Maximal relaxation rates of esophageal, nose, and mouth pressures during a sniff reflect inspiratory muscle fatigue

Maximal relaxation rate (MRR, % pressure fall/10 msec) of the inspiratory muscles is reduced with fatigue. We have investigated whether MRR of esophageal pressure (Pes) generated by voluntary sniffs is decreased by fatigue, and whether sniff nasopharyngeal (Pnp) and mouth (Pmo) MRR reflect these changes. In 10 normal subjects, control MRR of sniff Pes correlated closely to Pnp MRR (r = 0.977, p less than 0.001) and Pmo MRR (r = 0.947, p less than 0.001). To produce inspiratory muscle fatigue, four highly motivated subjects breathed to exhaustion (3 to 6 min) through a high inspiratory resistance. MRR was determined from 10 sniffs for Pes, Pnp, and Pmo before fatigue, and at intervals up to 10 min after fatigue. The subjects showed a mean decrease in sniff Pes MRR of 33% (range, 20 to 42) immediately after fatigue, which returned exponentially to control values within 3 to 4 min. The mean changes in Pes MRR were reflected by similar changes in Pnp MRR, 32% (range, 18 to 43) and Pmo MRR, 33% (range, 21 to 42). Studies were repeated in the four subjects with closely similar results. We conclude that fatigue of the inspiratory muscles reduces MRR of sniff Pes, and that this is reflected in Pnp and Pmo. Sniff Pes, Pnp, and Pmo MRR measurements may provide a useful method for detecting and monitoring fatigue; Pnp and Pmo have the advantage of being less invasive.     

Noninvasive assessment of inspiratory muscle function during exercise

The use of esophageal and gastric balloons limits measurement of the tension-time index of inspiratory muscles (TTI) during exercise. The aim of this study was to assess whether a noninvasive tension-time index, TT(0.1), given by P(0.1)/PI(max) x TI/Ttot (where P(0.1) is mouth occlusion pressure, PI(max) is maximal inspiratory pressure, and TI/Ttot is duty cycle) could reliably assess TTI during exercise. In seven healthy young men and nine patients with COPD we measured TT(0.1) and TTI (i.e., Pes/Pes(max) x TI/Ttot where Pes is mean esophageal pressure and Pes(max) is maximal static Pes) at rest and during an incremental exercise test. A significant linear correlation (p < 0.02) was found between TT(0.1) and TTI in all normal subjects and patients with COPD. An equation for estimating TTI from TT(0.1) was established for each group. In the normal subjects there was good agreement between estimated and observed data. In five additional normal males studied prospectively, the agreement was also satisfactory and reproducible. In the COPD patients the agreement was poor. In conclusion, in young healthy subjects the changes in TT(0.1) during exercise reflect the changes in TTI, allowing satisfactory estimation of TTI from noninvasive measurements of TT(0.1).     

Effect of home mechanical ventilation on inspiratory muscle strength in COPD

BACKGROUND: The mechanism responsible for chronic hypercapnic respiratory failure (HRF) in patients with COPD remains unclear. In this study, we tested the hypothesis that chronic HRF in patients with COPD is associated with low-frequency fatigue (LFF) of the diaphragm. METHODS: To test this hypothesis, we measured the twitch transdiaphragmatic pressure (Tw Pdi) elicited by stimulation of the phrenic nerves in 25 patients with chronic HRF (mean [+/- SD] Paco(2), 55.2 +/- 5.2 mm Hg) due to COPD before and 2 months after the initiation of noninvasive mechanical ventilation (NIV) [pressure-cycled ventilation with inspiratory positive airway pressure of 19.0 +/- 2.5 cm H(2)O]. We reasoned that had LFF been present, Tw Pdi should rise after effective NIV. RESULTS: The treatment compliance with NIV was good (median of machine usage was 7.1 h per night). Paco(2) decreased from 55.2 +/- 5.2 to 48.8 +/- 5.9 mm Hg (p < 0.001), and Pao(2) increased from 53.1 +/- 5.9 to 57.7 +/- 7.0 mm Hg (p = 0.007). Mean Tw Pdi at baseline was 11.1 +/- 6.6 cm H(2)O and after treatment was 11.7 +/- 7.2 cm H(2)O (not significant). Also, maximal static inspiratory mouth pressure did not change significantly (44.3 +/- 15.9 cm H(2)O vs 46.5 +/- 19.7 cm H(2)O). CONCLUSION: LFF of the diaphragm does not accompany chronic HRF in patients with COPD.     

Maximal sniff mouth pressure compared with maximal inspiratory pressure in acute respiratory failure

Inspiratory muscle strength most often is better reflected by sniff Pes than PImax against occlusion. Furthermore, sniff Pes can be estimated noninvasively by the measurement of sniff Pmo in normal subjects and in patients with respiratory muscle weakness. The aim of this study was to compare sniff Pmo and P.PImax to assess inspiratory muscle strength in patients with acute respiratory failure. The highest pressure was produced by P.PImax in 61 percent of measurements, and by sniff Pmo in 39 percent. Above 35 cm H2O P.PImax yielded the highest pressure in 55 percent of cases and the ratio sniff Pmo/P.PImax was 1.20 +/- 0.54. Below 35 cm H2O, P.PImax yielded the highest pressure in 75 percent of cases and the ratio sniff Pmo/P.PImax was 0.76 +/- 0.35 (p less than 0.02). Thus, measurements of sniff Pmo and P.PImax complement one another for assessing inspiratory muscle strength. However, sniff Pmo underestimates inspiratory muscle strength in patients with severe inspiratory muscle weakness.     

Impact of impaired inspiratory muscle strength on dyspnea and walking capacity in sarcoidosis

BACKGROUND: Dyspnea and fatigue are frequent but poorly understood symptoms in sarcoidosis patients. This study was aimed at assessing the clinical impact of inspiratory muscle impairment on dyspnea and exercise tolerance. This is the first study using nonvolitional tests that are independent of the patient's cooperation and motivation in addition to volitional tests of inspiratory muscle strength in patients with sarcoidosis. METHODS: Peak maximal inspiratory mouth pressure (Pimaxpeak), maximal inspiratory pressure sustained for 1.0 s (Pimax1.0), twitch mouth pressure (TwPmo), lung function test results, blood gas measurements, 6-min walking distance (6MWD), and Borg dyspnea scale (BDS) scores were assessed in 24 male sarcoidosis patients and 24 healthy male control subjects matched for age and body mass index. RESULTS: Mean (+/- SD) Pimaxpeak (95.2 +/- 25.3% vs 124.6 +/- 23.4% predicted, respectively; p < 0.001) and Pimax1.0 (85.6 +/- 31.4% vs 125.8 +/- 26.8% predicted, respectively; p < 0.001) were lower in sarcoidosis patients compared to control subjects. TwPmo tended to be lower in sarcoidosis patients, and there were three patients who had TwPmo values of < 1.0 kPa, which is a strong indicator of inspiratory muscle weakness. The mean 6MWD was 582 +/- 97 m in sarcoidosis patients and 638 +/- 65 in control subjects (p = 0.025). The mean BDS score was higher in sarcoidosis patients (3.3 +/- 1.7 vs 0.2 +/- 0.5, respectively; p < 0.001). Compared to maximal inspiratory pressure, lung function parameters, and blood gas levels, TwPmo was the strongest predictor for 6MWD (r = 0.663; p = 0.003) and BDS score (r = 0.575; p = 0.012) in sarcoidosis patients following multiple linear regression analysis. CONCLUSIONS: Impairment of inspiratory muscle strength occurs in sarcoidosis patients, and has been suggested to be an important factor causing dyspnea and reduced walking capacity, but this is only reliably detectable when using nonvolitional tests of inspiratory muscle strength.     

Relationship between grip strength and global muscle strength in community-dwelling older people

There are still conflicting results regarding the association between grip and global muscle strength in older people. Therefore, the objective of the present study was to determine the association between grip strength and global muscle strength, as well as between grip strength and individual trunk, hip, knee and ankle muscle strengths.MethodsGrip strength was assessed using a manual dynamometer, and trunk, hip, knee and ankle muscle strength with an isokinetic dynamometer, in order to obtain the global muscle strength variable, in 150 older men and women from the community. The association between grip and global muscle strength and between grip strength and the strength of each muscle group was determined through the Pearson correlation test, followed by multivariate linear regression adjusted for sex, age, body mass index, level of physical activity and number of comorbidities.ResultsA positive significant association was found between grip strength and global muscle strength in older people (r = 0.690; β = 10.07; p < 0.001; R² = 0.604), even after adjustment. There was also a low to moderate association between all the muscle groups and grip strength. However, when the model was adjusted, the relationship between grip strength and ankle dorsiflexor peak torque lost significance (p = 0.924).ConclusionGrip strength can represent global muscle strength in younger older people in the community, even when confounding variables are considered in the statistical model. However, grip strength does not eliminate the need for specific assessment of different muscle groups, when indicated.     

Effect of inspiratory muscle fatigue on inspiratory muscle relaxation rates in healthy subjects.

Simple methods to diagnose inspiratory muscle fatigue in the clinical setting would be of considerable benefit. Inspiratory muscle relaxation rates are known to slow following induction of fatigue. Inspiratory muscle relaxation rates have been measured following a short sharp inspiratory effort against an occluded airway (sniffmouth) or through the unoccluded nostrils (sniffnostrils). Relaxation rates in the absence of fatigue are faster when sniffs are performed through the unoccluded nostrils. While both methods have been shown to be capable of detecting inspiratory muscle fatigue, there may be quantitative or qualitative differences between the two techniques in their ability to detect fatigue similar to the differences observed in the fresh state. Accordingly, we measured relaxation rates with the two sniff techniques in five healthy naive male subjects before and after induction of fatigue. Inspiratory muscle fatigue was induced by threshold loading at 80 percent of Pesmax until the subjects were unable to generate the target pressure. For those trials in which sniffnostrils were performed, the maximum relaxation rate from the esophageal pressure curve (MRRes) was significantly decreased following induction of fatigue in nine of ten trials, while the exponential time constant (taues) was significantly increased in all ten trials. In contrast, for those trials in which sniffmouth were performed, the MRRes was significantly decreased following induction of fatigue in only six of ten trials. Similarly, taues was significantly increased following induction of fatigue in only six of ten trials. In addition, the magnitude of change in the MRR or tau following induction of fatigue was quantitatively greater with sniffnostrils compared with sniffmouth. Similar findings were obtained when relaxation rates were measured from the diaphragmatic pressure tracing. In conclusion, changes in relaxation rate following induction of fatigue were quantitatively greater and more consistently observed when sniffs were performed through the unoccluded nostrils rather than against an occluded airway.     

Non-invasive assessment of inspiratory muscle performance during exercise in patients with chronic heart failure

Aims The aim of this study was to assess inspiratory performanceat rest and during exercise in patients with chronic heart failurein comparison with healthy controls using a non-invasive index:the tension-time index of inspiratory muscles (TTMUS). Methods We studied 13 patients with chronic heart failure (57±7years) and 10 control subjects (58±6 years) at rest andduring an incremental maximal exercise test. Measurements includedbreathing pattern (inspiratory time, total time of respiratorycycle, minute ventilation, tidal volume and respiratory frequency),mouth occlusion pressure and mean inspiratory pressure (calculatedas follows: 5xmouth occlusion pressurexinspiratory time). Themaximal inspiratory pressure was measured at rest. TTMUS wascalculated from the equation: TTMUS=PI/PIMAXxTI/TTOT, wherePI/PIMAX is the ratio of mean inspiratory pressure to maximalinspiratory pressure and TI/TTOT is the ratio of mean inspiratorytime to total time of the respiratory cycle. Results At rest, the results in patients showed non-significantly highermouth occlusion pressure, lower maximal inspiratory pressure(PP<0·001), and a higher ratio of mean inspiratorypressure to maximal inspiratory pressure (PP<0·01).There was no difference in the breathing pattern. TTMUS wasthus significantly higher in the patients with chronic heartfailure (PP<0·001). At maximal exercise (77±16Wfor patients with chronic heart failure vs 142±27W forcontrols,PP<0·001), the ratio of mean inspiratorytime to total time of respiratory cycle, the mouth occlusionpressure and the ratio of mean inspiratory pressure to maximalinspiratory pressure were not different. TTMUS was thus comparablein the two groups. During exercise, at comparable workloads(20, 40 and 60W), the patients showed higher mouth occlusionpressure (PP<0·01) and a higher ratio of mean inspiratorypressure to maximal inspiratory pressure (PP<0·001),whereas the ratio of mean inspiratory time to total time ofthe respiratory cycle was similar. TTMUS was thus higher inthe patients at each workload (PP<0·05). Conclusion This study shows that the determination of TTMUS at rest andduring exercise allows the observation of alterations in inspiratorymuscle performance as a result of both reduced inspiratory strength,as measured by the maximal inspiratory pressure, and increasedventilatory drive, as reflected by the mouth occlusion pressurein patients with chronic heart failure. The non-invasivenessof this new index is an additional argument for its use in aclinical setting.     

Resting pressures in the lower esophageal sphincter

Interdeglutive pressures of the lower esophageal sphincter (LES) were studied by nonpullthrough techniques in 36 volunteers. Resting pressures did not remain at the high levels observed when the LES was entered. In 30 studies a gradual protracted decay ranging from 5 to 18 mm Hg was observed. These studies suggest that the LES may not pose a continuous barrier to reflux of gastric contents.     

Electromyographic validation of the mouth pressure-time index: a noninvasive assessment of inspiratory muscle load

The pressure-time index (PTI = Pmouth/Pi max x Ti/Ttot) has been validated by Ramonatxo (J. Appl. Physiol. 78 (1995) 646 and by Jabour (Am. Rev. Respir. Dis. 144 (1991) 531 as a noninvasive tool for the assessment of inspiratory muscles load. Nobody until now has evaluated the correlation between the PTI and diaphragmatic activity. Further, the PTI has not been compared with another measures of respiratory muscle load such as the transdiaphragmatic pressure index or TTdi. The purpose of our study was to test the hypothesis that the PTI measured at the mouth (PTIm) is a noninvasive reflection of TTdf and electromyographic activity of the diaphragm (EMGdf). We studied 6 patients with COPD and 5 normal individuals at rest and during a CO2 rebreathing trial and simultaneously measured PTIm, TTdi and EMGdi. The curves of PTIm and EMGdi follows the same trend during the CO2 rebreathing trial with strong and significant correlation between these parameters (r = 0.89 P < 0.05 and r = 0.82 P < 0.05 for PaCO2 of 45 and 53 mmHg respectively). We conclude that PTIm measured as Pmouth/Pi max x Ti/Ttot is an adequate noninvasive method that reflect not only the diaphragmatic activity but also the inspiratory muscles load.     

Postural variation of the maximum inspiratory and expiratory pressures in normal subjects

The maximum static inspiratory and expiratory pressures (MIP and MEP, respectively) were measured in 15 normal male subjects (average age, 27.14 years) in standing and sitting position. The MIP was determined at RV and FRC and MEP was determined at TLC and FRC. No significant differences were found for these parameters between the two postures. Our study proves that the posture adopted by the subject when these two maneuvers are performed does not influence the results obtained.     

Is serial determination of inspiratory muscle strength a useful prognostic marker in chronic heart failure?

BACKGROUND: Little data exists on the prognostic role of inspiratory muscle strength (PImax) in chronic heart failure (CHF). Training studies, however, frequently use it as a therapeutic target and surrogate marker for prognosis. The prognostic value of changes of PImax that allow this extrapolation is unknown. DESIGN: Patients with stable CHF were prospectively included and 1-year and all-time event rates recorded for endpoint analysis. METHODS: In 158 patients (85% men; New York Heart Association functional class: 2.4+/-0.6), PImax was measured along with clinical evaluations at two visits, the initial visit and the second visit, 6.4+/-1.4 months apart. The mean follow-up was 59+/-34 months. RESULTS: Overall, 59 patients (37%) reached the primary endpoint of death or hospitalization (endpoint positive), and overall mortality rate (secondary endpoint) was 26% (42 patients). PImax did not differ between endpoint-negative and endpoint-positive patients, both at the initial and at the second visit (8.3+/-5.6 vs. 7.3+/-3.4 kPa and 8.8+/-6.0 vs. 7.9+/-3.6 kPa, respectively; P=NS), and both groups showed increased PImax (0.6+/-2.6 vs. 0.6+/-2.8 kPa; P=NS). Cox analyses found neither the absolute nor the relative change of PImax to be significant predictors for the primary and secondary endpoints (P=NS for both), both for the 1-year and for the all-time event rates. Endpoint rates did not differ between patients showing increasing or decreasing PImax (P=NS; relative risk (RR): 0.77; 95% confidence interval: 0.47-1.27). CONCLUSION: Trials focusing on inspiratory muscle function should use the actual levels of PImax as a surrogate marker to represent prognostic information, rather than relative or absolute changes. This is the first study to investigate the prognostic information of the changes of PImax over time, regarding both short-term and long-term morbidity and mortality in patients with stable CHF.     

Effect of inspiratory resistance and theophylline on respiratory muscle strength in patients with amyotrophic lateral sclerosis

The effect of breathing through inspiratory flow resistive loads ranging between 4.5 and 27.0 cm H2O/L/s was assessed in eight patients with amyotrophic lateral sclerosis (ALS) and in eight control subjects. ALS patients developed respiratory muscle fatigue manifested by significant declines in negative inspiratory pressure (18.3%), vital capacity (7.2%), and peak inspiratory flow rate (5.5%). Control subjects did not fatigue with these resistances. In ALS patients, theophylline increased respiratory muscle strength after resistive breathing as manifested by an increase in negative inspiratory pressure (28.2%), vital capacity (10%), and peak inspiratory flow rate (11.8%). It is concluded that in patients with ALS, the already weakened respiratory muscles are easily fatigued. Furthermore, theophylline can strengthen loaded respiratory muscles in patients with ALS.     

Maximal inspiratory and expiratory pressures in adolescents. Normal values

The measurement of maximal inspiratory and expiratory pressures at the mouth (MIP and MEP, respectively) provides a noninvasive clinical method for evaluating the strength of respiratory muscles. In an attempt to reconcile the widely divergent normal values reported in the literature for healthy adolescents, we have measured, using simple manometry, MIP and MEP in 112 white subjects, 76 adolescents and 36 healthy adults. For female adolescents the values for MIP and MEP were 76 +/- 25 and 86 +/- 22 cm H2O, respectively, and were significantly less than those for male adolescents (p less than 0.01), whose mean values were 107 +/- 26 and 114 +/- 35 cm H2O, respectively. Mean values for adolescents were comparable to values measured in adult control subjects, and for both adolescents and adults, mean values approximated the lower end of the previously reported ranges of normal values in healthy subjects. Thus, MIP and MEP in healthy adolescents are significantly greater in male subjects than female subjects, but are comparable to those of healthy adults of the same sex. Furthermore, these studies suggest that the choice of normal values for MIP and MEP must take into account significant methodologic differences among laboratories.     

Postural variation of the maximum inspiratory and expiratory pressures in obese patients.

Evaluating whether weight is a factor responsible for the decrease of muscle force in the supine position with respect to the upright and sitting positions, we measured maximum inspiratory and expiratory pressures (PImax and PEmax) in these postures in ten obese patients (age 38.7 +/- 9.1 years; height 168.7 +/- 8.9 cm; and weight 139.3 +/- 28.4 kg) and ten normal control subjects (age 38.4 +/- 8.2 years; height 169.3 +/- 7.9 cm and weight 66.9 +/- 11.9 kg.) In both study groups, PImax and PEmax values decreased in the supine posture with respect to upright and sitting positions. Differences between pressures in the various postures were similar for both groups with the exception of PEmax in obese females. The decrease in PEmax values from the sitting to supine positions was greater in obese than control females. Weight, sex and postural changes can influence the generation of maximal expiratory force.     

Clinical manifestations of inspiratory muscle fatigue

Twelve patients exhibiting difficulties during discontinuation of artificial ventilation permitted us to investigate physical examination techniques used in diagnosing inspiratory muscle fatigue. Diaphragmatic and intercostal electromyographic tracings, arterial blood gases, rate and depth of ventilation, and thoracoabdominal motion were monitored during spontaneous breathing. Six patients showed electromyographic evidence of inspiratory muscle fatigue. A sequence of events leading to respiratory acidemia emerged--namely electromyographic evidence of fatigue, accompanied or followed by an increased respiratory rate, in turn followed by alternation between abdominal and rib cage breathing (respiratory alternans), paradoxical inward abdominal motion during inspiration (abdominal paradox), and finally an increase in PaCO2 associated with a fall in minute ventilation and respiratory rate, and worsening of respiratory acidemia. The abnormalities of respiratory movements may be reliable clinical signs of inspiratory muscle fatigue, particularly when accompanied by tachypnea and hypercapnia.