Biomedical subjects

D K McKenzie

Publications and source records attributed to D K McKenzie.

At least 19 recordsLinked to original sources

PubMed1998

Twitch interpolation of the elbow flexor muscles at high forces.

We investigated factors affecting maximal voluntary torque and the assessment of the level of voluntary drive in the elbow flexor muscles. First, the effective compliance of the system was tested by using single, paired, and trains of four stimuli to measure voluntary activation. At high voluntary torques the responses to all these stimuli were identical, suggesting that single stimuli are adequate for estimating voluntary drive. Second, the contribution of torque from synergist elbow flexor muscles was assessed. In attempted maximal voluntary contractions (MVCs), the voluntary activation of brachioradialis (median 91.5%, range 68.9-100%) was lower than for biceps brachii (median 99.1%, range 78.5-100%; P < 0.01). This suggests extra torque may be generated by brachioradialis during elbow flexion, beyond the torque where biceps brachii is maximally activated. Finally, lengthening of the elbow flexors occurred during MVCs, due to slight shoulder movements. This would allow force to increase independently of an increase in voluntary drive.

Adult↗

PubMed1998

Human respiratory muscles: sensations, reflexes and fatiguability.

1. Given the importance of the ventilatory 'pump' muscles, it would not be surprising if they were endowed with both sensory and motor specializations. The present review focuses on some unexpected properties of the respiratory muscle system in human subjects. 2. Although changes in blood gas tension were long held not to influence sensation directly, studies in subjects who are completely paralysed show that increases in arterial CO2 levels elicit strong sensations of respiratory discomfort. 3. Stretch reflexes in human limb muscles contain a monosynaptic spinal excitation and a long-latency excitation. However, inspiratory muscles show an initial inhibition when tested with brief airway occlusions during inspiration. This inhibition does not depend critically on input from pulmonary or upper airway receptors. 4. Human inspiratory muscles (including the diaphragm) have been considered to fatigue during inspiratory resistive loading. However, recent studies using phrenic nerve stimulation to test the force produced by the diaphragm show that carbon dioxide retention (hypoventilation) and voluntary cessation of loading occur before the muscles become overtly fatigued.

Carbon Dioxide↗

PubMed1997

Contraction of the human diaphragm during rapid postural adjustments.

1. The response of the diaphragm to the postural perturbation produced by rapid flexion of the shoulder to a visual stimulus was evaluated in standing subjects. Gastric, oesophageal and transdiaphragmatic pressures were measured together with intramuscular and oesophageal recordings of electromyographic activity (EMG) in the diaphragm. To assess the mechanics of contraction of the diaphragm, dynamic changes in the length of the diaphragm were measured with ultrasonography. 2. With rapid flexion of the shoulder in response to a visual stimulus, EMG activity in the costal and crural diaphragm occurred about 20 ms prior to the onset of deltoid EMG. This anticipatory contraction occurred irrespective of the phase of respiration in which arm movement began. The onset of diaphragm EMG coincided with that of transversus abdominis. 3. Gastric and transdiaphragmatic pressures increased in association with the rapid arm flexion by 13.8 +/- 1.9 (mean +/- S.E.M.) and 13.5 +/- 1.8 cmH2O, respectively. The increases occurred 49 +/- 4 ms after the onset of diaphragm EMG, but preceded the onset of movement of the limb by 63 +/- 7 ms. 4. Ultrasonographic measurements revealed that the costal diaphragm shortened and then lengthened progressively during the increase in transdiaphragmatic pressure. 5. This study provides definitive evidence that the human diaphragm is involved in the control of postural stability during sudden voluntary movement of the limbs.

Adult↗

PubMed1997

Task failure with lack of diaphragm fatigue during inspiratory resistive loading in human subjects.

Task failure during inspiratory resistive loading is thought to be accompanied by substantial peripheral fatigue of the inspiratory muscles. Six healthy subjects performed eight resistive breathing trials with loads of 35, 50, 75 and 90% of maximal inspiratory pressure (MIP) with and without supplemental oxygen. MIP measured before, after, and at every minute during the trial increased slightly during the trials, even when corrected for lung volume (e.g., for 24 trials breathing air, 12.5% increase, P < 0.05). In some trials, task failure occurred before 20 min (end point of trial), and in these trials there was an increase in end-tidal PCO2 (P < 0.01), despite the absence of peripheral muscle fatigue. In four subjects (6 trials with task failure), there was no decline in twitch amplitude with bilateral phrenic stimulation or in voluntary activation of the diaphragm, even though end-tidal PCO2 rose by 1.6 +/- 0.9%. These results suggest that hypoventilation, CO2 retention, and ultimate task failure during resistive breathing are not simply dependent on impaired force-generating capacity of the diaphragm or impaired voluntary activation of the diaphragm.

Adult↗

PubMed1997

Pulmonary afferents are not necessary for the reflex inhibition of human inspiratory muscles produced by airway occlusion.

In contrast to limb muscles, the usual response of human inspiratory muscles to sudden loading consists of an initial marked reduction of electromyographic activity (EMG) followed by a subsequent increase in EMG. To determine definitively whether pulmonary receptors are necessary for this short-latency reflex inhibition produced by airway occlusion, we studied five subjects with complete pulmonary denervation due to bilateral transplantation of the lungs and five matched control subjects. Subjects with pulmonary denervation were studied between 10 and 50 days after transplantation (median 21 days). Brief airway occlusion during inspiration (i.e., loading; duration 250 ms) produced short-latency reduction in EMG in the inspiratory muscles of all subjects with acute pulmonary denervation (scalenes and parasternal intercostal muscles; mean onset of inhibition 27 and 29 ms, respectively). The ongoing EMG was reduced by an average of 50% in scalenes and 36% in parasternal intercostal muscles. The size and the magnitude of the initial response did not differ significantly from those in control subjects. After the occlusion (i.e., unloading), activity of the inspiratory muscles was transiently reduced in control subjects and patients after bilateral lung transplantation. Given that the initial responses to airway loading and unloading were preserved after bilateral lung transplantation, we conclude that these reflex responses are not critically dependent on the discharge of intrapulmonary receptors. The results support the view that the short-latency inspiratory responses to loading and unloading can be mediated by inspiratory muscle afferents. They suggest a functionally different organization of the reflex pathways for inspiratory compared with limb muscles.

Adolescent↗

PubMed1997

Neural drive to the diaphragm in patients with severe COPD.

Patients with severe chronic obstructive pulmonary disease (COPD) have a greater neural drive to the parasternal intercostal and scalene muscles and greater inspiratory expansion of the rib cage than do healthy individuals. However, such patients also have a reduced outward displacement or a paradoxical inward displacement of the ventral abdominal wall during inspiration. This has led to the suggestion that they may have less use of the diaphragm, possibly secondary to chronic muscle fatigue. To assess the effect of COPD on the neural drive to the diaphragm, we inserted needle electrodes into the costal part of the right hemidiaphragm in eight patients with severe disease (mean [+/- SD] FEV1: 0.82 [+/- 0.27] L) and six control subjects of similar age, and measured the discharge frequencies of single motor units during resting breathing. A total of 115 diaphragmatic motor units were recorded in the control subjects and 122 in the patients. All motor units discharged rhythmically in phase with inspiration. However, whereas 95% of the units in the control subjects had a peak discharge frequency between 7 and 14 Hz, 79% of the units in the COPD patients had a peak discharge frequency greater than 15 Hz. As a result, the discharge frequency of all units averaged 10.5 [+/- 2.4] Hz in the control subjects, but 17.9 [+/- 4.3] Hz in the patients (p < 0.001). These observations indicate that patients with severe COPD have an increased neural drive not only to the rib cage inspiratory muscles, but also to the diaphragm. Consequently, the reduced inspiratory expansion of the abdomen in severe COPD results from mechanical factors alone.

Aged↗

PubMed1996

Reduced voluntary drive to the human diaphragm at low lung volumes.

Maximal inspiratory and transdiaphragmatic pressures vary with lung volume but the possibility that some of this variability reflects variable voluntary drive to the diaphragm has not been investigated systematically. We assessed the influence of lung volume on the ability to activate the diaphragm with voluntary effort during maximal Mueller manoeuvres. Voluntary activation of the diaphragm was assessed using twitch interpolation with bilateral phrenic nerve stimulation in 6 subjects. Each performed 10 maximal efforts at lung volumes around functional residual capacity (FRC) and additional efforts at volumes above and below FRC. Voluntary activation of the diaphragm was higher at lung volumes above FRC (> 60% TLC; activation 98.3 +/- 2.6%) than at lung volumes around FRC (45-60% TLC; activation 95.5 +/- 3.5%) and below FRC (< 45% TLC; activation 83.3 +/- 15.8%; p < 0.05). Submaximal diaphragm activation at low lung volumes may reflect differences in the length-tension relationships of the various inspiratory muscles and/or reflex inhibition of phrenic motoneurones at low lung volumes.

Adult↗

PubMed1996

Impaired reflex responses to airway occlusion in the inspiratory muscles of asthmatic subjects.

BACKGROUND: Asthmatic subjects have an impaired capacity to activate the diaphragm during attempted maximal inspiratory efforts. Limb muscles require reflex facilitation to achieve maximal force. The reflex responses of inspiratory muscles to airway occlusion in asthmatic subjects were measured and compared with those in non-asthmatic control subjects. METHODS: Nine healthy asthmatic subjects breathed at a constant inspiratory flow through a low resistance valve. Random inspirations were transiently occluded for 250 ms. Surface electromyographic activity (EMG) was recorded over the scalene muscles, parasternal intercostal muscles, and the lateral chest wall overlying the diaphragm. The asthmatic subjects were studied with and without bronchoconstriction. Responses were compared with data from a matched group of 12 control subjects. RESULTS: Compared with the reflex responses to airway occlusion in control subjects, the duration of the initial short latency inhibition of inspiratory muscles was prolonged by 50% in asthmatic subjects and the size of the subsequent excitation was reduced by 30%. Bronchoconstriction reduced the time to the peak of the excitatory response in asthmatic subjects, although the values remained longer than in the control subjects. CONCLUSIONS: This study reveals impaired reflex excitation of inspiratory motoneurones in asthmatic subjects which could contribute to the reduced ability of these subjects to drive the diaphragm during maximal volitional efforts.

Adolescent↗

PubMed1996

Discharge frequencies of parasternal intercostal and scalene motor units during breathing in normal and COPD subjects.

To determine whether patients with chronic obstructive pulmonary disease (COPD) contract the inspiratory muscles of the rib cage more strongly than do healthy subjects, we recorded the discharge frequencies of single motor units in the scalene and second parasternal intercostal muscles of seven patients with stable COPD (FEV1 = 33 +/- 13% predicted, mean +/- SD) and seven control subjects. Recordings were made with insulated monopolar electrodes during resting breathing, and single motor-unit discharges were identified with a customized method based on "template" matching. A total of 211 motor units were recorded in the control subjects and 260 in the patients. The inspiratory discharge frequencies were greater in the COPD patients than in the control subjects for both the parasternal (13.4 versus 10.1 Hz, p < 0.05) and scalene (11.4 versus 8.5 Hz, p < 0.02) muscles. Recording sites at which no motor units were recruited were more common in the control subjects than in the patients (p < 0.001). The sternomastoid muscle was silent in both subject groups. Therefore, effective central neural drive is increased to both the scalene and parasternal intercostal muscles but not to the sternomastoid muscle in patients with COPD.

Action Potentials↗

PubMed1995

Role of airway receptors in the reflex responses of human inspiratory muscles to airway occlusion.

1. This study was designed to investigate the role of airway receptors in the responses of a range of inspiratory muscles to airway occlusion. The occlusion had a rapid onset (< 10 ms), lasted 250 ms and produced only a slight impediment to inspiration. 2. Based on analysis of single trials and averaged rectified electromyographic responses (EMG) in six subjects, there was a major inhibition (IR) with an onset at 34 +/- 2 ms and a trough at 65 +/- 2 ms, and an excitation (ER) with a peak at 105 +/- 2 ms. These two responses are reflex given that voluntary reaction times to a tap on the chest wall occurred at latencies longer than the peak of ER. 3. The responses to airway occlusion did not appear in limb muscles which contracted phasically with inspiration. 4. Anaesthesia of the surface receptors of the upper airway did not attenuate the responses to occlusion. Because this procedure does not eliminate the inputs from muscle and deep laryngeal pressure receptors, two subjects were tested when intubated with a cuffed endotracheal tube so that the occlusion was delivered only to structures below the level of the trachea. Responses to airway occlusion were preserved when all upper airway receptors were 'bypassed'. 5. Responses to airway occlusion also remained after prolonged inhalation of nebulized lidocaine (lignocaine) sufficient to block the cough reflex. 6. The receptors mediating the responses to airway occlusion are therefore likely to reside in inspiratory muscles acting on the chest wall. If so, the short-latency inhibition contrasts with the excitatory stretch reflex responses observed in limb muscles.

Adaptation, Physiological↗

PubMed1995

Reliability of measurements of muscle strength and voluntary activation using twitch interpolation.

We investigated the reproducibility of measurements of maximal voluntary torque and maximal voluntary activation using twitch interpolation. On 5 days, each of 5 subjects performed 10 maximal voluntary isometric contractions of their elbow flexors. Single supramaximal stimuli were delivered over biceps brachii at the measured peak torque during each effort, and in the relaxed muscle 5 s later. A voluntary activation score was calculated from the size of twitches evoked by the stimuli (resolution < 0.15 Nm). Although all subjects were able to drive the stimulated elbow flexor muscles maximally in some trials, they did not do so in 75% of all contractions. Maximal voluntary torques did not vary significantly within a subject between sessions. There were consistent differences in the level of maximal voluntary activation between subjects (P < 0.01), but no differences in voluntary activation within an individual across days in 4 of 5 subjects. Failure to drive the stimulated elbow flexor muscles maximally was not associated with inadvertent co-contraction of the antagonist muscles.

Adult↗

PubMed1995

Respiratory muscle fatigue.

Ventilatory failure may accompany a variety of pulmonary and neuromuscular diseases. There has been much controversy about whether this failure is due to respiratory muscle fatigue at peripheral sites or a failure of drive at sites within the central nervous system. The chapter reviews this topic.

Animals↗

PubMed1995

Software compensation for lung volume in assessment of inspiratory muscle strength and endurance.

BACKGROUND: To increase the sensitivity of measurements of maximal inspiratory pressure (MIP) as a test of inspiratory muscle strength and endurance, software was developed to correct for variation in lung volume. METHODS: Using a body plethysmograph to determine absolute lung volume during each manoeuvre, values for MIP were expressed as a percentage of the pressure "expected" from the unfatigued MIP/lung volume relation in each subject. RESULTS: The method reduced the variance in peak and average pressure during a series of 18 MIP manoeuvres of 10 seconds duration separated by rest intervals of 10 seconds. CONCLUSIONS: The correlation between average pressure and contraction number was improved significantly by the MIP/lung volume correction. This simple correction has many applications in measurements of the MIP.

Adult↗

PubMed1995

Central fatigue. Critical issues, quantification and practical implications.

Central fatigue during exercise is the decrease in muscle force attributable to a decline in motoneuronal output. Several methods have been used to assess central fatigue; however, some are limited or not sensitive enough to detect failure in central drive. Central fatigue develops during many forms of exercise. A number of mechanisms may contribute to its development including an increased inhibition mediated by group III and IV muscle afferents along with a decrease in muscle spindle facilitation. In some situations, motor cortical output is shown to be suboptimal. A specific terminology for central fatigue is included.

Exercise↗

PubMed1994

Dynamic changes in the zone of apposition and diaphragm length during maximal respiratory efforts.

BACKGROUND: Clinical tests of diaphragmatic strength are limited by the wide normal variation in maximal pressure which result, in part, from changes in diaphragmatic length. During relaxation at different lung volumes diaphragmatic length (LDI) can be estimated from the length of the zone of apposition (LZapp) and the transverse diameter of the rib cage (DRC). A study was carried out in two subjects using sequential digital radiography at six frames/second to determine whether these relations apply during maximal respiratory efforts which distort the rib cage and diaphragm. METHODS: The length of the anteroposterior contour of the diaphragm and DRC were determined by curve fitting. LZapp was measured with a millimetre rule. RESULTS: A significant correlation was found between LDI and LZapp during both maximal inspiratory and expulsive manoeuvres (R2 = 0.88 and 0.52). LDI was estimated from the measurements of LZapp and DRC using a multiple regression equation derived from measurements during static relaxation. Despite the complex dynamic events at the onset of these "static" manoeuvres, actual LDI correlated strongly with derived LDI using all data for the two manoeuvres in each subject (R2 = 0.95 and 0.84). Measurements with ultrasonography (12 cm linear probe) and magnetometers confirmed the changes in LZapp and DRC during inspiratory and expulsive efforts. CONCLUSIONS: Non-invasive measurements of LZapp and DRC can be used to derive an accurate estimate of diaphragmatic length under dynamic conditions.

Adult↗

PubMed1994

Impaired voluntary drive to breathe: a possible link between depression and unexplained ventilatory failure in asthmatic patients.

BACKGROUND: Although psychological distress predicts mortality in asthma, an underlying physiological link has not been shown. This study examined relations between impaired voluntary drive to breathe and measures of mood states. METHODS: The level of maximal voluntary activation of the diaphragm and elbow flexors was measured in a previous study using a sensitive modification of the twitch interpolation technique in 11 asthmatic and 10 control subjects. In this study psychological distress was assessed using the Profile of Mood States questionnaire and measures of distress were compared with the muscle voluntary activation results. RESULTS: For the asthmatic subjects, depressed mood increased the risk of impaired maximal voluntary activation of the diaphragm by 3.5 times (95% CI 1.09 to 11.3). No such association was observed in control subjects. CONCLUSIONS: These results suggest that depressed mood may predispose an asthmatic patient to impaired voluntary activation of the diaphragm. Such individuals would be at increased risk of rapidly developing ventilatory failure if faced with severe airway narrowing.

Asthma↗

PubMed1993

Shoulder dystocia: an analysis of risks and obstetric maneuvers.

OBJECTIVE: The purpose of this study was to determine whether there is a risk profile for predicting or preventing shoulder dystocia and whether any of the obstetric maneuvers to disimpact a shoulder reduce the likelihood of permanent injury. STUDY DESIGN: A retrospective analysis of 14,297 parturients with 12,532 vaginal deliveries and 1765 cesarean sections (12.4%) from January 1986 through June 1990 was performed. A total of 204 maternal and infant charts, related to shoulder dystocia or neonatal injury, were reviewed in depth for age, parity, episiotomy, type of delivery, hemorrhage, maternal obesity, diabetes, weight gain, fetal weight, sex, and Apgar scores. In addition, the type of maneuver or combination thereof used to relieve the dystocia, type of injury to the infant, and follow-up of the injury were reviewed. RESULTS: The 185 coded episodes of shoulder dystocia represent 1.4% of all vaginal deliveries (12,532). There were 42 injuries recorded: 14 fractured clavicles and 28 brachial plexus injuries. An additional 19 patients, not coded for shoulder dystocia, sustained 14 fractured clavicles and five brachial plexus injuries. All but one of the brachial plexus injuries resolved by 6 months. The occurrence of shoulder dystocia increased in direct relationship to the birth weight and becomes significant in newborns over 4000 gm (p < 0.01). The occurrence of a previous large infant was also a significant risk factor (p < 0.01). Diabetes and midforceps delivery become significant factors only in the presence of a large fetus. Obesity, multiparity, postdate pregnancy, use of oxytocin, low forceps delivery, episiotomy, and type of anesthesia were unrelated to shoulder dystocia. No delivery method was without injury. CONCLUSIONS: This study clearly indicates that most of the traditional risk factors for shoulder dystocia have no predictive value, shoulder dystocia itself is an unpredictable event, and infants at risk for permanent injury are virtually impossible to predict. In addition, no delivery method in shoulder dystocia was superior to another with respect to injury. Thus no protocol should serve to substitute for clinical judgment.

Delivery, Obstetric↗

PubMed1993

Reduced voluntary drive to breathe in asthmatic subjects.

Maximal voluntary drive to the diaphragm and a non-respiratory muscle group (elbow flexors) was compared in 10 control subjects and 11 asthmatics who were studied when well. The degree of voluntary activation during repeated attempted maximal quasi-static efforts was determined using the twitch interpolation technique in the absence of contractile fatigue under both control conditions and following bronchial challenge with histamine. Diaphragm activation was assessed using bilateral phrenic stimulation at the normal resting end-expiratory lung volume after exhalation from TLC. Asthmatic subjects showed lower and more variable voluntary activation than control subjects for both diaphragm (82.0% +/- 18.4 [SD], vs 87.8% +/- 12.0, P < 0.01) and elbow flexors (91.3% +/- 7.6 vs 95.8% +/- 4.1, P < 0.01). Histamine challenge decreased FEV1 in asthmatic subjects to 50% of the initial value, but had no significant effect on voluntary activation in either subject group. The decreased voluntary drive to the diaphragm observed in some asthmatic subjects may predispose to rapid development of ventilatory failure during severe airway narrowing.

Adult↗