Sniff Nasal Inspiratory Pressure A Noninvasive Assessment of Inspiratory Muscle Strength

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1 Sniff Nasal Inspiratory Pressure A Noninvasive Assessment of Inspiratory Muscle Strength FRANCIS HERITIER, FRANCOIS RAHM, PHILIPPE PASCHE, and. JEAN-WILLIAM FITTING Division de Pneumologie and Service d'oto-rhino-laryngologie, Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland The measurement of esophageal pressure during maximal sniffs (sniff Pes) has been shown useful to assess inspiratory muscle strength. The aim of this study was to validate a noninvasive method for estimating sniff Pes. The sniff nasal inspiratory pressure (SNIP) was measured through a plug occluding one nostril during sniffs performed through the contralateral nostril. Sniff Pes was simultaneously measured with an esophageal balloon. Ten normal subjects performed 338 sniffs of variable intensity. The correlation coefficient of SNIP and sniff Pes was 0.99 ± 0.01 (p < 0.001). The ratio SNIP/sniff Pes was 0.91 (range, 0.82 to 0.99) and the mean difference between the two measures (SNIP - sniff Pes) was cm H 2 0 (-1.2to -8.6 cm H 2 0 ). Twelve patients with neuromuscular or skeletal disorders performed 181 maximal sniffs. The correlation coefficient of SNIP and sniff Pes was 0.96 ± 0.04 (p < 0.001). The ratio SNIP/sniff Pes was 0.93 (0.77 to 1.07) and the mean difference (SNIP - sniff Pes) was cm H 2 0 (+0.47 to cm H 2 0 ). Nasal mucosal congestion was induced by nebulization of increasing doses of histamine in four normal subjects. The ratio SNIP/sniff Pes was 0.93 (0.72 to 1.02) when nasal peak flow was> 100 Umin, and 0.49 (0.36 to 0.57 L/min) when nasal peak flow fell below 100 Umin. We conclude that SNIP provides a reliable and noninvasive estimation of sniff Pes in normal subjects and in patients with neuromuscular or skeletal disorders. The validity of this method may by impaired by severe nasal congestion. Heritier F, Rahm F, Pasche ~ Fitting J-W. Sniff nasal inspiratory pressure: a noninvasive assessment of inspiratory muscle strength. Am J Respir Crit Care Med 1994;150: The amplitude of pressure change measured at the mouth during a maximal inspiratory effort performed against an occlusion (Pl max) is classically used in the assessment of inspiratory muscle strength (1). However, the Plmax maneuver is both demanding and unpleasant. Thus, if low Plmax values can reflect true muscle weakness, they may also be due to lack of motivation or coordination. A useful alternative method for measuring inspiratory muscle strength consists of asking the subjects to perform short sniffs of maximal intensity (2). The sniff maneuver is natural and probably easier to perform than the Plmax maneuver for most individuals. Thus, inspiratory muscle strength is often better reflected by the change in esophageal pressure measured during maximal sniffs (sniff Pes) than by Plmax (3). However, the clinical usefulness of sniff Pes is limited by the invasive character of its measurement, requiring an esophageal balloon. Less invasive methods have been described for estimating sniff Pes, using balloons either in the mouth or in the nasopharynx (4). The aim of this study was to determine whether sniff Pes could be estimated by measuring changes in nasal pressure in an entirely noninvasive manner. METHODS We studied 11 normal healthy Caucasian volunteers and 12patients with neuromuscular or skeletal disorders (11 Caucasians and one black Afri- (Received in original form September 13, 1993 and in revised form February 23, 1994) Correspondence and requests for reprints should be addressed to Dr. J. W. Fitting, Division de Pneumologie, CHUV, CH-l0ll lausanne, Switzerland. Am J Respir Crit Care Med Vol 150. pp , 1994 can). The normal volunteers comprised eight males and three females, two of them accustomed to performing the sniff maneuver. The patients consisted of seven males and five females, none of them accustomed to the sniff maneuver. Lung volumes were measured by body plethysmographyand spirometry with a mass flow sensor (SensorMedics 6200 Autobox, YorbaLinda, CA). Maximal inspiratory pressure (Plmax) and expiratory pressure (PEmax) were measured in the sitting position using a standard flanged mouthpiece connected to a pressure transducer (Micro Switch 126PC; Honeywell, Freeport, IL)by a polyethylene catheter. A 1-mmleakwasadded in order to preventparticipation of orofacial muscles. The pressures were amplified and recorded on paper at a speed of 10mm/s(Hewlett-Packard 7758A, Palo Alto, CA; or Gould EasyGraf, Valley View, OH). The transducer was calibrated beforeeach study using a U-tubewater manometer. Plmax was measuredfrom residual volume (RV) and PEmax from total lung capacity (TLC), with a noseclip. Three technically acceptable recordings were obtained for each maneuver. The largest change of pressure maintained for 1 s was used for analysis. Esophageal pressure (Pes) was measured with a 5-cm-long balloon filled with 0.5ml of air and positioned in the midesophagus (5). The sniff nasal inspiratory pressure (SNIP) was measured through a plug occluding one nostril during a sniff performed through the contralateral nostril. The plug was made of two to three waxed ear plugs (Calmor, Neuhausen am Rheinfall, Switzerland) hand-fitted around the tip of a catheter (Figure 1).The absence of air leak around the plug wasascertained byoccluding the contralateral nostrilduring an inspiratoryeffort.the esophageal balloon and the nasal plug were connected to two pressure transducers (Micro Switch 126PC, Honeywell, Freeport, IL) via identical100-cm polyethylenecatheters. SNIPandsniff Peswererecorded simultaneously, using the same equipment as used for measuring Plmax and PEmax. The normal subjects and the patients were studied in the sitting position while breathing through the patent nostril and keeping the mouth closed. Without a prior training period, the normal subjects were asked to perform short, sharp sniffs of variable intensity from functional resid-

2 Heritier, Rahm, Pasche, et al.: Sniff Nasal Inspiratory Pressure 1679 Figure 1. Method of measurement of sniff nasal inspiratory pressure (SNIP). (Left) Nasal plug hand-fitted around the tip of the catheter. (Right) Plug and catheter inserted into one nostril. ual capacity (FRC), including several maximal maneuvers (Figure 2). Only maximal sniffs were performed by the patients. No visual feedback was provided. Successive sniff maneuvers were separated by two to four normal breaths. In normal subjects, the criteria used to select sniffs suitable for analysis were: (1) a pressure tracing showing a smooth upstroke and a sharp peak for Pes and SNIP; (2) a total sniff duration of less than 500 ms for Pes; (3) a sniff performed from FRC as judged by pre-sniff Pes. The patients met criteria (1) and (3) in a manner similar to the normal subjects. On some occasions, total sniff duration exceeded 500 ms, while the shape of the pressure tracing appeared otherwise suitable. For this reason, we chose a time to peak of less than 400 ms as the upper limit in the patients. To assess the validity of SNIP for estimating sniff Pes in the presence of various degrees of nasal congestion, maximal SNIP and sniff Pes were measured simultaneously in four healthy subjects before and after nasal histamine challenge tests. Prior to these tests, major nasal abnormalities were excluded by anterior rhinoscopy. Mucosal congestion was achieved by nebulization of increasing doses of histamine. Doses of 0.1,0.2,0.4, and 0.8 mg of histamine were administered in that order into each nostril through a metered dose inhaler. Before the histamine challenge and 5 min after each application of histamine, the following measurements were performed: (1)assessment of the nasal patency under static conditions, by measuring the cross-sectional area at the head of the inferior turbinate by acoustic rhinometry (Stimotron Rhinoklack RK 1000,Acutronic, Grasbrunn, Germany) (6); (2) assessment of the dynamic nasal patency by measuring the nasal peak inspiratory flow rate (Youlten Peak Nasal Flow Meter, Clement Clarke Int. Ltd, London, UK); (3) assessment of the effect of nasal congestion on the relationship between SNIP and sniff Pes, by measuring these two variables simultaneously during four to six maximal sniffs. The three sniffs showing the largest swings of Pes were considered for analysis and their values were averaged. Nasal and esophageal pressures are subatmospheric during sniffs. SNIP and sniff Pes designate the amplitudes of pressure change from atmospheric pressure, and are expressed in absolute values. The values for both measures are expressed as mean ± SO.Correlation coefficients (r) between SNIP and sniff Pes were calculated and linear regression analysis of the two measures was performed. The agreement between SNIP and sniff Pes was assessed by the method of differences against the means, according to Bland and Altman (7). RESULTS Normal Subjects Descriptive data for the normal subjects were age, 31.8 ± 5.2 yr; height, 1.74 ± 0.1 m; and weight, 66.0 ± 11.2 kg. Spirometric data were FEV., ± 14.6% predicted; FVC, ± 13.4% predicted; and FEV./FVC, 98.8 ± 6.3% predicted. In 10 of the 11 normal subjects, SNIP and sniff Pes were compared during sniffs of intensities ranging from 10 to 140 cm H 20. Criteria for analysis were met by 338 sniffs (rejection rate 13%). The correlation coefficient of SNIP and sniff Pes was 0.99 ± 0.01; p < (Figure 3). Regression analysis showed that SNIP = Pes. Considering all measurements of each subject, the mean difference (SNIP - sniff Pes) was cm H 20 (range, -1.2 to -8.63) (Table 1). For all sniffs, the mean value of the ratio SNIP/sniff Pes was 0.91 (0.82 to 0.99). Considering the five maximal values of each SUbject, the ratio SNIP/sniff Pes was 0.92 (0.89 to 0.97), and the mean pressures were 85 cm H 20 (65 to 123 cm H 20) for SNIP and 93 cm H 20 (74 to 135 cm H 20) for ~c...~_,i~ c.:.: =- sniff Pes =,...,...,...,,,...,,-,, ::...:..._--'-_.._-_._--~'~_.._.---~-,-- ',.=-=-=-- -' :--'-::-:-~==-',_c-:.~",~~::.::~~ 20 em H20. ~ll j;~-,~<~;-j[- Figure 2. Example of pressure tracings during sniffsof variable intensity performed by a normal subject. (Top): Sniff nasal inspiratory pressure (SNIP). (Bottom): Sniff esophageal pressure (sniff Pes). Upward deflections reflect subatmospheric pressure swings o~ ,~-..., o sniff Pes (em H20) Figure 3. Relationship between sniff nasal inspiratory pressure (SNIP) and sniff esophageal pressure (sniff Pes) during 338 sniffs of different intensity performed by 10 normal subjects. The solid line represents the line of identity.

3 1680 AMERICAN JOURNAL OF RESPIRATORY AND CRITICAL CARE MEDICINE VOL TABLE 1 VALUES OF SNIFF NASAL INSPIRATORY PRESSURE (SNIP) AND SNIFF ESOPHAGEAL PRESSURE (SNIFF PES) DURING SNIFFS RANGING FROM VERY LOW TO MAXIMAL INTENSITY PERFORMED BY NORMAL SUBJECTS All Sniffs Age All Sniffs All Sniffs Mean Difference Five Highest Values Five Highest Values Five Highest Values Subject No. (yr) No. of Sniffs SNIP/Pes (r) SNIP - Pes SNIP/Pes SNIP (em H 2O) Pes (em H 2O) Mean ± SD 32 ± 5 34 ± ± ± ± ± ± ± 20 Pes (Table1). Figure 4 presents a plot of the difference between SNIP and sniff Pesagainst their mean for the five maximal values of each subject. The mean difference (d) was ± 4.94 cm H 20. The limits of agreement were: Upper limit: d + 2SD, (2 x 4.94) = 2.05 cm H 20 Lower limit: d - 2SD, (2 x 4.94) = cm H 20 The 95% confidence interval (CI) of the bias was: d SEM, (2.01 x 0.699) = cm H 20 d SEM, (2.01 x 0.699) = cm H 20 The SEM of the upper and lower limits was: 'Ii 3SD2/n = 1.21cm H 20 The 95% CI of the upper limit was: SEM, (2.01 x 1.21) = 4.48 cm H Mean + 2SD 2 ~... 0 O- N + ~++ = -2- * e -4- ~ u "'-'" Mean CI.l -8- T.,. ~ 4.» =...p ~ t: = + + CI.l = L Z Mean - 2 SD ~ I I I I (SNIP + sniff Pes) I 2 (em H2O) Figure4. The difference between sniff nasal inspiratory pressure (SNIP) and sniff esophageal pressure (sniff Pes) against the mean of these two variables. The five largest values for 10 normal subjects are presented SEM, (2.01 x 1.21) = cm H 20 The 950/0 CI of the lower limit was: SEM, (2.01 x 1.21) = cm H SEM, (2.01 x 1.21) = cm H 20 Patients Descriptive data for the patients were: age, 49.7 ± 19.2yr; height, 1.66 ± 0.07m; and weight, 72.4 ± 20.5kg. Spirometric data and lung volumes were: FE'ft, 83.2 ± 24.1% predicted; FVC, 82.7 ± 25.00/0 predicted; FE\4/FVC, ± 9.80/0 predicted; TLC, 83.5 ±' 19.70/0 predicted; and FRC, 93.2 ± 30.1% predicted. Maximal respiratory pressures were: Pl max, 61.9 ± 33.4 cm H 20 and PE max, 66.9 ± 27.3 cm H 20. Diagnoses are given in Table 2. Criteria for analysiswere met by 181 sniffs (rejection rate, 110/0). The correlation coefficient of SNIP and sniff Peswas 0.96 ± 0.04; p < Regression analysis showed that SNIP = 2.'Jl Pes. Considering all measurements for each patient, the mean difference (SNIP - sniff Pes) was cm H 20 (range, to cm H 20) (Table2). For all sniffs the mean value of the ratio SNIP/sniff Peswas 0.93(0.77to 1.07). Considering the three maximal values for each patient, the ratio SNIP/sniff Pes was 0.91 (0.72to 1.04)(Figure 5), and the mean pressures were 65 cm H 20 (28 to 101) cm H 20 for SNIP and 71 cm H 20 (32 to 109 cm H 20) for Pes (Table 2). Figure 6 presents a plot of the difference between SNIP and sniff Pes against their mean for the three maximal values for each patient. The mean difference (d) was -6.2 ± 7.07 cm H 20. The limits of agreement were: Upper limit: d + 2SD, = 7.94 cm H 20 Lower limit: d - 2SD, = cm H 20 The 95% CI of the bias was: d SEM, (2.03 x 1.178) = cm H 20 d SEM, (2.03 x 1.178) = cm H 20 The SEM of the upper and lower limits was: V380 2/n= 1.96 H 20 The 95% CI of the upper limit was: SEM, (2.03 x 1.96) = cm H SEM, (2.03 x 1.96) = 3.96 cm H 20 The 95% CI of the lower limit was:

4 Heritier, Rahm, Pasche, et 01.: Sniff Nasal Inspiratory Pressure 1681 TABLE 2 VALUES OF SNIFF NASAL INSPIRATORY PRESSURE (SNIP) AND SNIFF ESOPHAGEAL PRESSURE (SNIFF PES) DURING MAXIMAL SNIFFS PERFORMED BY PATIENTS All Sniffs Three Three Three Age All Sniffs All Sniffs Mean Difference Highest Values Highest Values Highest Values Patient No. Diagnosis (yr) No. of Sniffs SNIP/Pes (r) SNIP - Pes SNIP/Pes SNIP (em H 2O) Pes (em H 2O) 1 Steinert's disease Myasthenia gravis Decompression sickness Multiple sclerosis Post-poliomyelitis sequelae Myasthenia gravis Myasthenia gravis? Still's disease Pompe's disease Kyphoscoliosis Myopathy Guillain-Barre syndrome Mean ± SD 49 ± ± ± ± ± ± ± ± SEM, (2.03 X 1.96) = cm H SEM, (2.03 X 1.96) = cm H 2 0 Nasal Histamine Challenge The influence of nasal mucosal congestion on SNIP was studied in four of the 11 normal subjects. Before the histamine challenge, the cross-sectional area at the level of the lower turbinate was 0.76 ern- (range, 0.45 to 1.0 ern"), nasal peak inspiratory flow was 224 Umin (170to 270 Umin), and the ratio SNIP/sniff Pes was 0.96 (0.90 to 1.02). The individual response to the histamine challenge was variable. At the end of the challenge test, the nasal cross-sectional area was 0.33 cm 2 (0.20 to 0.43 cm-), nasal peak flow was 74 Umin (10 to 120 Umin), and the ratio SNIP/sniff Pes was 0.01(0.36to 0.99). Figure 7Apresentsthe relationship between the SNIP/sniff Pes ratio and the nasal cross-sectional area. Despite a similar cross-sectional area in the four subjects at the end of the test, two of them showed a markedly decreased SNIP/sniff Pes ratio, whereas the other two did not. Figure 7B presents the relationship between SNIP/sniff Pes ratio and nasal peak flow. The two subjects showing a decrease in the SNIP/sniff Pes ratio also had a larger decrease in nasal peak flow than the other two subjects. Thus, SNIP reflected sniff Pes adequately when nasal peak flow was only moderately decreased, but failed to do so when nasal peak flow fell markedly. The ratio SNIP/sniff Pes was 0.93(0.72 to 1.02) when nasal peak flow was> 100 Umin, and 0.49 (0.36 to 0.57) when nasal peak flow fell below 100 Umin ,, M 75 == ecj '-" 50 ~ I-C fi.) Z '0----, ,,---- o sniff Pes (em H20) 125 Figure 5. Relationship between sniff nasal inspiratory pressure (SNIP) and sniff esophageal pressure (sniff Pes) during maximal sniffs in 12 patients. The three largest values of each patient are presented. The solid line represents the line of identity. 10 Mean + 2SD 8,, M= e -2 t- + *+ + + (J + + '-" fi) QI -8 Mean = t:: =fi) ~ -20 I-C -22 Z -24 Mean - 2SD fi.) (SNIP+sniff Pes) I 2 (em H2O) Figure 6. The difference between sniff nasal inspiratory pressure (SNIP) and sniff esophageal pressure (sniff Pes) against the mean of these two variables. The three largest values for 12 patients are presented.

5 1682 AMERICAN JOURNAL OF RESPIRATORY AND CRITICAL CARE MEDICINE VOL ),9 s l::.~ 0.6 _ 0.5 llo 0.4 Z(I) ,-----, nasal cross-sectional area (cm2) Figure Z Effect of nasal mucosal congestion induced by increasing doses of histamine in four normal SUbjects. (A) Relationship between the ratio SNIP/sniff Pes and the nasal cross-sectional area. (B) Relationship between the ratio SNIP/sniff Pes and the nasal peak inspiratory flow. Each subject is represented by a distinct symbol. DISCUSSION ~ In the search for a reliable and practical test of inspiratory muscle strength, considerable interest has recently focused on the sniff maneuver. Thus, Laroche and colleagues (3) showed that inspiratory muscle strength is better retlectedby the esophageal pressure change during a maximal sniff (sniff Pes) than by the mouth pressure change during a maximal inspiratory effort against an occluded airway (Pl max). This observation is at first sight surprising because the sniff is a ballistic maneuver associated with some increase in lung volume and some distortion of the chest wall. During such a nonisometric contraction, a certain loss of muscle tension can be expected to occur because of the force-velocity relationship. Twofactors may, however,explain the larger pressure changes obtained during maximal sniffs than during the Plmax maneuver. First, although the Pl max maneuver is generally considered as static, it is associated with substantial changes in diaphragm length and configuration, and, like the sniff, is therefore also not an isometric contraction (8). Second, the sniff is a natural and easy maneuver for most subjects, whereas a sustained maximal inspiratoryeffort against an occlusion is unpleasantand even painful. Except for highly motivated volunteers, it is likely that many subjects, attempting to avoid chest discomfort, realize only a submaximal effort during the measurement of Pl max A greater activation of inspiratorymuscles may therefore occur during the sniff, and may outweigh the disadvantage of a dynamic maneuver. In our study, SNIP was measured through a plug occluding one nostril during sniffs performed through the contralateral nostril. By using this technique in normal subjects, we found a close agreement between SNIP and sniff Pes over a wide range of pressures. A similar agreement was found during maximal sniffs in a group of patients with neuromuscular or skeletal disorders. In both groups, we very rarely observed the occurrence of SNIP/sniff Pes ratios greater than unity, and in these cases, SNIP only slightly overestimated Pes. The similarity between SNIP and sniff Pes is explained by a phenomenon of nasal collapse. During an inspiration through the nose, the airflow is regulated by a resistance located in the nostrils (9, 10).The nasal flow-limiting segment, or nasal valve, is situated in the first 2.5 em from the external orifice, and its main component corresponds to the isthmus nasi (10). Nasal airflow increases proportionately with transnasal pressure and reaches a plateau when this pressure attains a critical value (9). Then, the nasal valve partlycollapses and a greater inspiratoryeffort does not increase nasal airflow. This flow limitation occurs for a trans ,.--., , , nasal peak inspiratory now (Ifmin) nasal pressure of 10to 15em H 20 and for a ventilation of approximately 30 Umin (9, 10). During a sharp sniff, the transnasal pressure elicits the collapse of the nasal flow-limiting segment, and only a small pressure gradient exists between the extra- and intrathoracic airways beyond the point of occlusion. This was demonstrated by two previous studies. Using balloons positioned either in the nasopharynx (Pnp) or in the mouth (Pmo), Koulouris and colleagues (4) showed a close correlation between Pnp, Pmo, and Pes during sniffs. Similarly, in a previous study we found a good agreement between Pmo as measured by a standard mouthpiece and Pes during sniffs over a wide range of pressures (11). However,these previous techniques have some limitations. The insertion of a nasopharyngeal balloon is fairly invasive and necessitates topical anesthesia. On the other hand, a balloon placed in the oral cavity may elicit oral reflexes that can prevent measurements (4). Similarly, we observedoccasionaloverestimations of sniff Pes by Pmo as measured by a mouthpiece, because of an agonist activation of orofacial muscles (11). We did not observe the same kind of limitations with the SNIP technique. Indeed, the insertion of the nasal plug is easy, and the measurements are not impaired by movements of oral structures. However, it is crucial to eliminate any air leak round the plug before the measurements are made. This is easily achieved by hand-fitting waxed ear plugs to accommodate the shape of the nasal vestibule. The use of a nasal plug in the SNIP technique offers several advantages. First, the measurement of nasal pressure through the plug in the nasal vestibule is entirely noninvasive. Second, the distortion of the nasal valve by the plug prevents collapse of the valve during the sniff, and permits transmission of the pressure. from the rhinopharynx to the catheter. Third, sniffs performed through a single nostril may offer an advantage in the assessment of inspiratory muscle strength. In a previous study, we observed that the ratio sniff Pmo/sniff Pes was greater if one nostril was occluded (11). A similar observation was reported by Heijdra and coworkers (12), who found larger values of sniff Pmo when the sniffs were performed with one instead of two open nostrils. This could be of particular value in the case of sniffs of low intensity, such as in patients with severe muscular weakness (13). Indeed, the collapse of the nasal valve occurs at a lower transnasal pressure gradient when only a single nostril is open during inspiration (10). Several limitations of the SNIP technique should be borne in mind. Although the mean difference between SNIP and sniff Pes was small, the limits of agreement (2 SO above and below the mean difference) were relatively large. This is of no practical significance in normal subjects, in whom large pressure swings are generated during maximal sniffs. On the other hand, it can be argued that the range between the limits of agreement may represent a sizeable proportion of maximal pressure swings in patients with severe neuromuscular weakness, and this could limit the usefulness of the technique. However, as shown in Figures 5 and 6, the lowest values of sniff Pes were adequately reflected by SNIP. Anatomic abnormalities may potentially hinder the measurement of nasal pressure in some subjects. For instance, a major septal deviation or septal defect is likely to impair pressure transmission from the rhinopharynx. Finally, nasal mucosal congestion predisposes to the same kind of limitation. This was demonstrated in the two subjects who showed the largest decrease in nasal peak flow after the histamine challenge. However, our data suggest that nasal congestion that creates only a moderate decrease in nasal peak flow does not impair the estimation of sniff Pes by SNIP.One may conclude that in the absence of nasal peakflow measurement, the SNIP technique should not be used in

6 Heritier, Rahm, Pasche, et 01.: Sniff Nasal Inspiratory Pressure cases of obvious nasal congestion. Furthermore, we observed that the shape of the pressure tracing was altered when nasal congestion was associated with underestimation of sniff Pes by SNIP. Indeed, the characteristic smooth upstroke and sharp peak were most often replaced by an irregular tracing with a truncated peak. Significant underestimation of sniff Pes by SNIP should be suspected in cases of such an unusual wave pattern. In summary, we found that measuring SNIP in one occluded nostril during sniffs performed through the contralateral nostril provides a reliable estimation of esophageal pressure in normal subjects and in patients with neurologic and skeletal disorders. Being a simple and fully noninvasive test, the SNIP may prove useful in the assessment of patients for suspected respiratory muscle weakness. Maximal inspiratory and expiratory pressures (Pl max and PE max ) are physiologically the most adequate tests of respiratory muscle strength and should be used as first choices. When they yield normal values, no further test is necessary. However, low values of Pl max may be related either to inspiratory muscle weakness or to difficulty in performing the maneuver. In the latter situation, the measurement of sniff esophageal pressure may reveal a normal inspiratory muscle strength. The same information can be obtained by the SNIP, with the advantage of its being a noninvasive technique. References 1. Black LF, Hyatt RE. Maximal respiratory pressures: normal values and 1683 relationship to age and sex. Am Rev Respir Dis 1969;99: Miller JM, Moxham J, Green M. The maximal sniff in the assessment of diaphragm function in man. Clin Sci 1985;69: Laroche CM, Mier AK, Moxham J, Green M. The value of sniff esophageal pressures in the assessmentof global inspiratory muscle strength. Am Rev Respir Dis 1988;138: Koulouris N, Mulvey DA, Laroche CM, Sawicka EH, Green M, Moxham J. The measurement of inspiratory muscle strength by sniff esophageal, nasopharyngeal, and mouth pressure. Am Rev Respir Dis 1989;139: Milic-Emili J, Mead J, Turner JM, Glauser EM. Improved technique for estimating pleural pressure from esophageal balloons. J Appl Physiol 1964;19: Lenders H, Pirsig W. Diagnostic value of acoustic rhinometry: Patients with allergic and vasomotor rhinitis compared with normal controls. Rhinology 1990;28: Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986;1: Gandevia SC, Gorman RB, McKenzie DK, Southon FCG. Dynamic changes in human diaphragm length: maximal inspiratory and expulsive efforts studied with sequential radiography. J Physiol (london) 1992;457: Bridger GP, Proctor DF. Maximum nasal inspiratory flow and nasal resistance. Ann Otol Rhinol Laryngol 1970;79: Haight JSJ, Cole P. The site and function of the nasal valve. Laryngoscope. 1983;93: Hsritier F, Perret C, Fitting JW. Esophageal and mouth pressure during sniffs with and without nasal occlusion. Respir Physiol1991 ;86: Heijdra YF, Dekhuijzen PNR, van Herwaarden CLA, Folgering HTM. Differences between sniff mouth pressures and static maximal inspiratory mouth pressures. Eur Respir J 1993;6: Heritier F, Perret C, Fitting JW. Maximal sniff mouth pressure compared with maximal inspiratory pressure in acute respiratory failure. Chest 1991;100:175-8.