Norman Comtois

Neural control of mechanical ventilation in respiratory failure

Mechanical ventilation is a life-saving intervention for the management of acute respiratory failure. Its objective is to reduce excessive respiratory effort while improving gas exchange. By applying positive pressure to the airway, the mechanical ventilator assumes to a varying extent the work necessary to breathe, thereby unloading the respiratory muscles. In its most basic form, called controlled mechanical ventilation, a pre-set tidal volume is delivered at a fixed rate, irrespective of the patient’s own breathing pattern. If the mechanical and natural respiratory cycles are not matched, however, the patient ‘fights’ the ventilator, causing discomfort, gas exchange deterioration and cardiovascular impairment 1 . To avoid discoordination between the patient and the ventilator, it is often necessary to suppress the patient’s intrinsic respiratory drive with the use of hyperventilation, sedation or even muscle paralysis, which increase the risk of complications due to excessive ventilation 2‐3 , drug-related adverse effects

Effects of diaphragm shortening on the mean action potential conduction velocity in canines.

1. The present study was designed to test if the mean muscle fibre action potential conduction velocity (VAPC) in the costal diaphragm changes with muscle length, in spontaneously breathing mongrel dogs. 2. VAPC was determined by the electromyogram (EMG) power spectrum ‘dip’ method, which is based on the bipolar electrode transfer function. A bipolar EMG electrode with a 20 mm fixed interelectrode distance was sutured to the costal diaphragm in the fibre direction, and in a region with a low density of motor endplates. Diaphragm length was measured with piezoelectric crystals positioned next to the EMG electrode. Seven dogs were vagotomized and spinally anaesthetized in order to increase diaphragmatic shortening, reduce velocity of shortening and abolish possible cross‐talk signals from adjacent muscles. 3. Our results showed that VAPC in the canine costal diaphragm was 3.4 m s‐1 and was not significantly related to diaphragmatic shortening.

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.

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.

Functional consequences of eccentric contractions of the diaphragm

Abstract 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 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

a b s t r a c t 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 H2O to 6.3 (0.9) cm H2O, P<.01; Pdi, from 12.1 (2.0) cm H2O to 15.4 (1.8) cm H2O, P<.05)), and these values increased even further with the rigid cast (Pga, to 12.6 (1.5) cm H2O; Pdi, to 20.2 (2.3) cm H2O; 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.

Respuesta diafragmática en función de la actividad previa del músculo

Objetivo La actividad previa puede modificar la contractilidad muscular, lo que puede conducir a la infra o supraestimacion de la fuerza en las determinaciones funcionales. El presente trabajo se ha propuesto como objetivo valorar cambios en la presion maxima generada por el diafragma tras diferentes series de contracciones isometricas espontaneas y cuasi maximas. Metodos Se estudiaron por duplicado 6 perros con un peso medio ± desviacion estandar de 26 ± 7 kg. Se obtuvo la respuesta supramaxima del diafragma –presiones gastrica (Pgatw) y esofagica (Pestw) inducidas por estimulacion frenica bilateral, y acortamiento muscular (sonomicrometria)— por estimulacion simultanea supramaxima de ambos nervios frenicos, tanto en situacion basal como tras series cortas (5), medianas (10) y largas (20) de esfuerzos inspiratorios espontaneos contra la via aerea ocluida, antes y despues de administrar anestesia subaracnoidea (elimina la contribucion ventilatoria de los musculos intercostales). Resultados La serie corta y, sobre todo, la serie mediana provocaron la potenciacion de la respuesta contractil (Pgatw de 3,2 ± 0,4 a 3,7 ± 0,3, y de 3,5 ± 0,3 a 3,9 ± 0,3 cmH2O, respectivamente; p < 0,05 ambas). La potenciacion fue algo superior con anestesia subaracnoidea (un 21 frente al 11% sin anestesia, para la Pgatw tras las series medianas). La serie larga provoco, sin embargo, una disminucion de la respuesta (Pgatw: 3,2 ± 0,4 a 2,5 ± 0,3 cmH2O; p < 0,05), probablemente por predominio de la fatiga sobre la potenciacion. Conclusiones Los esfuerzos previos determinan la capacidad contractil del diafragma y resulta dificil predecir el predominio de fatiga o de potenciacion en la respuesta. Este factor deberia tenerse en cuenta al determinar las presiones respiratorias maximas en la clinica diaria.

Diaphragmatic Response Is Influenced by Previous Muscle Activity

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.