during He-O2 inhalation but no changes in VI and breathing pattern were detectable; during exercise, sustained He-induced hyperventilation was observed without any change in the absolute value of P0.1; increase in P0.1 between the resting period and exercise (delta P0.1) was significantly higher during He-O2-breathing than during air breathing; this He-induced hyperventilation was associated with a sustained increase in VT/tI, but with constant tI/tTOT. Helium-breathing during exercise cannot be a simple situation of resistance unloading, as has been suggested. We conclude that He-O2-breathing, after the initial compensation period, induces reflex changes in ventilatory control with an increase in inspiratory neural drive. Moreover, it appears that exercise P0.1 is not a legitimate index of inspiratory neural drive whenever rest P0.1 changes according to the nature of the inhaled gas mixture.

Volume acceleration as an index of neuromuscular output

At the transition from expiration to inspiration, when flow and volume changes are small, changes in the respiratory system driving pressure could determine the degree of volume acceleration (AI), which, in turn, could reflect the degree of respiratory center output. To test this hypothesis, we calculated AI occurring in each respiratory cycle at the transition from expiration to inspiration during CO2 rebreathing in 4 healthy supine subjects. To minimize the flow and volume change over the measurement interval, we measured AI just prior to inspiration within the limits of an expiratory flow of 0.2 L . sec -1 to zero flow using digital differentiation. We also measured mouth pressure 100 msec after the onset of inspiration (P0.1) during intermittent transient inspiratory airway occlusions. During CO2 rebreathing AI increased significantly with both increasing PCO2 and P0.1. We also compared pairs of rebreathing studies, performed without and with an alinear (16 cm H2O . L -1 . sec -1) inspiratory resistor (IR), repeated twice in the 4 subjects. IR markedly decreased delta VE/delta PCO2 and the slope of the increase in mean inspiratory flow rate with PCO2 (delta VT/TI/delta PCO2) but did not significantly alter either delta AI/delta PCO2 or the increase in P0.1 with PCO2 (delta P0.1/delta PCO2). However, the effects of IR on AI and P0.1 differed between the early and late phases of each rebreathing run; early in the rebreathing runs (PCO2 = 55 Torr) IR increased both AI and P0.1 by a similar amount, but near the end of rebreathing (PCO2 = 60 Torr) IR increased P0.1 but not AI. Our results are consistent with the possibility that AI reflects neuromuscular output under the conditions of the study. Hence this approach justifies further evaluation to determine its general applicability.

Effects of aminophylline on respiratory drive and neuromuscular coupling in normal man and in patients with chronic airflow obstruction

In order to evaluate the separate effects of aminophylline on the neural and muscular components of the respiratory control system, assessed by electromyographic activity of the diaphragm (EMGd) and mouth occlusion pressure (P0.1), respectively, 6 normal subjects and 14 patients with mild or moderate chronic airflow obstruction (8 asthmatics and 6 COPD) were studied during CO2 rebreathing, before and after administration of a therapeutic dose of aminophylline 5.6 mg/kg. Compared to normal subjects, before aminophylline administration both asthmatic and COPD patients exhibited a significantly greater value in EMGd response slope to CO2. In no group did aminophylline modify P0.1 or EMGd activity response slope to CO2, nor did it significantly affect neuromuscular coupling, assessed by plotting change in P0.1 against change in EMGd activity with increasing CO2. The data appear to indicate that aminophylline in therapeutic concentrations does not modify respiratory drive or neuromuscular coupling in normal subjects, or in patients with mild or moderate chronic airflow obstruction.

Pulmonary function and ventilatory response to chemical stimuli in familial myopathy

We studied the pulmonary function and ventilatory response to carbon dioxide and hypoxia in three sisters aged 16, 13, and 10 years who presented with droopy eyelids, external ophthalmoplegia, hearing loss, speech difficulty, and truncal muscular weakness. Pulmonary function test results showed decreased maximum static pressure, reduced vital capacity and total lung capacity, normal functional residual capacity, elevated residual volume, and reduced dynamic pulmonary volumes. The degree of functional abnormality paralleled the severity of clinical manifestations. The characteristic picture of pulmonary functional abnormality was distinct from either restrictive disorders of pulmonary origin or obstructive pulmonary diseases. The ventilatory response to hypoxia was markedly diminished and hypercapnic response was moderately diminished in all three patients.

Oesophageal multipurpose monitoring probe

An oesophageal probe which incorporates facilities for pressure, temperature, electrocardiographic and stethoscopic monitoring is described. Actual and potential uses are outlined.

Comparison of the respiratory responses to external resistive loading and bronchoconstriction

The effects of resistive loads applied at the mouth were compared to the effects of bronchospasm on ventilation, respiratory muscle force (occlusion pressure), and respiratory sensations in 6 normal and 11 asthmatic subjects breathing 100% O2. External resistive loads ranging from 0.65 to 13.33 cm H2O/liter per s were applied during both inspiration and expiration. Bronchospasm was induced by inhalation of aerosolized methacholine. Bronchospasm increased ventilation, inspiratory airflow, respiratory rate, and lowered PACO2. External resistive loading, on the other hand, reduced respiratory rate and inspiratory flow, but left ventilation and PACO2 unaltered. FRC increased to a greater extent with bronchospasm than external flow resistive loads. With both bronchospasm and external loading, occlusion pressure increased in proportion to the rise in resistance to airflow. However, the change in occlusion pressure produced by a given change in resistance and the absolute level of occlusion pressure at comparable levels of airway resistance were greater during bronchospasm than during external loading. These differences in occlusion pressure responses to the two forms of obstruction were not explained by differences in chemical drive or respiratory muscle mechanical advantage. Although the subjects' perception of the effort involved in breathing was heightened during both forms of obstruction to airflow, at any given level of resistance the sense of effort was greater with bronchospasm than external loading. Inputs from mechanoreceptors in the lungs (e.g., irritant receptors) and/or greater stimulation of chest wall mechanoreceptors as a result of increases in lung elastance may explain the differing responses elicited by the two forms of resistive loading.

Effect of assisted ventilation on respiratory drive of normal anesthetized dogs

We measured the diaphragm electromyogram (EMGd) and airway occlusion pressure (P100) of anesthetized dogs at the ends of 30-min of alternating spontaneous breathing and assisted mechanical ventilation (inspiratory positive pressure triggered by a minimal inspiratory effort; IPPB). Peak airway pressures (Paw) of 5, 10, 15, and 20 cm H2O were applied in random order during IPPB. The initial rate of rise of EMGd (4 experiments) and P100 (9 experiments) decreased during IPPB, both in proportion to the Paw. At Paw of 20 cm H2O, PaCO2, EMGd and P100 decreased on the average 5.2 Torr, 58% and 56%, respectively. When PaCO2 was kept the same during IPPB as during spontaneous breathing (8 experiments), IPPB reduced EMGd and P100 15% and 24% respectively, both statistically significant. During progressive cooling of both cervical vagi, the effect of IPPB (Paw=20 cm H2O) on EMGd and P100 decreased progressively, near maximal at 6--4 degrees C and after vagotomy. However, the decrease in PaCO2 during IPPB was not affected. These data suggest that increasing tidal volume during IPPB depresses respiratory drive (EMGd and P100), partially from decreased PaCO2 and partially from stimulating pulmonary stretch receptors. The drop in PaCO2 during IPPB does not appear to be vagally mediated.

Respiratory neuromuscular response to CO2 rebreathing with inspiratory flow resistance in humans

The effects of inspiratory flow resistance on mouth occlusion pressure (P0.15) and diaphragmatic EMG (EMGdi) responses to CO2 rebreathing were studied in normal subjects. Occlusion pressures were measured 150 msec after onset of an inspiratory effect; EMGdi was analyzed as a moving time average and quantified in terms of peak activity and rate of rise of activity. After a control CO2 response was obtained in each subject, rebreathing was repeated 30 min later with either of two inspiratory flow resistive loads, 5 cm H2O/L/sec (IR5) and 14 cm H2O/L/sec (IR14). With IR5 (6 subjects), the P0.15 response was decreased in two subjects, unchanged in two, and increased in two; peak EMGdi was unchanged in all, while rate of rise of EMGdi response decreased in 4 of the 6 subjects. With IR14 (6 subjects, 9 runs), the P0.15 response was not decreased in any subject, remained unchanged in 4, and increased in 5; peak EMGdi response to rebreathing in all runs was, again, unchanged by this load, but rate of rise of EMGdi was decreased in 3 and unchanged in 6. The inspiratory off-switch threshold as reflected by peak diaphragmatic activity was not changed by inspiratory flow resistance, whereas inspiratory neural drive as reflected by the the rate of rise of activity was decreased in some subjects. The decrease in inspiratory drive without change in inspiratory off-switch threshold resulted in prolongation of inspiration in an attempt to effect efficient lung expansion. However, the defense of ventilation during rebreathing with both resistances appeared to mainly depend on the response of inspiratory muscle force (P0.15), since in 7 of the 7 runs in which the P0.15 response was significantly increased from control, the ventilatory response was not decreased.