Sniff Nasal Inspiratory Pressure in Healthy Japanese Subjects: Mean Values and Lower Limits of NormalKamide N.a · Ogino M.b · Yamashina N.a · Fukuda M.a
aFaculty of Rehabilitation, School of Allied Health Sciences, and bDepartment of Neurology, School of Medicine, Kitasato University, Sagamihara, Japan Corresponding Author
Background: When assessing respiratory muscle strength using sniff nasal inspiratory pressure (SNIP), it is important to consider ethnic differences. Therefore, it is necessary to determine the mean values and lower limits of normal for SNIP in the Japanese population. Objective: To determine the mean values and lower limits of normal for SNIP, which is used as an assessment of inspiratory muscle strength, in healthy Japanese subjects. Methods: A total of 223 healthy Japanese volunteers (112 men, 111 women), aged 18–69 years, were studied; none had a history of pulmonary disease, heart disease, neuromuscular disease or sinusitis. To measure SNIP, a nasal plug was inserted into one nostril and the mouth was kept closed. Each subject was asked to take short, sharp sniffs with maximal effort from functional residual volume. Results: Based on the intraclass correlation coefficient, SNIP measurements showed good reproducibility in both men and women. The mean SNIP values were 76.8 ± 28.9 cm H2O in men and 60.0 ± 20.0 cm H2O in women; the values were significantly higher in men than in women (p < 0.01). On stepwise multiple linear regression analysis, the SNIP values were negatively related to age in men and positively related to body mass index (BMI) in women. The lower limits of normal for SNIP were 32.9 cm H2O in men and 28.8 cm H2O in women. Conclusions: In healthy Japanese subjects, the mean SNIP value was higher in men than in women. In Japanese subjects, SNIP values appear to be related to age in men and BMI in women.
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Vital capacity (VC), forced expiratory volume in 1 s, arterial oxygen saturation, maximal inspiratory mouth pressure (PImax) and maximal expiratory mouth pressure (PEmax) are used to noninvasively assess respiratory function. In particular, PImax and PEmax are used to assess respiratory muscle strength [1,2,3], as well as to judge the effect of respiratory muscle training . Sniff nasal inspiratory pressure (SNIP) is also used to assess respiratory muscle strength . In fact, it has been reported that sniffing is strongly related to the activity and strength of the diaphragm [6, 7], and SNIP can be used to assess inspiratory muscle strength or fatigue in patients with neuromuscular or skeletal disease [8, 9]. In patients with amyotrophic lateral sclerosis in whom it is impossible to measure VC or PImax, it is possible to measure SNIP , which has a high sensitivity to predict ventilatory failure .
However, the mean values and lower limits of normal for SNIP in Asian populations, such as the Japanese population, have not been reported, as little research has been done in these populations. On the other hand, PImax and PEmax have been shown to be lower in Asian populations than in Caucasian populations . Therefore, when assessing respiratory muscle strength, it is important to consider ethnic differences. It is also necessary to determine the mean values and lower limits of normal for SNIP in Asian populations.
The aim of the present study was to determine the mean values and lower limits of normal for SNIP, which is used as an assessment of inspiratory muscle strength, in healthy Japanese subjects.
Two hundred and twenty-three healthy Japanese volunteers (112 men, 111 women), aged 18–69 years, were included in the study. The subjects included students, teachers, office workers, manual laborers (garbage collectors, security guards) and hospital workers (doctors, nurses, physical therapists, occupational therapists). The exclusion criteria included: pulmonary disease (chronic obstructive pulmonary disease or emphysema); heart disease (myocardial infarction or heart failure); neuromuscular disease (Parkinson disease, amyotrophic lateral sclerosis or muscular dystrophy); cerebrovascular disease, and pregnancy. None of the patients had any history of sinusitis, regular medication use or known exposure to respiratory irritants or allergens. Competitive athletes were excluded. The researchers confirmed the exclusion criteria during face-to-face interviews with all of the subjects. Written informed consent was obtained from all subjects. The study was approved by the institutional faculty and was conducted in accordance with the ethical standards for research involving human subjects described in the Helsinki Declaration.
To measure SNIP, a nasal plug (NPLG00; Micro Medical Ltd., Kent, UK), to which a catheter made from polyethylene was connected, was inserted into one nostril of the subject, who was sitting on a chair. The choice of right or left nostril was decided by each subject. In all patients, the presence of pain or nasal polyps was confirmed during face-to-face interviews before measurement. The end of the catheter was connected to a pressure meter (RPM01; Micro Medical), and the pressure meter was connected to a personal computer using an RS-232 cable; the pressure signal was displayed on the computer screen, and the data were collected using the personal computer. According to the operating manual prepared by Micro Medical, the pressure meter has an accuracy of ±3%. Each subject was encouraged to relax and breathe normally while keeping the mouth closed. Then, the subject was instructed to take short, sharp sniffs with maximum effort from functional residual capacity. The contralateral nostril was not occluded during sniffing. The pressure signal was displayed on the computer screen during sniff performance and eupnea. The researchers confirmed that the subjects performed the sniffs properly from functional residual capacity by monitoring the computer screen.
The subjects sniffed several times before the measurements were obtained to take into account the learning effect . The pressure generated during sniffing after the initial trials was measured 5 times using the pressure meter and then recorded by the personal computer; consecutive tests were separated by 30-second intervals. In addition, each subject was asked about current and past smoking, and anthropometric data (height and body weight) were obtained for each subject, from which the body mass index (BMI) was calculated.
The intraclass correlation coefficient was used to assess the reproducibility of SNIP measurements for the 5 measurements in each subject. The maximum value of the 5 measurements in each subject was used to calculate the mean values and standard deviations (SDs) for men and women. Gender differences in the mean values were analyzed using the unpaired t test. Differences in the mean values between subjects who did and did not smoke were investigated using analysis of covariance adjusted for age, height, body weight and BMI by gender. Stepwise multiple linear regression analysis was performed to assess the contributions of age, height, body weight and BMI to the mean values by gender. Lower limits of normal were defined by subtracting 1.64 times the residual SD from the predicted value in the regression model . Ninety-five percent of healthy subjects lie above the lower limits. The statistical analyses were performed using SPSS for Windows, version 11.0J (SPSS Japan Inc., Tokyo, Japan). The significance level was set at 5%.
The subjects’ mean age was 41.5 ± 16.2 years in men and 39.3 ± 16.7 years in women; the mean height was 169.2 ± 6.1 cm in men and 157.3 ± 6.3 cm in women, and the mean weight was 65.9 ± 8.8 kg in men and 53.1 ± 6.3 kg in women; the mean BMI was 23.0 ± 2.6 in men and 21.5 ± 2.6 in women. No adverse events related to SNIP measurement were reported.
The intraclass correlation coefficient was 0.92 (95% CI 0.91–0.94; p < 0.01) in all subjects, 0.93 (95% CI 0.91–0.95; p < 0.01) in men and 0.88 (95% CI 0.84–0.91; p < 0.01) in women. In addition, there were no significant differences from the first SNIP to the fifth SNIP measurement in the present study (repeated-measures analysis of variance, data not shown).
Table 1 shows the mean and SD values for SNIP by age group and gender. The mean SNIP value was 76.8 ± 28.9 cm H2O for men and 60.0 ± 20.0 cm H2O for women; the mean SNIP value was significantly lower for women than for men (p < 0.01). Furthermore, when gender differences were examined by age group, women had significantly lower mean SNIP values than men in subjects below 50 years of age, but there were no significant gender differences in subjects above 50 years of age (table 1).
|Table 1. Mean and SD of SNIP by age group|
Fifty-two men and 9 women had a past or current history of smoking. However, there were no significant differences in the mean SNIP values between those with a history of smoking and those without in either men or women.
On stepwise multiple linear regression analysis, the SNIP values were negatively related to age in men and positively related to BMI in women (table 2, fig. 1 and 2). Thus, SNIP decreased with increasing age in men and increased with increasing BMI in women. Lower limits of normal for SNIP could be determined using the predicted value and the residual SD of the regression model. The residual SD was 26.7 for men and 19.1 for women; thus, the mean lower limit of normal for the predicted SNIP was 32.9 cm H2O in men and 28.8 cm H2O in women (table 2).
|Table 2. Stepwise multiple linear regression analysis for the SNIP value in men and women|
|Fig. 1. Correlation between SNIP and age in men and women.|
|Fig. 2. Correlation between SNIP and BMI in men and women.|
The mean values for SNIP obtained in the present study were 76.8 ± 28.9 cm H2O for men and 60.0 ± 20.0 cm H2O for women. Uldry and Fitting  determined the mean values in a Caucasian population, aged 20–80 years, and reported mean values of 91.0–117.0 cm H2O for men and 75.5–94.0 cm H2O for women (table 3). Comparing the Japanese and Caucasian populations, the mean SNIP values tended to be lower in the Japanese population than in the Caucasian population for both men and women. In addition, the lower limits of normal in the present study were calculated to be 32.9 cm H2O in men and 28.8 cm H2O in women, but Uldry and Fitting  reported lower limits of normal of 52–78 cm H2O in men and 47.5–66 cm H2O in women. Therefore, the lower limits of normal also tended to be lower in the Japanese population than in the Caucasian population. There appear to be ethnic differences in SNIP, as has been found for PImax and PEmax . Accordingly, the mean SNIP values obtained in the present study may be useful for assessing inspiratory muscle strength in Japanese subjects. Furthermore, SNIP values below 32.9 cm H2O in men and 28.8 cm H2O in women would be considered as indicating inspiratory muscle weakness. However, these values may be low for diagnosing inspiratory muscle weakness. In the American Thoracic Society/European Respiratory Society statement on respiratory muscle testing, it was mentioned that SNIP values greater than 70 cm H2O in men and 60 cm H2O in women are unlikely to be associated with significant inspiratory muscle weakness . Based on ethnic differences and the SNIP values obtained in the present study and those published in the American Thoracic Society/European Respiratory Society statement, we propose that in the Japanese population, SNIP values below 50 cm H2O in men and 40 cm H2O in women are clinically associated with inspiratory muscle weakness.
|Table 3. Comparison of SNIP values between the present study and a previous study|
Smoking history had no effect on the SNIP value. Previous studies that investigated PImax in healthy volunteers also showed that smoking had no effect on PImax [15, 16]. It is thought that smoking has no effect on inspiratory muscle strength; thus, there is no need to consider the effect of smoking on SNIP.
The results of the present study showed that the SNIP values in Japanese subjects were related to age in men and BMI in women. A previous study involving Caucasian subjects found that the SNIP values were related to age in both men and women . This finding agrees with our results in men, but not in women. However, this is not surprising, since PImax is thought to be affected by both age and physique [1, 15, 16]. In particular, it was reported that PImax was more affected by physique than by age in Caucasian women . Furthermore, the gender difference in body composition may also have an effect. For example, previous studies reported that the mass of the diaphragm was related to the lean body mass or the fat-free mass [17, 18]. There are gender differences in lean body mass and fat-free mass, and body composition has an effect on physical performance . Gender differences in body composition may have affected the relationship between SNIP and age or BMI in the present study. Thus, the SNIP values of Japanese subjects can be predicted by the BMI in women and age in men.
The present study had several limitations. Firstly, it did not include data on subjects ≥70 years of age. Therefore, the SNIP values of elderly subjects ≥70 years of age must be interpreted carefully. For instance, the gender difference may not need to be considered in the elderly. In fact, the present study found very little gender difference among subjects above 50 years of age. Therefore, further data on SNIP values in the elderly need to be accumulated. Furthermore, other lung function tests, such as VC or PImax, were not measured, because healthy subjects without pulmonary disease were included in the present study. Therefore, the lung functions of the studied subjects are not known. In addition, differences in the SNIP values between Japanese and Caucasian populations cannot be clearly demonstrated, since the Caucasian population was not studied in the present study. Furthermore, the mean values and the lower limits of normal obtained from the present study are not applicable to other Asian populations, because anatomical characteristics differ between the Japanese and other Asian populations.
It has been reported that a combination of multiple tests has a higher precision than a single test for diagnosing respiratory muscle weakness . Therefore, based on the mean values and the lower limits of normal for SNIP obtained in the present study, SNIP can be used in combination with PImax to assess respiratory muscle strength. In addition, the results of the present study provide clinically useful information for the assessment of inspiratory muscle strength in patients with pulmonary disease or neuromuscular disease in the Japanese population.
The results of the present study suggest that the mean SNIP values differ between Japanese and Caucasian populations, and the following specific mean values for the Japanese population were obtained: 76.8 ± 28.9 cm H2O in men and 60.0 ± 20.0 cm H2O in women. The SNIP values in Japanese subjects were related to age in men and BMI in women; smoking history had no effect on SNIP. Thus, the present study provides useful data for assessing inspiratory muscle strength in the Japanese population.
Faculty of Rehabilitation, School of Allied Health Sciences
1-15-1 Kitasato, Sagamihara, Kanagawa 228-8555 (Japan)
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Received: March 20, 2008
Accepted after revision: August 18, 2008
Published online: October 28, 2008
Number of Print Pages : 5
Number of Figures : 2, Number of Tables : 3, Number of References : 19
Respiration (International Journal of Thoracic Medicine)
Vol. 77, No. 1, Year 2009 (Cover Date: January 2009)
Journal Editor: Bolliger C.T. (Cape Town)
ISSN: 0025-7931 (Print), eISSN: 1423-0356 (Online)
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