Reverse Triggering Dyssynchrony 24 h after Initiation of Mechanical Ventilation.

BACKGROUND: Reverse triggering is a delayed asynchronous contraction of the diaphragm triggered by passive insufflation by the ventilator in sedated mechanically ventilated patients. The incidence of reverse triggering is unknown. This study aimed at determining the incidence of reverse triggering in critically ill patients under controlled ventilation. METHODS: In this ancillary study, patients were continuously monitored with a catheter measuring the electrical activity of the diaphragm. A method for automatic detection of reverse triggering using electrical activity of the diaphragm was developed in a derivation sample and validated in a subsequent sample. The authors assessed the predictive value of the software. In 39 recently intubated patients under assist-control ventilation, a 1-h recording obtained 24 h after intubation was used to determine the primary outcome of the study. The authors also compared patients' demographics, sedation depth, ventilation settings, and time to transition to assisted ventilation or extubation according to the median rate of reverse triggering. RESULTS: The positive and negative predictive value of the software for detecting reverse triggering were 0.74 (95% CI, 0.67 to 0.81) and 0.97 (95% CI, 0.96 to 0.98). Using a threshold of 1 µV of electrical activity to define diaphragm activation, median reverse triggering rate was 8% (range, 0.1 to 75), with 44% (17 of 39) of patients having greater than or equal to 10% of breaths with reverse triggering. Using a threshold of 3 µV, 26% (10 of 39) of patients had greater than or equal to 10% reverse triggering. Patients with more reverse triggering were more likely to progress to an assisted mode or extubation within the following 24 h (12 of 39 [68%]) vs. 7 of 20 [35%]; P = 0.039). CONCLUSIONS: Reverse triggering detection based on electrical activity of the diaphragm suggests that this asynchrony is highly prevalent at 24 h after intubation under assist-control ventilation. Reverse triggering seems to occur during the transition phase between deep sedation and the onset of patient triggering.

Work of Breathing in Premature Neonates: Noninvasive Neurally-Adjusted Ventilatory Assist versus Noninvasive Ventilation.

BACKGROUND: We tested whether work of breathing in premature newborns estimated by phase angle (θ) by using respiratory inductance plethysmography is decreased during neurally-adjusted ventilatory assist (NAVA) noninvasive ventilation (NIV) versus NIV alone. METHODS: NAVA NIV and NIV were applied in random order while using respiratory inductance plethysmography to measure the phase angle. RESULTS: Patient-ventilator asynchrony was decreased during NAVA NIV; however, the phase angle was not different between the modes. A large number of repeated assists with switches to backup were found when using NAVA NIV. Results of the analysis indicated these were due to the apnea alarm limit set during NAVA NIV. CONCLUSIONS: The improvement in patient-ventilator synchrony supports the hypothesis that work of breathing may be decreased with NAVA NIV; however, we were unable to demonstrate this with our study design. Short apnea time settings with NAVA NIV led to a large number of switches to backup and repeated assists during the same neural effort. (ClinicalTrials.gov registration NCT02788110.).

[Functional consequences of eccentric contractions of the diaphragm]. / Consecuencias de las contracciones excéntricas del diafragma sobre su función.

INTRODUCTION AND OBJECTIVES: Eccentric contractions are those that occur after a muscle has been stretched, and they can predispose the muscle to damage. Most previous studies have been performed on limb muscles, and the potential consequences of eccentric contractions on the respiratory muscles are therefore unknown. The aim of this study was to evaluate the effects of repeated eccentric contractions on diaphragmatic function. METHODS: In 6 dogs, the diaphragm was stretched by applying pressure on the abdominal wall, and consecutive series of eccentric contractions were induced by bilateral supramaximal stimulation. The effect of these contractions on the diaphragm was then evaluated by applying bilateral twitch and tetanic stimulation of the phrenic nerves and measuring the changes in abdominal pressure and the shortening of the right and left hemidiaphragms (by sonomicrometry). Structural study of the muscle was also performed in 4 animals. RESULTS: Eccentric contractions were successfully achieved in all cases. Stimulation-induced diaphragmatic pressures became lower immediately after these contractions: twitch pressure fell by 53% and tetanic pressure by 67% after the first 10 eccentric contractions (P<.001 in both cases). Tetanic stimulation also demonstrated an early deterioration in contractility, which fell by 29% in the right hemidiaphragm (P<.05) and by 14% in the left hemidiaphragm (P<.001). Functional impairment was persistent, lasting at least 12 hours, and was associated with sarcomeric and sarcolemmal damage. CONCLUSIONS: This experimental model, which enabled the effects of eccentric contractions to be studied in the diaphragm, revealed a deterioration of muscle function that persisted for hours and that appeared to be partly due to structural damage. In the clinical setting, physiologic or therapeutic maneuvers that increase the resting length of the diaphragm should be used with caution.

[Modifications of diaphragmatic activity induced by midline laparotomy and changes in abdominal wall compliance]. / Modificaciones en la actividad del diafragma inducidas por laparotomía media y cambios en la rigidez de la pared abdominal.

INTRODUCTION AND OBJECTIVE: Diaphragmatic activity varies with the initial length of the muscle. Our objective was to evaluate the influence of surgery and changes in abdominal wall compliance on diaphragmatic activity. METHODS: Both phrenic nerves in 7 mongrel dogs were stimulated electrically with single supramaximal pulses (twitch). The gastric (Pga) and transdiaphragmatic (Pdi) pressures generated and muscle shortening (sonomicrometry) were used to evaluate diaphragmatic activity, which was determined at baseline, after midline laparotomy, with an elastic abdominal bandage, and with a rigid circular cast. Abdominal pressure was then gradually increased in order to induce progressive lengthening of the diaphragm. RESULTS: After laparotomy, the pressures were somewhat lower (by 12%) than at baseline. The elastic bandage produced a slight increase in the pressure generated by the diaphragm (mean [SE] values: Pga, from 4.2 [0.3]cm H(2)O to 6.3 [0.9]cm H(2)O, P<.01; Pdi(tw), from 12.1 [2.0]cm H(2)O to 15.4 [1.8]cm H(2)O, P<.05]), and these values increased even further with the rigid cast (Pga, to 12.6 [1.5]cm H(2)O; Pdi, to 20.2 [2.3]cm H(2)O; P<.01 for both comparisons); this occurred despite smaller degrees of muscle shortening: by 57% [5%] of the initial length at functional residual capacity at baseline, by 49% [5%] with the bandage (P<.05), and by 39% [6%] with the cast (P<.01). With progressive lengthening of the muscle, its contractile efficacy increased up to a certain point (105% of the length at functional residual capacity), after which it began to decline. CONCLUSIONS: Abdominal wall compliance plays an important role in the diaphragmatic response to stimulation. This appears to be due mainly to changes in its length at rest.

Tidal volume transmission during non-synchronized nasal intermittent positive pressure ventilation via RAM® cannula.

BACKGROUND: Nasal intermittent positive pressure ventilation (NIPPV) is a widely used mode of support in neonates, during which ventilator inflations may or may not coincide with spontaneous breathing. OBJECTIVE: We tested the hypothesis that inflations delivered with NIPPV via RAM® cannula and not accompanied by patient effort produce minimal tidal volume as measured by respiratory inductance plethysmography. DESIGN/METHODS: Fourteen subjects were monitored while receiving NIPPV. We compared tidal volumes during ventilator-supported breaths, unsupported breaths, and ventilator inflations not accompanied by patient effort (defined using electrical activity of the diaphragm). RESULTS: Mean tidal volumes in arbitrary units were 0.30 ± 0.22 in NIPPV inflations associated with patient effort and 0.27 ± 0.15 in spontaneous breaths without ventilator assistance (p = 0.82). Tidal volumes during ventilator-only inflations were 0.06 ± 0.04 (p < 0.005 vs. both ventilator-assisted and unassisted efforts). CONCLUSIONS: NIPPV via RAM cannula produces minimal, clinically insignificant tidal volumes during non-spontaneous inflations.

[Diaphragmatic response is influenced by previous muscle activity]. / Respuesta diafragmática en función de la actividad previa del músculo.

OBJECTIVE: Previous muscle activity can alter muscle contractility and lead to strength underestimation or overestimation in functional measurements. The objective of this study was to evaluate changes in the maximum pressure produced by the diaphragm after different series of spontaneous near-to-maximal isometric contractions. METHODS: Duplicate studies were performed on 6 dogs with a mean (SD) weight of 26 (7) kg. The supramaximal response of the diaphragm was achieved by simultaneous supramaximal stimulation of both phrenic nerves, both under basal conditions and after series of 5, 10, and 20 spontaneous inspiratory efforts against the occluded airway, performed before and after spinal anesthesia (which eliminates the ventilatory contribution of the intercostal muscles). The response was measured using the twitch gastric pressure (Pga) and twitch esophageal pressure (Pes) and by muscle shortening (sonomicrometry). RESULTS: The short series of 5 inspiratory efforts and, in particular, the medium series of 10 efforts produced potentiation of the contractile response, with a rise in the Pga from 3.2 (0.4) cm H(2)O to 3.7 (0.3) cm H(2)O, and from 3.5 (0.3) cm H(2)O to 3.9 (0.3) cm H(2)O, respectively (P=.05 in both cases). The potentiation was somewhat greater after subarachnoid anesthesia (an increase in the Pga of 21% after the medium series of 10 efforts with anesthesia vs 11% without anesthesia). However, the long series of 20 efforts produced a fall in the response, with a decrease in the Pga from 3.2 (0.4) cm H(2)O to 2.5 (0.3) cm H(2)O (P< .05), probably due to fatigue overcoming the effect of potentiation. CONCLUSIONS: Previous effort affects the contractile capacity of the diaphragm and it is difficult to predict the predominance of fatigue or potentiation in the response. This factor must be taken into account when determining the maximum respiratory pressures in daily clinical practice.