Respiratory and laryngeal contributions to maximum phonation duration

Journal of Voice Vol. 14, No. 3, pp. 331-340 © 2000 The Voice Foundation

Respiratory and Laryngeal Contributions to Maximum Phonation Duration Nancy Pearl Solomon, Shannon J. Garlitz, and Rochelle L. Milbrath University of Minnesota, Minneapolis, Minnesota

Summary: Maximum phonation duration (MPD) is a common assessment procedure in speech-language pathology. However, the specific contributions of the respiratory and phonatory components of the speech-production mechanism to this task are not typically assessed. Six women and 6 men with normal speech and voice were monitored for lung volume during a standard MPD task, and for laryngeal airway resistance (Rlaw) during a modified MPD (stow syllable-repetition) task. On average, subjects used 90% of their vital capacity (VC) for their best MPD trial. There was no systematic relation between MPD and VC for these subjects. Rlaw was strongly correlated with MPD for the men (rs = 0.886 for/o/; r~,= 0.829 for/i/), but not for the women. Rlaw increased linearly as lung volume decreased (slope > 0.15) for a subset of trials (32%). This was a common pattern for four of the subjects. The clinical utility of MPD to assess breathing for speech is questioned because of the lack of association between MPD and VC, and some atypical laryngeal-valving strategies. Key Words: Maximum phonation duration--Lung volume--Vital capacity--Laryngeal airway resistance Chest-wall kinematics--Aerodynamics--Laryngeal-valving strategies.

M a x i m u m phonation duration (MPD) is a c o m m o n task for the clinical assessment of the respiratory and phonatory components of the speech-production mechanism. The client is told to inhale as much as possible, and then to prolong a vowel as long as possible. The maximum duration obtained over several trials is compared to normative data, and is presumed to provide a rough indication of respiratory "support"

(ie, lung volume, alveolar pressure) and phonatory function for speech. Because of the heavy reliance on client performance, the validity of MPD as a clinical assessment procedure has been questioned, t-4 Kent et al 2 emphasized the importance of quantitative instrumental procedures to analyze specific characteristics of the response. Using M P D as a prime example, they stated that "measures of maximum performance are not always interpretable in and of themselves." The proportion of the vital capacity (VC) used by individual speakers for the M P D task is an obvious source of variability. Ranges reported in the literature include 49-78% VCfi 33-78% VC, 6 and 69-95% VC 7 in normal speakers, and 27-97% VC in speakers with laryngeal pathology. 8 It is not surprising that the proportion of the VC is less than 100%. One reason is that a small portion o f the VC may be wasted at the

Accepted for publication October 14, 1999. Portions of this paper were presented at the Annual Convention of the American Speech-Language-Hearing Association, November 1998, San Antonio, Texas, and at the Voice Foundation Symposium, June 2000, Philadelphia, Pennsylvania. Address correspondence and reprint requests to Nancy Pearl Solomon, Department of Communication Disorders, University of Minnesota, Minneapolis, MN 55455, USA. e-mail: [email protected]

331

332

NANCY PEARL SOLOMON ET AL

beginning of the task before phonation begins 7 because of high passive alveolar pressure and/or incomplete laryngeal approximation: Additionally, some amount of lung volume remains at the end of phonation or the chest wall would not be able to generate adequate pressure to sustain vocal fold oscillation. 5 Nonetheless, the extent of unused lung volume reported previously seems excessive and could reflect submaximal performance or measurement error. Yanagihara et al5 carefully examined lung volume during an MPD task. They recognized that VC, alveolar pressure, and laryngeal valving of the airstream are the key factors that regulate MPD. In this and a subsequent series of papers, they demonstrated that airflow was the most important determinant of MPD. 6-12 Although the importance of collecting airflow data to interpret MPD productions has been demonstrated, it is rarely done in clinical settings. Some studies of MPD in clinical populations have supported the usefulness of this task for assessment of the respiratory and phonatory systems. Such support has been provided for persons with voice disorders, 8,9,13 spastic dysarthria associated with cerebral palsy, 14-16 and bulbar amyotrophic lateral sclerosis (ALS). 17 Results generally have indicated that MPD is inversely related to the severity of the disorder, especially pertaining to the degree of vocal fold adduction 9,12 or respiratory impairment. 17 Other studies using MPD have been unsuccessful at differentiating speakers with certain speech and voice disorders. Gamboa et a118 were unable to differentiate speakers with Parkinson disease from normal speakers on the basis of MPD. Trudean and Forrest 3 compared MPD in speakers with and without vocal fold lesions, and Treole and Trudean4 examined MPD before and after successful voice therapy for vocal nodules. Neither study found statistically significant differences. These studies raise questions about the clinical efficacy of MPD as an assessment procedure, except perhaps in cases of severe impairment. In typical clinical practice, MPD is included in a larger battery of tests, but it is still not known whether it contributes valid information to the overall evaluation. Variability in MPD data that is not explained by VC is assumed to be due to laryngeal resistance to airflow, 17,19 but this has never been assessed directly. Previous instrumental studies of MPD have meaJournal of Voice, Vol. 14, No. 3, 2000

sured lung volume and airflow. The present study adds the assessment of air pressure, allowing calculations of resistance (pressure/flow). Using a well-accepted noninvasive method to estimate laryngeal airway resistance (Rlaw), 20 the present study provides this missing link in the literature. This measure, when examined throughout trials, should provide an indication of laryngeal-valving strategies for maximally sustaining phonation. Although this method involves slow syllable repetitions rather than a sustained vowel, it was presumed that speakers would use similar laryngeal-valving strategies. The general goal of this study was to examine respiratory and laryngeal contributions to maximum phonation duration. The study addressed two mechanistic issues related to MPD task performance: 1. What proportion of the vital capacity (VC) do normal speakers use during a standard maximum phonation duration (MPD) task? 2. What resistance by the laryngeal airway (Rlaw) is provided during a modified MPD task? In addition, this study addressed three interpretive issues: 1. What is the correlation between MPD and VC? 2. What is the correlation between Rla w and MPD? 3. How does Rlaw vary throughout individual trials?

METHOD Subjects Twelve university students (6 women, 6 men) participated in this study. They were nonsmokers; had negative histories for respiratory, neurologic, speech, Or hearing disorders; and had no vocal training. Pulmonary function was screened such that at least 80% of a forced vital capacity was expired in 1 s (FEV1/FVC > 0.80). Table 1 lists the age, height, weight, and vital capacity of each subject. Subjects provided informed written consent but were naive to the specific purposes of the experiment and were unfamiliar with the tasks and equipment. They were compensated $10 for their participation (-1 h).

Instrumentation and calibration procedures Lung volume Lung volume was determined with respiratory inductive plethysmography (RIP, Respitrace, Ambula-

RESPIRATORY AND LARYNGEAL CONTRIBUTIONS TO MPD

333

TABLE 1. Physical Characteristics of Subjects Subject

Age (yr)

Height(em)

Weight(kg)

Vital Capacity (L)

F1

19

165

64

3.90

F2

26

163

48

3.16

F3

28

163

64

4.05

F4

19

170

59

3.83

F5

22

173

79

4.47

F6

29

178

61

4.39

M1

25

180

61

4.58

M2

21

178

84

6.52

M3

21

185

68

5.62

M4

21

180

75

5.16

M5

21

178

70

4.14

M6

23

180

111

4.87

tory Monitoring) calibrated against a flow-based spirometer (pitot-tube pneumotachometer; Medical Graphics) and accompanying software (Breeze; Medical Graphics). Before each session, the spirometer was calibrated with a 3-L syringe (5 repetitions). Two RIP bands ("Respibands"), selected for size according to the circumferences of the chest and abdomen, were placed on the subject's bare skin and covered with elastic netting. The upper band encircled the rib cage; its upper edge was placed just under the axillae. The lower band encircled the abdomen; its lower edge was placed at about the pubic hairline and its upper edge was below the lowest rib. To further ensure secure placement of the Respibands, they were taped carefully (so as not to interfere with the wires sewn into the fabric) to the skin in several places. In addition, washable ink was used to mark the location of the bands on the skin, and these marks were checked periodically throughout the sessions. Over the Respibands and netting, subjects wore a loose-fitting shirt. Seated upright in a chair, subjects performed relaxation, isovolume, and vital capacity maneuvers for calibration, according to standard kinematic method.2~ The relaxation maneuver involved having subjects breathe quietly for several cycles, hold their breath at the resting expiratory level (REL), and then relax the chest wall against the closed airway (instruction: "Let your belly flop out."). For the isovolume maneuver, subjects again closed their airway at REL,

then displaced abdominal and rib cage volume back and forth ("Pull your belly in and then let it flop out.") for two complete cycles. This maneuver allowed calibration for relative volume displacement of the 2 parts of the chest wall (rib cage and abdomen; see Data analysis). Relaxation and isovolume maneuvers at REL were repeated several times before and after spirometric calibration and once after every 2 phonation trials during the experiment to confirm a stable body position and consistent calibration of the chest wall. Subjects performed at least 2 VC maneuvers with the spirometer while wearing the Respibands to calibrate the kinematic signals for lung volume. For this maneuver, subjects wore nose clips and breathed through the mouthpiece of the spirometer for several resting cycles, then inspired and expired maximally.

Laryngeal airway resistance Oral air pressure and airway-opening airflow (ie, through the mouth and nose) were determined for the eventual estimation of RI~w. instrumentation included a circumferentially vented pneumotachograph face mask and a small flexible tube (length = 14 cm inside diameter = 3 ram), which was passed through the mask, each coupled to a differential pressure transducer and accompanying filters and electronics (PTL-1 transducer, 30-Hz low-pass filter for pressure; PTW-1 transducer, 3-kHz low-pass filter for flow; Glottal Enterprises MS-100). Pressure was calJournal of Voice, Vol. 14, No. 3, 2000

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NANCY PEARL SOLOMON ET AL

ibrated with an incline manometer (Meriam Instrument M231) and flow was calibrated with a microprocessor-driven 2-L bellows (Glottal Enterprises, MCU-4).

Phonation Phonation was sensed with an externally powered (Shure PS 1A) head-mounted condenser microphone (AKG C420) and amplifier (Shure FPI 1). Subjects monitored sound pressure level (SPL) during task productions by watching the digital display on a sound level meter (Exteck 407740, C-scale) placed 60 cm from the mouth. Tones for cueing pitch were generated by a pure-tone generator (General Radio 1312), digitized (Vibra 16-bit soundcard), and played over loudspeakers (Altec Lansing ACS 40). Data recording and monitoring The 5 data signals (rib cage motion, abdominal motion, pressure, flow, and speech) were digitized on-line to a laboratory computer (Gateway P5-133) at a sampling rate of 800 Hz per channel using data acquisition hardware and software (Dataq Instruments, CODAS, WinDaq/200, Model DI-205). Data were monitored on-line in an x-y display (rib cage v. abdomen) on a digital oscilloscope (Tektronix TDS 210) and a time-based display on the computer monitor (all 5 signals), and backed-up on data tape (Vetter 820).

Data collection Phonation tasks Subjects performed 3 repetitions of 2 tasks with 2 vowels each, resulting in a total of 12 productions. The tasks were maximally sustained phonation and slow syllable repetitions, and the vowels were/a/and /i/. Sustained/ct/is the most common utterance used clinically to assess MPD, 2 and slow repetitions of the consonant-vowel (CV) syllable/pi/is the standard stimulus used for assessing Rlaw.20 Both vowels were used for both tasks because MPD has been found to vary across vowel. 13,22 During the standard MPD task, performed first, only chest-wall kinematic and phonation data were collected so that no equipment was placed on the face. Subjects maintained a comfortable body position with arms placed on the chair's armrests and feet flat on Journal of Voice, Vol. 14, No. 3, 2000

the floor throughout data collection. During trials, they were reminded not to shift position, raise the shoulders, or flex the torso. Although restricting torso movement could axtificially reduce VC, this procedure was necessary for valid kinematic assessment. Before each trial, subjects were asked to "Take in as much air as you possibly can, and say [/o/,/i/] as long as you possibly can, until you are completely out of air." For the slow syllable-repetition task, kinematic, aerodynamic, and acoustic signals were recorded. The experimenter held the pneumotachographic mask firmly against the face and the subject placed the pressure tube lightly between the lips such that the open end of the tube was in the mouth; body position was as before. The instructions were the same except that the speech stimuli were/po/and/pi/repeated legato at a rate of 1.5 syllables/s. Rate was trained with a digital metronome (Seiko DM-10). The order of the vowels within each task was alternated within and across subjects. Because MPD may be affected by vocal fundamental frequency (F0) and sound pressure level (SPL), 1,5,23,24 F 0 was prescribed to be 210 Hz for women and 120 Hz for men, and SPL was 65 dB.

Data analysis Lung volume Each subject's rib cage and abdominal kinematic signals were calibrated for volume displacement by plotting the signals in an x-y display (x = rib cage, y = abdomen) and adjusting the line generated during the isovolume maneuver to a slope of -1.21 This was done by calculating the slope of the isovolume lines using data-analysis software (CODAS, WINDAQ), confirming that this slope was constant from several isovolume maneuvers performed throughout the session, and multiplying the abdominal signal by this slope. Thus, equivalent volume displacements by the rib cage and abdomen were represented numerically (in volts). The rib cage and adjusted abdominal signals were then summed and the resulting values were placed into a new channel for lung volume. Kinematically derived lung volume (in volts) was calibrated for spirometrically derived lung volume (in liters) by matching the maximum VC maneuver to the corresponding kinematic data. Reducing the lung volume data for the experimental tasks involved marking the maximum and minimum

RESPIRATORY AND LARYNGEAL CONTRIBUTIONS TO MPD lung volume levels during each trial. Lung volume excursion (LVE) was the difference between these values. The maximum LVE of the 3 trials for each sustained vowel was used for data analysis of the MPD task. Lung volume levels during the syllable repetition task were determined midvowel for each syllable.

Laryngeal airway resistance Peak oral pressure (Po) during productions o f / p / and airway opening flow (Fao) during the intervening v o w e l s / a / o r / i / w e r e measured from the computer files that had been calibrated before each session. Po was measured if: (1) Fao was < 20 ml/s during the closed portion of the/p/consonant, indicating that no or negligible air was leaking through the mouth or nose; and (2) the pressure peak was acceptable in its morphology (not especially sharp or shallow, not double-peaked, etc). When two adjacent pressure peaks met these measurement criteria, their values were noted and averaged to provide an interpolation of Po during the vowel. In addition, 240 ms taken from the middle of the vowel between the pressure peaks was selected. The data points from this portion of the vowel were averaged for the pressure and flow signals to derive pharyngeal pressure (Pph) and Fao, respectively. The difference between the interpolated Po and the Pph for the same vowel was the estimate of translaryngeal pressure (Ptl). Ptl was divided by average Fao to derive Rlaw for each syllable in the trial. 2° To assess variations in Rlaw over time during the modified MPD task, Rlaw values per syllable were plotted against lung volume (from maximum to minimum levels) for each trial. Trials were included in this analysis if at least 50% of the syllables were measurable for Rlaw. Nineteen trials were excluded for this reason.* Each remaining trial (n = 53) was plotted and trimmed for unreliable or questionable data. All of the initial syllables in each trial were eliminated based on Rlaw measurement criteria. In addition, the Respibands occasionally failed to track the chest-wall motions at extremely low lung volumes, 25 evidenced by lung volume signals that appeared unchanged or slightly increased despite continued phonation. As a conservative estimate, all data

335

< 5%VC were considered suspicious and were eliminated. This procedure eliminated 5.8% of the available data from the 53 trials. An additional 0.4% of LV data were suspicious but were not captured by the < 5% VC cutoff; these were eliminated as well. The trimmed data plots were analyzed by fitting a regression line to each plot of Rlaw against lung volume. If the correlation coefficient r > 0.50 and the fit of the regression equation was significant at P < 0.05, then the linear model was accepted and the slopes of the regression lines were analyzed further. Slopes < -0.15 were categorized as decreasing and slopes > 0.15 were increasing. Slopes between these values were described as relatively steady. The boundaries of the categories were based on visual inspection of the data and were somewhat arbitrary.

Maximum phonation duration The durations of the maximally sustained phonations were measured using acoustic analysis software (CSpeech 4.0). The longest trial out of three attempts was used for analysis of MPD. Statistical analysis of each dependent variable (MPD, LVE, and Rlaw) involved a repeated-measures analysis of variance (ANOVA) with sex as a between-subjects factor and vowel as a within-subjects factor. The associations between MPD and VC and between MPD and Rlaw were analyzed with Spearman rank order correlation coefficients. RESULTS Table 2 lists lung-volume excursion as a percentage of each subject's vital capacity during the longest of three maximally sustained/o/and/i/vowels. On average, subjects used 89.9% of the VC during this standard MPD task; values ranged from 75 to 97%. No statistically significant differences for LVE were detected across sex, F(1,10) = 0.193, P -- 0.670, or vowel, F(1,10) = 0.022, P --- 0.884. MPD averaged 22 s and ranged from 17 to 33 s. No statistically significant differences for MPD were detected across sex, F(1,10~ = 3.104, P = 0.109, or vowel, F(l,10) = 4.572, P = 0.058. Rlaw, estimated from the syllable repeti-

*Individual syllable tokens often did not meet measurement criteria especially toward the beginning of trials. Some subjects had more difficulty than others producing qualifying data for the modified MPD task. Specifically, F5 was eliminated entirely from analysis of Rlaw because of missing data, M5 only produced 2 trials with _>50% of measurable Rlaw data, and F2, F4, and M4 produced 3 usable trials. All other subjects (n = 7) provided usable R1,w data for at least 50% of the syllables in all 6 trials.

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NANCY PEARL SOLOMON ET AL T A B L E 2. Performance on the Maximum Phonation Tasks* LVE (% VC)

M P D (s)

R1. w (emH20/L/s)

Subject

/cd

/i/

/a/

/i/

/a/

/i/

F1

86.4

94.9

30.8

32.9

35.9

41.6

F2

95.2

96.8

17.6

18.2

33.3

24.4

F3

92.5

75.0

19.7

29.4

43.0

60.1

F4

95.1

91.8

24.2

24.7

73.1

57.2

F5

94.1

93.2

21.2

22.9

--

--

F6

87.2

83.2

22.6

22.6

23.5

26.4

Mean

91.8

89.2

22.7

25.1

41.8

42.0

SD

3.9

8.4

4.6

5.3

18.9

16.7

M1

86.4

91.7

21.3

25.3

48.4

37.3

M2

97.1

95.5

17.9

16.8

23.1

21.5

M3

89.8

93.6

18.7

17.5

25.7

20.3

M4

86.0

81.7

18.2

17.7

29.4

27.5

M5

94.0

87.8

17.0

22.1

22.9

23.5

M6

76.3

90.8

22.2

25.4

31.5

36.7

Mean

88.3

90.2

19.2

20.8

30.2

27.8

7.3

4.9

2.1

4.0

9.6

7.5

SD

*Note: Lung volume excursion (LVE) expressed as a percentage of the vital capacity (%VC) and maximum phonation duration (MPD, in seconds) were determined from the best performance on maximally sustained vowels. Laryngeal airway resistance (Raaw, in cmHzO/L/s) was determined as the average across all slow syllable-repetitiontrials for each vowel. SD = standard deviation.

tion task, a v e r a g e d 41.9 c m H 2 0 , / L / s for w o m e n a n d 29.0 c m H 2 0 ] L / s for m e n a c r o s s trials (Table 2). B e c a u s e o f substantial variability, e s p e c i a l l y a c r o s s w o m e n , this d i f f e r e n c e d i d n o t r e a c h statistical significance, F(1,9 ) = 2.842, P = 0.126. Rla w d i d n o t differ s i g n i f i c a n t l y a c r o s s v o w e l , FO,9) = 0.142, P = 0.715. N o s t a t i s t i c a l l y s i g n i f i c a n t s e x - b y - v o w e l intera c t i o n s w e r e f o u n d for a n y o f t h e s e variables. No significant correlations were found between V C a n d M P D for g e n d e r o r v o w e l (Table 3). C o r r e lations b e t w e e n Rla w a n d M P D w e r e n o t s i g n i f i c a n t for the w o m e n , b u t a s t a t i s t i c a l l y s i g n i f i c a n t c o r r e l a tion w a s i d e n t i f i e d for the m e n f o r / a / a n d a s i m i l a r t r e n d w a s f o u n d f o r / i / ( s e e Table 3). T h e l i n e a r r e g r e s s i o n m o d e l r e v e a l e d an i n c r e a s i n g pattern o f Rla w v a l u e s a c r o s s l u n g v o l u m e for 17 tri-

Journal of Voice, Vol. 14, No. 3, 2000

T A B L E 3. Spearman Rank Order Correlation

Coefficients (rs) and Significance Values (P) for Vital Capacity (VC) by Maximum Phonation Duration (MPD) and Laryngeal Airway Resistance (Rlaw) by MPD VC × MPD

Rlaw × MPD

rs

P

rs

P

/a/

0.086

0.919

0.300

0.683

/i/

0.086

0.919

0.700

0.233

/a/

-0.029

0.999

0.886

0.033

/i/

-0.771

0.103

0.829

0.058

Women

Men

RESPIRATORY AND LARYNGEAL CONTRIBUTIONS TO MPD

337

of the VC used was not a limitation for MPD task performance in the laboratory. These data for LVE compare well with the range reported by Isshiki et al, 7 but the values exceed those reported by Yanagihara et al 5 and Yanagihara and Koike. 6 One difference in measurement technique might explain a portion of this discrepancy. Previous lung-volume measures were made at the beginning and end of phonation, and for that reason are called "phonation volume." In the present study, we measured lung volume excursion as the difference between the maximum and minimum lung volume levels. These were not exactly the same as the phonation volume, which could represent a smaller excursion because of air loss at the beginning and end of trials. Our measurement procedure was selected to examine whether subjects attempted to use all or most of their VC when performing the MPD task, not to reflect the efficiency of using the lung volume. In addition, measurement error may have inflated some LVE values. We found an estimated error of - 5% at the low

als, decreasing for 3, and relatively steady for 12. Rlaw values were too variable in the remaining 21 trials to fit the model at r > 0.5. One example of each pattern is illustrated in Figure 1. The slopes for all trials that fit the model and the associated subject identifiers are plotted in Figure 2. Two women and 2 men each demonstrated the increasing Rlaw pattern for at least 3 trials (F2, F3, M4, M6). No other participant demonstrated the increasing pattern on more than one trial, nor did any participant demonstrate the decreasing pattern more than once (see Figure 2). DISCUSSION Twelve experimentally naive young adults who had normal speech and respiratory function used a large proportion of their vital capacity (75-97% VC) to perform a maximum phonatio n duration task. Their performance on the task for duration was within normal limits. 2 Thus, in response to the first research question, we can conclude that the proportion

70 M1 A

6O

0

=£ E 0 0 e,,,,

50

40.

.~_ 0

~,

30.

< 0

M4

20.

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10.

0 100

90

80

70

60

50

40

30

20

10

Lung Volume (%VO)

F I G . l . Laryngeal airway resistance plotted against lung volume, trom maximum to m i n i m u m levels, for three modified M P D (slow syllable-repetition) trials. Regression lines for each trial are dashed. Based on the slope of the line, the trial for M 4 (repeated/po/) was categorized as increasing (slope = 0.332, r = 0.939, P < 0.001), M1 (/pi/) as decreasing (slope = -0.253, r = 0.616, P = 0.005), and F6 (/pi/) as relatively steady (slope = 0.052, r = 0.773, P < 0.001). Journal of Voice, Vol. 14, No. 3, 2000

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NANCY PEARL SOLOMON ET AL

0.50.

~a 0.45,

0.30 •

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0.15 -

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-0.30

-0.45 FIG. 2. Slopes of the regression lines for each modified MPD (slow syllable-repetition) trial that fit the model (r > 0.5 and P < 0.05) for Riaw data plotted over lung volume as in Figure 1. Subject identifiers are listed for each data point. Note that three or four trials each for M4, M6, F2, and F3 had slopes > 0.15 or a linearly increasing pattern of Rlaw during the task.

extreme of the vital capacity because of instrumentation 25 (see Methods). Thus, we caution the reader to interpret < 5% differences in LVE data as unreliable. A unique contribution of this research to the MPD literature was an examination of resistance to airflow by the laryngeal airway. To answer the second research question, we used a modified MPD task so that Rlaw could be estimated. Rlaw averaged 35 cmH20/ L/s across trials. This value is similar to average values documented previously for normal young adults producing this syllable repetition task but only in the midrange of the vital capacity. 2°,26 Although not statistically significant, Rlaw for women tended to be greater and more variable than Rlaw for men, observations that are consistent with previous research. 27 Not surprisingly, Rlaw did not differ across vowels. The vowel/i/was included in the study based on the reasoning offered by Smitheran and Hixon 20 that complete velopharyngeal closure was more likely with the high vowel. 28 However, aerodynamic data that did not meet the measurement criteria were as likely to be f r o m / p a / a s from/pi/repetitions (26% and 29%, respectively). The substantial amount of unanalyzable data reveals that the modified MPD task was difficult for these normal speakers.

Journal of Voice, Vol. 14, No. 3, 2000

To examine the associations between MPD, VC, and Rlaw, correlational analysis was conducted. Based on this analysis, the answer to the first interpretive research question is an emphatic n o - - M P D was not at all systematically associated with vital capacity for these subjects. In a study with a much larger sample size 22 (N = 103), phonation volume was weakly correlated with MPD (r = 0.41 for/ol). Similarly, Schmidt et a124 reported significant correlations (r not reported) between VC and MPD for 28 women singers and nonsingers, but o n l y a similar tendency for 22 men. The lack of association between VC and MPD in the present study could be due to limitations in subject sampling and statistical power. For our purposes, however, this was not problematic. In fact, these data lent themselves perfectly to the investigation of alternate predictors of MPD. The second interpretive research question was posed because Rlaw was expected to partially explain the MPD results. The correlations of 0.886 and 0.829 between MPD and Rlaw for the men indicate that 78% and 69% of the variance in the MPD data for/a/ and/i/productions, respectively, could be predicted by Rlaw. However, Rlaw data did not correlate significantly with MPD for the women. These results must be interpreted with caution because the variables were

RESPIRATORY AND LARYNGEAL CONTRIBUTIONS TO MPD

determined from different tasks. The modified MPD task was designed to be as comparable to the standard MPD task as possible, but perhaps another noninvasive method such as an externally driven flow interrupter would have allowed comparison of MPD and Rla w during the same trial. The technique for determining Rlaw designed by Smitheran and Hixon 20 essentially uses the lips as an internally driven flow interrupter, but other aspects of the productions (eg, aspiration for/p/) confounded durational data from these trials. Although no previous studies have calculated Rlaw during an MPD task, studies assessing airflow5-12 and vocal breathiness 19 are useful for inferring laryngeal valving. As discussed above, airflow has been determined to be a useful predictor of MPD. 5-12 The expected inverse relationship between breathiness and MPD also has been demonstrated. 19 However, the associations reported have not been particularly strong. The sex difference found when correlating MPD and Rlaw was not expected and is not easily explained. Contrary to the present results, Ptacek and Sander 19 reported that the association between MPD and breathiness, which supposedly is directly related to Rlaw, was greater for females than males. The variation in Rla w o v e r trials also failed to explain this sex difference. Of the 4 subjects who demonstrated a preferred laryngeal-valving strategy of increasing Rlaw, two were women and two were men. The final interpretive research question addressed variations in Rlaw over time to reveal laryngeal-valving strategies used during an MPD task. This question was addressed by calculating the slope of the regression line for each trial. Approximately one third of the trials (17 of 53) were produced with a pattern of R~aw increasing linearly as lung volume decreased. No other systematic patterns were noted with regularity. Because SPL was controlled in the present study, variations in Rlaw throughout trials often were the result of changes in air flow rather than air pressure. Alternately, Iwata et a112 reported that persons with normal voices maintained relatively steady airflow throughout the MPD task produced with comfortable pitch and loudness. However, SPL was not controlled and subglottal pressure was not measured in their study. There is a large change from positive to negative passive alveolar pressure when progressing from

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high to low lung volumes. Therefore, it is reasonable to expect adjustments in flow and/or pressure at the extremes of the VC. During typical speech, we rarely impinge upon the highest and lowest levels of lung volume. Thus, strategies used during this modified MPD task are not likely to represent strategies used during typical speech. The strategies used during the typical MPD task may differ as well. Terasawa et a111 and Prathanee et a122 compared the tasks of taking a "comfortable" breath to taking the "deepest" breath. Results from a variety of phonatory function measures indicated that the comfortable breath was preferred. Terasawa et al 11 found that airflow was higher, and Prathanee et a122 found stronger correlations between phonatory volume and mean flow rate for the comfortable-breath task than for the MPD task. The present data reveal some laryngeal-valving strategies during the modified MPD task that are not considered typical of normal speech. In addition, although patterns of airflow over time during the MPD task may be predictable in normal speakers, they have been shown to vary widely in persons with voice disorders. 12 Previous research also has demonstrated that lung volume level influences various aspects of phonatory function, including selfselected mean Fo,29 variability of F0,29 SPL, 29 vertical laryngeal position, 3° and voice onset time. 31 These studies, along with the others discussed above,3, 4,18 should prompt clinicians to seriously reconsider including the MPD task in their assessment protocols. Instead, a sustained phonation of approximately 5 seconds in the midrange of the vital capacity should provide the data necessary to assess phonation for speech purposes and the adequacy of respiratory function for phonation. In conclusion, maximum phonation duration was not correlated with VC for the 12 normal persons studied despite verification that they used most of their VC to perform the task. A modified MPD task, consisting of slow legato repetitions o f / p i / o r / p a / throughout the VC, was used to estimate laryngeal airway resistance. The task mimicked the MPD task except for the brief interruptions in phonation by/p/ that allowed noninvasive assessment of alveolar pressure. Presuming that the tasks are comparable, the Rlaw data were correlated with results from the standard MPD. Rla w appeared to account for a substantial amount of the variance in MPD for the men but not Journal of Voice, Vol. 14, No. 3, 2000

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the women. Furthermore, approximately one third of the trials were produced with increasing laryngeal airway resistance from high to low lung volume levels. This laryngeal-valving strategy and lung volume excursion are not typical of speech. Given these results, we seriously question the clinical utility of this maximum performance task for assessing speech breathing and vocal function.

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