Inspiratory Flow Resistance Research Articles

0769 Comparison of Expiratory Pressures Generated by ULTepap, Provent, Bongo Rx, and Theravent EPAP Devices: A Laboratory Bench Testing

Abstract Introduction Expiratory positive airway pressure (EPAP) has been a treatment option for patients with obstructive sleep apnea (OSA). Among devices like Provent, Theravent, and Bongo, ULTepap is a new FDA-cleared EPAP device that seals around the nares with a nasal pillow interface. Comparisons among the expiratory pressures that these devices might generate are not available. Research Question: Are expiratory threshold therapy devices equivalent in generating backpressure in an expiratory direction? Methods A test rig was designed and fabricated to test the pressures generated by ULTepap, Provent, Theravent, and Bongo Rx. Airflow was generated by a linear actuator-driven piston in a syringe, and a range of flow rates was provided by varying the voltage input to the actuator. The resulting pressures were measured in an expiratory and inspiratory direction. Results The backpressures produced by ULTepap and Provent were comparable at flow rates of 99/142/212 ml/sec (average 3.5/7.5/13.8 cmH2O for ULTepap vs 4.5/8.5/14.5 cmH2O for Provent, p=0.7918). Bongo Rx and Theravent devices produced substantially lower backpressures than ULTepap devices (average 0.8/1.8/3.5 cmH2O for Bongo Rx and 0.9/2.2/5.3 cmH2O for Theravent at flow rates of 99/142/212 ml/sec, p=0.0138 and 0.0404, respectively). In comparison, all four devices presented very low inspiratory flow resistance, with all generating 0.5 cmH2O or less at all flow rates. Conclusion Not all FDA-cleared EPAP devices produce similar expiratory pressure profiles. ULTepap generated backpressures closest to that of Provent. Clinical trials comparing the efficacy, tolerance, and role of these EPAP devices in patients with OSA are warranted. Support (If Any)

Comparison of expiratory pressures generated by four different EPAP devices in a laboratory bench setting

Objective/backgroundExpiratory positive airway pressure (EPAP) has been a treatment option for patients with obstructive sleep apnea (OSA). ULTepap is a new FDA-cleared EPAP device that seals the nares with a nasal pillow interface. Comparisons of expiratory pressures generated by ULTepap and other EPAP devices like Provent, Bongo Rx, and Theravent are not available. We aimed to compare the backpressures created by these devices in an in vitro laboratory bench setting. MethodsA test rig was designed and fabricated to test the expiratory pressures generated by ULTepap, Provent, Bongo Rx, and Theravent. Airflow was generated by a linear actuator-driven piston in a syringe, and a range of flow rates was provided by varying the voltage input to the actuator. The resulting expiratory and inspiratory pressures were measured and resistances were calculated. ResultsThe backpressures generated by ULTepap and Provent were comparable at all flow rates. For flow rates at 99/142/212 ml/s, the expiratory pressures were 3.5/7.5/13.8 cmH2O for ULTepap and 4.5/8.5/14.5 cmH2O for Provent. Bongo Rx and Theravent devices produced substantially lower backpressures compared to ULTepap devices (0.8/1.8/3.5 cmH2O for Bongo Rx and 0.9/2.2/5.3 cmH2O for Theravent at flow rates of 99/142/212 ml/s). All four devices presented very low inspiratory flow resistance, with all generating 0.5 cmH2O or less at all flow rates. ConclusionNot all FDA-cleared EPAP devices produce similar expiratory pressure profiles. ULTepap generated backpressures closest to that of Provent. Clinical trials comparing the efficacy, tolerance, and adherence of these EPAP devices in patients with OSA are warranted.

Columellar strut grafts versus septal extension grafts during rhinoplasty for airway function, patient satisfaction and tip support

Different techniques exist to provide tip support in rhinoplasty. There is little evidence to provide a consensus on the most effective choice.Evaluating columellar strut graft (CSG) and septal extension grafts (SEG) for their influence on airway function, patient satisfaction and tip support.A retrospective cohort study was undertaken on 165 adult patients who underwent open rhinoplasty with either a CSG or SEG, from February 2012 to August 2019 in a single tertiary facial-plastic practice in Sydney, Australia. Operations were for both cosmetic and functional indications, and both primary and revision cases were assessed. Airway testing and patient-reported outcomes (PROMs) were performed preoperatively and at least 6 months following the procedure. Photographic tip analysis was taken from approximately 4 and 12-month postoperative photographs.Nasal peak inspiratory flow (NPIF) and total nasal airway resistance (NAR) were the primary airway functional outcomes. The primary PROMs analysed were a visual analogue scale (VAS) for nasal obstruction and 13-point Likert scale for global cosmesis, the Nose Outcome Symptom Evaluation (NOSE), and the nasal obstruction score. Tip support was determined by the nasolabial angle (NLA) and Simon's ratio as assessed by Rhinobase developed by Apaydin et al. on lateral Frankfort plane photographs. Data normalised as an improvement over preoperative baseline, accounting for individual variability.A total of 165 patients was assessed (35.2 ± 12.9 yrs, 72% female), 100 (61%) of which received SEG. There were similar nasal airway assessments between CSG and SEG groups, with ΔNPIF (20.0 ± 42.1 L/min v 19.9 ± 44.9 L/min, p = 0.983) and Δ "obstructed" NAR (-1.13 ± 1.90 v -1.02 ± 4.33 Pa/cm3/s, p = 0.849). Amongst PROMs, a greater cosmetic outcome was seen in the SEG group (7.20 ± 2.97 v 5.69 ± 3.45, p < 0.01) with all other assessments similar between CSG and SEG techniques. Photographic analysis of tip projection showed reduced NLA distortion in the SEG.While greater patient-perceived cosmesis was seen in patients with a SEG, there were similar airflow and patient-reported nasal function between groups. Photographic analysis of tip projection showed SEG patients additionally benefited from less NLA distortion and greater tip maintenance.

Effect of electroacupuncture on lung dysplasia in rats with intrauterine growth restriction induced by maternal food restriction

To investigate the protective effect of electroacupuncture (EA) at "Zusanli" (ST 36) in pregnant rats on lung dysplasia of newborn rats with intrauterine growth restriction (IUGR) induced by maternal food restriction. Twenty-four female SD rats were randomly divided into a control group, a control+EA group, a model group and a model+EA group, 6 rats in each group. From the 10th day into pregnancy to the time of delivery, the rats in the model group and the model+EA group were given with 50% dietary restriction to prepare IUGR model. From the 10th day into pregnancy to the time of delivery, the rats in the control+EA group and the model+EA group were treated with EA at bilateral "Zusanli" (ST 36), once a day. The body weight of offspring rats was measured at birth, and the body weight and lung weight of offspring rats were measured on the 21st day after birth. The lung function was measured by small animal lung function detection system; the lung tissue morphology was observed by HE staining; the content of peroxisome proliferator activated receptor γ (PPARγ) in lung tissue was detected by ELISA. Compared with the control group, the body weight at birth as well as the body weight, lung weight, lung dynamic compliance (Cdyn) and PPARγ at 21 days after birth in the model group were significantly decreased (P<0.01), and the peak inspiratory flow (PIF) and inspiratory resistance (RI) were significantly increased (P<0.01); the number of alveoli was significantly decreased, and the alveolar area and alveolar septal thickness were significantly increased, and some alveoli were ruptured and fused. Compared with the model group, the body weight at birth as well as the body weight, lung weight, Cdyn and PPARγ at 21 days after birth in the model+EA group were significantly increased (P<0.01, P<0.05), and the PIF and RI were significantly reduced (P<0.05); the number of alveoli was significantly increased, and the alveolar area and alveolar septal thickness were significantly reduced, and the rupture and fusion of alveolar was improved. EA at "Zusanli" (ST 36) may protect the lung function and lung histomorphology changes by regulating the level of PPARγ of lung in IUGR rats induced by maternal food restriction.

Effects of Air Stacking on Dyspnea and Lung Function in Neuromuscular Diseases

ObjectiveTo investigate whether the decrease in dyspnea in neuromuscular diseases after air stacking (AS) occurs mostly in patients with decreased inspiratory muscle force and ensuing chest wall restriction or heterogeneous ventilation across the lungs. DesignInterventional, before-after study. SettingA neurorehabilitation inpatient and outpatient center. ParticipantsFifteen consecutive adult patients affected by neuromuscular diseases (N=15). InterventionsAS treatment. Main Outcome MeasuresPatients had vital capacity (VC) and sniff nasal inspiratory pressure (SNIP) measured. We measured Borg score, oxygen saturation, and ventilation heterogeneity across the lung as estimated from the difference between respiratory resistance at 5 and 19 Hz (R5-19) with the forced oscillation technique before and 5, 30, 60, and 120 minutes after applying AS. ResultsBefore AS, Borg score was significantly related to R5-19 (r2 0.46, P<.05) but not to VC % predicted, SNIP % predicted, and time since symptom onset. After AS, average Borg score gradually decreased (P=.005), whereas inspiratory flow resistance at 5 Hz, R5-19, and inspiratory reactance at 5 Hz tended to improve, despite not reaching statistical significance. The decrease in dyspnea at 60 and 120 minutes after AS significantly correlated with baseline R5-19 (r2 0.49, P<.01 and r2 0.29, P<.05, respectively), but not with VC % predicted, SNIP % predicted, time since symptom onset, and clinical severity score for patients affected by amyotrophic lateral sclerosis. ConclusionsThese findings suggest that dyspnea in neuromuscular diseases is related to heterogeneous ventilation rather than inspiratory muscle force and/or lung volumes decrease. Restoring ventilation distribution across the lungs with AS appears to improve dyspnea.

Effect of the external nasal dilator on adolescent athleteswith and without allergic rhinitis

ObjectivesThe ability to effectively breathe through the nose is an important component of physical exercise. The goal of this study is to evaluate the effect of the external nasal dilator (END) on healthy adolescent athletes and those with allergic rhinitis. MethodsClinical trial, double-blind, crossover, in which we evaluated healthy adolescent athletes with allergic rhinitis, using experimental and placebo ENDs, submitted to a maximum cardio-respiratory test in randomized order. Predicted values for peak nasal inspiratory flow (PNIF%) and nasal resistance (NR) were obtained, and the rating of perceived exertion (RPE) was also assessed after the race test. Results65 adolescents participated in the study, 30 of whom had allergic rhinitis. The use of experimental ENDs demonstrated a statistically significant improvement in peak nasal inspiratory flow values (predicted %), nasal resistance, maximal oxygen uptake value (VO2Max.) and rating of perceived exertion, both in the healthy group and the one with allergic rhinitis. ConclusionResults suggested that END reduces nasal resistance, improves maximal oxygen uptake and rating of perceived exertion after a maximum cardio-respiratory test on healthy adolescents and those with allergic rhinitis.

Use of siliconised infant endotracheal tubes reduces work of breathing under turbulent flow

The high resistance of an infant endotracheal tube (ETT) can markedly impair ventilation and gas exchange. Since some manufacturers cover the inner surface of their ETTs with a silicon layer in order to diminish deposition and ease mucous evacuation from airway, via surface roughness decrease, we assessed whether the silicon layer may affect tube resistance, work of breathing and other parameters of ventilation.We compared SUMI (Poland) non-siliconised and siliconised polyvinyl chloride ETTs (2.5, 3.0 and 4.0mm ID), twenty of each type and size combination. Simulating volume-controlled ventilation with the hybrid (numerical–physical) lung models of a premature infant and a 3-month-old baby peak inspiratory pressure (PIP), peak inspiratory and expiratory flow (PIF, PEF), (patient+ETT) inspiratory and expiratory airway resistance (Rins, Rexp) and work of breathing by ventilator (WOBvt) were measured. Additionally, images of the both type surfaces were taken using Hitachi TM-1000 electron microscope.When 2.5 and 3.0mm ID ETTs were examined, laminar flow (Re <2300) across the tube was observed, and there were no clinically significant differences in the ventilation parameters between non-siliconised and siliconised tubes. Whereas, when 4mm ID ETTs were tested, turbulent flow was observed, and PIP, Rins, Rexp and WOBvt were significantly lower (5%, 17%, 17%, and 7%, respectively) (P<0.05), but PIF and PEF were significantly higher (8%, 14%) (P<0.05). Thus, the silicone inner surface of ETT offers less resistance and WOBvt in presence of turbulent flow. However, artifacts observed on the surface of non-siliconised and siliconised ETTs can potentially impair ventilation.

Airway Ventilation Pressures During Bronchoscopy, Bronchial Blocker, and Double-Lumen Endotracheal Tube Use: An In Vitro Study

To quantify inspiratory flow resistance of instrumented single-lumen and double-lumen endotracheal tubes. Bench-top in vitro experiments. Laboratory of a university hospital. In vitro lung simulator. A lung simulator was ventilated mechanically via several single- and double-lumen endotracheal tubes (ETT) that were instrumented with adult and pediatric bronchoscopes as well as bronchial blockers. While ventilating with a square-flow wave and increasing peak inspiratory flow from 10-100 L/min, the pressures proximal and distal to the instrumented ETT were measured. Flow (Q) and the pressure loss (∆P) were related with regression of the quadratic equation: ∆P=k1Q+k2Q2. With all combinations of single-lumen endotracheal tubes, double-lumen endotracheal tubes, bronchial blockers, and adult and pediatric bronchoscopes, ∆P was accurately related to Q using the quadratic equation with excellent fit, R2>0.99 for all combinations. The regression parameters k1 and k2 were statistically significant for all combinations except k1 with a bronchoscope through 37-Fr double-lumen endotracheal tube. Parameter k2 was dominant at flows above 10 L/min for uninstrumented airways and 20 L/min for instrumented airways. ∆P increased dramatically with flow, and increased with decreasing endotracheal tube size or addition of instrumentation in a quantitatively predictable manner. Pressure loss across instrumented endotracheal tubes follows a predictable flow-dependant quadratic pattern. Using the quantitative in vitro results of this study, a clinician can maximize inspiratory ventilation pressures during these situations without delivering excessive airway pressures to the patient.

Comparative evaluation of nasal blood flow and airflow in the decongestant response to oxymetazoline

BackgroundNasal blood flow (NBF) plays a crucial role in many physiological and pathological processes but its regulation and relation to other rhinological outcomes is poorly understood. ObjectivesWe measured nasal airway patency, nasal blood flow, and subjective and objective measures of decongestion and assessed their reproducibility and responsiveness. Methods19 healthy adults attended twice. A dose-response curve was constructed using doubling doses of oxymetazoline of 25μg, 50μg, 100μg, and 200μg at 20 minute intervals. Peak nasal inspiratory flow (PNIF) and nasal airway resistance (NAR) were measured at baseline and after each successive dose, NBF using laser Doppler flowmetry at baseline, 50 μg and 200 μg and a decongestion visual analogue scale after the final dose. ResultsAfter the final dose of oxymetazoline, NBF decreased by a mean (95% CI, P value) of 139.6 (108.3–170.8, P < .001) units and 99.4 (68.1–130.7, P < .001) units, PNIF increased by 48.9 (22.0–75.8, P < .001) L.min-1 and 38.9 (12.0–65.8, P = .003) L.min-1, and NAR decreased by 0.1 (0.02–0.15, P < .001) Pa/s/cm3 and 0.09 (0.02–0.15, P = .002) Pa.s.cm-3 at the first and second visits respectively. The area under the curve of decongestion was not significantly different between visits for each variable. The standardized response means for the decongestant response were as follows: NBF, 1.41; PNIF, 1.03; and NAR, 0.97. ConclusionsNasal blood flow using laser Doppler flowmetry is a sensitive and reproducible outcome to decongestion with oxymetazoline, similar to nasal patency and symptoms. Clinical Trials Registrationwww.clinicaltrials.gov (NCT 00487032).

The Nose as a Target Organ in the Diagnosis of Severe Aspirin-exacerbated Respiratory Disease

The nasal provocation test (NPT) with lysine aspirin is a useful tool in the diagnosis of aspirin-exacerbated respiratory disease (AERD), previously reffered to as Samter's disease. The aim of the present study focuses on methodological interventions to show the usefulness of the NPT with lysine aspirin in differentiating AERD patients from aspirin-tolerant asthma (ATA) patients to improve the diagnostic efficacy and minimize the risk for adverse reactions compared with the gold standard. Thirty AERD patients comprised the active study group while the control group consisted of 25 ATA patients. A combination of objective nasal aerodynamic response (i.e., nasal inspiratory flow and nasal inspiratory resistance) was evaluated by active anterior rhinomanometry and the subjective clinical nasal and extranasal symptoms (including forced expiratory volume) were monitored throughout the challenge. Fifty-five NPTs were successfully completed: sensitivity, 87%; specificity, 100%; positive predictive value, 100%; negative predictive value, 86%; global efficacy, 92.72%. No severe adverse reactions were recorded. The present NPT with lysine aspirin proved to be a safe, efficient, and a timesaving method in the diagnosis of patients with AERD, even in those with severe rhinitis-rhinosinusitis and/or recurrent nasal polyposis.

Effect of salbutamol on respiratory muscle function and ventilation in awake canines

The effect of the beta-agonist bronchodilator salbutamol on respiratory muscles and ventilation is uncertain. The presence of beta2 receptors on skeletal muscles and increased diaphragm contractility in vitro with salbutamol predict a significant effect that has not been confirmed, in vivo in non-fatigued diaphragm or in clinical studies using standard bronchodilator dosages. Therefore, we infused salbutamol at a higher dosage (23.3 microg/min) used clinically for treatment of respiratory emergencies, while measuring directly the length, shortening and EMG activation of costal and crural diaphragm, parasternal intercostal and transversus abdominis muscles, in 10 awake canines. At this salbutamol dosage, ventilation and tidal volume increased significantly during both resting and CO2-stimulated breathing. Salbutamol elicited significant increases in respiratory muscle shortening with much smaller increases in EMG activity, so the proportionally greater muscle shortening per unit EMG showed increased muscle contractility. The effects of salbutamol were not extinguished by inspiratory flow resistance or fluid challenge but were reversed specifically by the beta-blocker, propranolol. This study demonstrates that, in sufficient intravenous dosage, the beta-agonist salbutamol elicits increased ventilation and a beta2 receptor-mediated increase in contractility of respiratory muscles.

The efficacy and safety of QVAR (hydrofluoroalkane-beclometasone diproprionate extrafine aerosol) in asthma (Part 2): Clinical experience in children.

QVAR [hydrofluoroalkane-134a beclometasone dipropionate (BDP)] produces equivalent asthma control to chlorofluorocarbon-based BDP inhalers, at approximately half the daily dose in adults, a probable consequence of the increased lung deposition of QVAR that results from its greater fine particle fraction. Recent studies have relied on the clinical experience with QVAR in adults as a basis for investigations in childhood asthma. Design considerations, such as the use of the breath-actuated Autohaler delivery device and measurement of direct health benefits, account for problems of variation in inspiratory flow, handling difficulties and low airways resistance that are associated with children. QVAR appears to be well tolerated in children with no clinically relevant adverse effects on adrenal function, bone metabolism or growth at recommended doses.

IDENTIFICATION OF COGNITIVE DEMANDS WITH A COMPETENCE MODEL

S188 Introduction: Physician error is an important factor in patient morbidity and mortality. Ventilation-related events (VRE) impose high cognitive demands on anesthesiologists. Cognitive errors result when anesthesiologists' resources are insufficient for these demands. [1] In this study, we analyzed a competence model of the respiratory system to identify the cognitive demands of VRE. Methods: A competence model of the respiratory system was constructed based on information obtained during interviews of board-certified anesthesiologists. This model is a detailed representation of the relationships between the functions of the respiratory system and the goals of the anesthesiologist problem-solver. Faults or VRE are represented as departures from normal functioning. Using the model, we identified three sub-spaces that correspond to three anesthesia goals of insuring adequate: 1) inspired oxygen concentration (FIO2); 2) delivered tidal volume; and 3) ventilation/perfusion matching. Cognitive demands of VRE within the delivered tidal volume sub-space were assessed by estimating the number of physiologic variables relevant to the event, the tightness of coupling between variables, the level of uncertainty in the relationships between available indicators and the event diagnosis, and the time pressure and risk of the event. [2] Results: Delivered tidal volume is influenced by characteristics and settings of the anesthesia machine, ventilator, breathing circuit, scavenger system, and patient airway, lungs, and thorax. In order to identify root causes of changes in delivered tidal volume, clinicians must assess several functional variables such as inspiratory flow, inspiratory pressure, circuit and airway resistance, and lung compliance. Important variables such as airway resistance, lung compliance, and physiologic dead space vary from patient to patient, but cannot be easily measured, leading to uncertainty. Inadequate delivered tidal volume quickly results in patient death or permanent injury. These factors place several cognitive demands on clinicians. Discussion: Characterizing the cognitive demands of intraoperative VRE is an important first step to aiding anesthesiologists in responding to such events. [3] This analysis was utilized to better understand how clinicians use their resources to cope with the demands of VRE. Results of the study will be used to guide the development of intraoperative cognitive aids. Supported by the Anesthesia Patient Safety Foundation

The role of the upper airway in contributing to respiratory responses during experimental seizures in piglets.

Recent data demonstrating stimulation of respiration during seizures are at odds with the frequent clinical finding of respiratory impairment in patients with generalized seizures. To determine the role of the upper airway in contributing to these clinical observations, a study was performed in 12 weanling piglets. An arterial catheter and epidural electrodes were placed in ketamine-anesthetized piglets. In intact piglets, all airflow was measured through a snug-fitting nasal mask whereas pressure was measured with a subglottic catheter. Tracheostomized piglets had airflow and pressure measured at the trachea. Seizures were induced with i.v. injections of pentylenetetrazol. Peak inspiratory flow resistance was calculated by averaging three epochs of five to seven consecutive breaths. Epochs of greater than 20 s in duration with a tidal volume of < 10 mL were termed apnea. Apnea was considered central when airway pressure was > -2 cm H2O and obstructive when < -10 cm H2O. After 20 min of untreated seizures, intact piglets had significant respiratory and metabolic acidosis, whereas tracheostomized piglets had significant increases in Ve and mild metabolic acidosis. Apnea and obstruction were observed frequently in intact piglets. Resistance was unchanged during seizures in tracheostomized piglets, whereas a nearly 4-fold increase in inspiratory resistance was observed in intact piglets. We conclude that upper airway patency and resistances are important determinants of respiratory responses during generalized seizures in piglets.

Evaluation of indexes predicting the outcome of ventilator weaning and value of adding supplemental inspiratory load.

To assess the diagnostic accuracy of several measured and calculated indexes for early prediction of weaning outcome, and to study the value of supplemental inspiratory load in improving the accurate prediction of successful weaning. Prospective study. ICU of a University Hospital. Thirty consecutive patients under prolonged mechanical ventilation and without chronic obstructive pulmonary disease (COPD). Forty weaning trials were performed. Data were recorded at 15, 30 (adding inspiratory flow resistance), 60 and 120 min. The threshold values and the accuracy of three indexes were determined: Inspiratory airway occlusion pressure at 0.1 sec. (P01) to maximum inspiratory pressure ratio (P01/MIP), inspiratory effort quotient (IEQ), and the ratio of respiratory frequency to tidal volume (F/Vt). All three were useful predictors for weaning success with a diagnostic accuracy between 82%-87%. At 15 min of spontaneous breathing, a P01/MIP ratio < 0.14 predicts weaning success with a sensitivity of 82% and specificity of 83%. In our group of patients no reintubation was necessary. The application of mechanical inspiratory load significantly increased P01 values (3.16 +/- 1.22 to 3.60 +/- 1.19, p < 0.001). The degree of the P01 increase did not provide prediction of weaning outcome. a) P01/MIP, IEQ and F/Vt ratio were accurate, early predictors of weaning outcome. b) The addition of a moderate mechanical inspiratory load did not enhance the diagnostic accuracy of P01 measurements. c) In our patients, a period of two hours seemed to be sufficient for development and detection of weaning failure.

Detection of Inspiratory Resistive Loads by Heart-Lung Transplant Recipients

Human heart-lung transplantation recipients (HL) are models for the effects of selective lower respiratory tract denervation. We used a series resistor to examine thresholds of inspiratory flow resistance perception among 16 HL in comparison with 15 normal control subjects (C) and 12 heart transplant recipients (H). HL and C were closely age matched. H subjects were older and had mildly reduced lung volumes compared with the other groups. The perception threshold was defined as the added resistance (delta R) detected by subjects at 50% of random, blinded presentations. These delta R were subsequently expressed as a fraction of the airway resistance of the subject and baseline resistance of the apparatus (delta R/Raw + Rapparatus), otherwise known as the Weber fraction. The mean +/- SEM Weber fractions were nearly identical for HL and C (0.32 +/- 0.05 versus 0.34 +/- 0.05, respectively). The Weber fraction of H (0.74 +/- 0.17) was significantly greater than those of both other groups (p less than 0.05 by Kruskal-Wallis test). We conclude that lower respiratory tract afferents do not play a significant role in the perception of inspiratory resistive loads. The finding of an abnormally high Weber fraction in the heart transplant population remains unexplained but may be a function of age or restrictive pulmonary defects.

Demand and Continuous Flow Intermittent Mandatory Ventilation Systems

A mechanical lung was used to evaluate the pressure and flow characteristics of four demand and two continuous flow intermittent mandatory ventilation (IMV) systems. The amount of negative pressure required to initiate inspiratory flow and peak expiratory resistance were measured. The inspiratory pressure required to initiate flow in the demand mode was also compared to pressures generated in the assist mode. In addition, the peak expiratory resistance was measured with four commercially available exhalation valves. Results showed that the ventilator manometer measuring internal machine pressures significantly underestimated the amount of negative pressure required to open the demand valve (p less than 0.01). There are major differences in the flow and pressure characteristics among demand and continuous flow IMV systems. Systems that impose high inspiratory elastic threshold loads and expiratory flow resistive loads may have a deleterious effect on the mechanics of breathing, and thereby limit weaning success and eventually impair the recovery of certain patients in respiratory failure. The basic methodology, especially the simple technique of inserting an aneroid manometer in line next to a patient's ET tube, for measuring proximal negative inspiratory force (NIF test) can be easily applied to any and all ventilators at any practitioner's individual institution.

Involvement of respiratory perception in the resistive load compensation.

Relation of threshold for perception of added inspiratory flow resistance to ventilatory and occlusion pressure response to hypercapnia during external flow-resistive loading was examined in 8 healthy volunteers and 14 patients with chronic obstructive pulmonary disease. Occlusion pressure response to hypercapnia without the load and the threshold for perception were similar between normal subjects and patients. Occlusion pressure at end-tidal PCO2 of 60 mm Hg increased to 134 +/- (SD) 23.4% (p less than 0.01) in normal subjects, but it did not change in patients (113.7 +/- 25.3%). Occlusion pressure response to hypercapnia increased to 155.4 +/- 60% (p less than 0.05) during ventilatory loading in normal subjects but it did not change in patients (105.9 +/- 33%). Inverse correlation between threshold values and changes after loading in occlusion pressure response to hypercapnia was seen in patients as well as in normal subjects. These results indicate in both normal subjects and patients with chronic obstructive pulmonary disease that occlusion pressure response to hypercapnia during the ventilatory loading includes signals from the higher central nervous system and that the lower the threshold, the higher the respiratory drive during the loaded ventilation.

Interrupter technique for measurement of respiratory mechanics in anesthetized cats.

In six spontaneously breathing anesthetized cats (pentobarbital sodium, 35 mg/kg ip), airflow, changes in lung volume, and tracheal and esophageal pressures were measured. Airflow was interrupted by brief airway occlusions during relaxed expirations (elicited via the Breuer-Hering inflation reflex) and throughout spontaneous breaths. A plateau in tracheal pressure occurred throughout relaxed expirations and the latter part of spontaneous expirations indicating respiratory muscle relaxation. Measurement of tracheal pressure, immediately preceding airflow, and corresponding volume enabled determination of respiratory system elastance and flow resistance. These were partitioned into lung and chest wall components using esophageal pressure. Respiratory system elastance was constant over the tidal volume range, divided approximately equally between the lung and chest wall. While the passive pressure-flow relationship for the respiratory system was linear, those for the lung and chest wall were curvilinear. Volume dependence of chest wall flow resistance was demonstrated. During inspiratory interruptions, tracheal pressure increased progressively; initial tracheal pressure was estimated by backward extrapolation. Inspiratory flow resistance of the lung and total respiratory system were constant. Force-velocity properties of the contracting inspiratory muscles contributed little to overall active resistance.

Respiratory muscle function during CO2 rebreathing with inspiratory flow-resistive loading.

We investigated the respiratory muscle contribution to inspiratory load compensation by measuring diaphragmatic and intercostal electromyograms (EMGdi and EMGic), transdiaphragmatic pressure (Pdi), and thoracoabdominal motion during CO2 rebreathing with and without 15 cmH2O X l-1 X s inspiratory flow resistance (IRL) in normal sitting volunteers. During IRL compared with control, Pdi measured during airflow and during airway occlusion increased for a given change in CO2 partial pressure and EMGdi, and there was a greater decrease in abdominal (AB) end expiratory anteroposterior dimensions with increased expiratory gastric pressure (Pga), this leading to an inspiratory decline in Pga with outward AB movement, indicating a passive component to the descent of the abdomen-diaphragm. The response of EMGic to IRL was similar to that of EMGdi, though rib cage (RC)-Pga plots did infer intercostal muscle contribution. We conclude that during CO2 rebreathing with IRL there is improved diaphragmatic neuromuscular coupling, the prolongation of inspiration promoting a force-velocity advantage, and increased AB action serving to optimize diaphragm length and configuration, as well as to provide its own passive inspiratory action. Intercostal action provides increased assistance also. Therefore, compensation for inspiratory resistive loads results from the combined and integrated effort of all respiratory muscle groups.