Introduction. Pulmonary impairment may be observed in patients with severe idiopathic scoliosis (IS). The
aim of the study was to analyze spirometry parameters and identify the pulmonary status of patients prior
to scoliosis surgery.
Materials and methods. Ninety-two adolescent girls with IS, aged 10-18 underwent a spirometry
examination and radiographic evaluation. The body height loss was calculated from Stokes’ formula. The
values of the pulmonary parameters were interpreted against the measured versus the corrected body
Results. The mean thoracic Cobb angle was 68.3°, range 50°-89°. A restrictive pattern was observed in over
40% of patients. The use of reference values calculated for corrected body height decreased the number
of patients presenting pulmonary parameters within the normal range (48.9% versus 56.5%) and increased
the number of patients with a possible restrictive pattern (42.4% versus 32.6%). Airway obstruction
was observed in 10.9% of patients when measured body height was used versus 8.7% of patients while
corrected body height was used.
Conclusions. More than half of patients with severe thoracic idiopathic scoliosis present pulmonary
impairment, mainly of a restrictive pattern. The calculations of reference values from the corrected body
height as a substitute for the measured one may influence the interpretation of the spirometry examination
Wstęp. Zaburzenia w funkcjonowaniu układu oddechowego można zaobserwować u pacjentów z ciężką
skoliozą idiopatyczną (SI). Celem pracy była analiza parametrów spirometrycznych oraz ocena stanu układu
oddechowego nastolatek przed leczeniem operacyjnym SI.
Materiał i metody. Dziewięćdziesiąt dwie nastoletnie dziewczęta z SI w wieku 10-18 lat poddano badaniu
spirometrycznemu i ocenie radiologicznej. Utratę wzrostu obliczono na podstawie wzoru Stokesa.
Wartości parametrów spirometrycznych obliczono na podstawie wzrostu rzeczywistego oraz wzrostu
Wyniki. Średnia kąta Cobba w odcinku piersiowym wynosiła 68,3°, zakres 50°-89°. U ponad 40% pacjentów
wyniki sugerowały obecność zmian o charakterze restrykcyjnym. Zastosowanie wartości referencyjnych
obliczonych dla wzrostu skorygowanego zmniejszyło liczbę pacjentów z prawidłowymi wartościami
parametrów spirometrycznymi (48,9% versus 56,5%), jednocześnie zwiększając liczbę pacjentów z
wynikami wskazujących na obecność zmian restrykcyjnych (42,4% versus 32,6%). Na podstawie parametrów
spirometrycznych obliczonych dla wzrostu rzeczywistego, obturację dróg oddechowych zaobserwowano
u 10,9% pacjentów, a u 8,7% pacjentów, gdy zastosowano wzrost skorygowany.
Wnioski. Ponad połowa pacjentów z ciężką skoliozą idiopatyczną w odcinku piersiowym kręgosłupa
wykazywała zaburzenia w funkcjonowaniu układu oddechowego, głównie o charakterze restrykcyjnym.
Obliczenie wartości referencyjnych na podstawie wzrostu skorygowanego, jako substytutu wzrostu
rzeczywistego, może mieć wpływ na interpretację wyników badania spirometrycznego.
Spirometry examination is a commonly used test to measure the volumes and flows as indicators of the respiratory function and is used to diagnose lung diseases and monitor lung status . Assessing the pulmonary status of patients with idiopathic scoliosis is a part of the preparation for surgical correction of spine deformity.
Lung restriction is a common respiratory pattern observed in adolescents with idiopathic scoliosis. The origin of decreased lung volumes in idiopathic scoliosis is multifactorial. Several parameters have been identified as related to respiratory impairment, including: the Cobb angle [2-6], the loss of thoracic kyphosis , the number of vertebrae involved in the curve [2,4,7], spinal rotation, the location of the curve  and chest mobility .
Airway obstruction is rarely observed in patients with idiopathic scoliosis . However, McPhail et al.  concluded that the prevalence of obstructive disease was 39% of patients scheduled for surgical spine correction of thoracic scoliosis. Mechanical airway obstruction reduced expiratory flows and increased airway resistance may occur as a consequence of thorax deformity. It is supposed that chest rotation can produce displacement or rotation of the intrathoracic and/or mainstem bronchi or compression of a mainstem bronchus against vertebra and mediastinal structures [12,13]. A reversibility test can help to determine whether the airway obstruction is caused by an undiagnosed lung disorder.
The aim of the study was to analyse the preoperative results of a spirometry examination in adolescent girls with severe idiopathic scoliosis.
Materials and methods
Ninety-two consecutive adolescent girls, aged 14.0 ±1.9 years, range 10-18, who were admitted for surgical treatment of idiopathic scoliosis, were enrolled. All patients met the inclusion criteria: no congenital, neuromuscular or syndromic spine deformity, no previous spine surgery and no pulmonary disorders diagnosed.
Thoracic and lumbar/thoracolumbar Cobb angles were measured on anteroposterior standing radiographs . The Lenke curve type was applied . The Risser sign was used to assess the patients’ bone maturity .
Corrected body height calculation
The corrected body height was calculated as a sum of the measured body height and the loss of body height due to spine deformity. Body height was measured in a standing position on a stadiometer by the first author. The measurement was performed prior to the spirometry examination in the morning hours.
Stokes’ formula was used to calculate the loss of body height. In the case of a single curve (Lenke 1 or Lenke 5), the following formula was used: 1.55 – 0.0471Cobb + 0.009Cobb2, while for the double curves (Lenke 2 or Lenke 3), the following formula was applied: 1.0 + 0.066Cobb + 0.0084Cobb2 .
Calculation of the predicted pulmonary
The predicted values of the FVC, FEV1 and FEV1/FVC, the lower limit of normal (LLN), the upper limit of normal (ULN), z-scores, and percentages of the predicted values of pulmonary parameters were calculated according to the Global Lung Function Initiative (GLI 2012) reference values . The predicted values of pulmonary parameters were calculated independently for the measured and corrected body height.
A spirometry examination was performed in a sitting position using the LungTest LT 250 spirometer (MES, Kraków, Poland). All examinations were done by the first author who is certified for spirometry examination. Prior to pulmonary testing, the technique of the spirometry examination was explained to the patient. A nose clip was used to prevent partial air exhalation through the nose . To prevent air from escaping, the patient was asked to tighten the seal around the mouthpiece and place the tongue below the mouthpiece to avoid blocking airflow. All patients performed a minimum of 3 maneuvers and up to a maximum of 8 maneuvers of maximal inhalation followed by maximal exhalation until plateau was reached. Enthusiastic coaching was provided to encourage the patient to make a maximum effort. Phrases such as “blowing 100 candles on a cake” were helpful to forcefully exhale until plateau was reached [19, 20]. If needed, after rest time, the patient repeated the examination until acceptable repeatability was achieved, defined as follows: the difference between the largest and the next largest FVC is ≤0.150 L, and the difference between the largest and next largest FEV1 is ≤0.150 L . The best single value of FVC and FEV1 was used for analysis [4, 19, 21-26]. In the case spirometry results indicated an obstruction pattern (FEV1/FVC < LLN), a reversibility test of airway obstruction was performed .
Intraclass Correlation Coeficient (ICC)
To determine repeatability (intraobserver agreement), 32 subjects underwent spirometry examinations twice within a one to three-day interval. The absolute value of FEV1 and FVC were analyzed. The Intraclass Correlation Coefficient (ICC), The Confidence Interval (C.I.) and the Standard Error of Measurement (SEM) were calculated.
The Shapiro-Wilk test was used to analyze data distribution. Mean, standard deviation and range were calculated. The significance of differences between pulmonary parameters: predicted values, LLN, ULN, the z-score, and the percentage of predicted FVC, FEV1, and FEV1/FVC, was determined by Student t-test. The Spearman correlation coefficient was used to calculate the correlation between the parameters: Cobb angle versus absolute values, pulmonary parameters, and loss of body height versus pulmonary parameters. Spirometry examination repeatability was determined for FVC and FEV1 using the Intraclass correlation (ICC), the Confidence Interval (CI), the Standard Error of Measurement (SEM). Analysis was performed with Statistica Software (TIBCO Software Inc., Palo Alto, the USA) and IBM SPSS Statistics 18 (Chicago, Illinois, the USA). The significance level was set at p = 0.05.
The thoracic and lumbar/thoracolumbar Cobb angle values are presented in Table 1. The median of the Risser sign was 3.
The Lenke 1 group consisted of 40 patients, the Lenke 2 group of 10 patients, the Lenke 3 group of 31 patients, and the Lenke 5 group of 11 patients.
Corrected body height calculation
The mean body height loss was 5.4 cm ±2.9 (2.0-13.2). The corrected body height was significantly higher than the measured body height (166.3 cm vs. 160.5 cm, p < 0.01).
Predicted values of the pulmonary parameters
The predicted values, upper limit of normal (ULN) and lower limit of normal (LLN) for FVC and FEV1 were significantly higher when calculated based on the corrected body height versus the measured body height. The FEV1/FVC values, on the other hand, were significantly lower when calculated according to the corrected body height, Table 2.
Pulmonary parameters obtained
during the spirometry examination
The absolute values of FVC, FEV1, and FEV1\FVC, as well as z-score values and the percentage of predicted values of pulmonary parameters are presented in Table 3.
When measured body height was used, z-score values of the FVC and FEV1 were within normal range (-1.64 ÷ 1.64), however when corrected body height was used,
z-score values decreased and revealed to be below normal, the expected range for both parameters (FVC -1.76 vs. -1.25; FEV1 -1.86 vs. -1.41). Also, the percentages of the predicted value of the FVC and FEV1 decreased when the corrected body height was used instead of the measured one (FVC 79.54% vs. 85.47%; FEV1 78.35% vs. 83.73%). In contrast, the FEV1\FVC z-score value revealed within the expected range, for both the measured and corrected body height. An increase of the parameter’s value was observed when corrected body height was used (-0.35 vs. -0.40). When percentages of the predicted value of FEV1\FVC were obtained for the corrected body height, values were higher comparing to the ones obtained based on the measured body height (96.87% vs. 96.52%), as presented in Table 3.
When the measured body height was used, in 56.5% of patients, pulmonary parameters revealed to be within the normal range; in 32.6% of patients, values of the pulmonary parameters suggested restriction, and in 10.9%, results pointed to the obstruction pattern. However, when pulmonary reference values were recalculated using the corrected body height, the number of patients presenting pulmonary parameter values within the normal range decreased to 48.9%, the number of patients with a possible restriction pattern increased to 42.4%. The number of patients presenting an airway obstruction pattern decreased to 8.7% when values were calculated based on the corrected body height, Fig. 1.
Also, in patients with a possible restriction pattern, using the corrected body height instead of the measured body height impacts severity classification. When the pulmonary parameters were calculated according to the corrected body height, the mild restrictive pattern decreased from 53.3% to 41.0%, while the moderate increased from 20.0% to 25.6%, and severely moderate restriction increased from 20.0% to 28.2%, as shown in Fig. 2.
Out of 92, 10 patients revealed an airway obstruction pattern when calculated for the measured body height:
5 patients presented mild, 2 patients moderate and 3 patients moderately severe airway obstruction. When using the corrected body height, 8 patients presented an airway obstruction pattern: 2 patients mild, 2 patients moderate, 3 patients moderately severe, and 1 patient severe airway obstruction. An example of a patient is presented in Appendix 1.
A significant negative correlation between the absolute FVC and the FEV1 values versus the thoracic Cobb angle (r = -0.2673, p = 0.016; r = -0.3087, p = 0.005,
respectively) was observed. No such correlation was revealed for the lumbar spine (FVC r = -0.3560, p = 0.490; FEV1 r = -0.3701, p = 0.400). Also, with increasing body height loss, the absolute values of both pulmonary parameters (FVC and FEV1) decreased, as presented in Figures 3 and 4.
The Intraclass Correlation Coefficient
The repeatability of pulmonary parameter measurements FEV1 and FVC was excellent. Values of ICC, CI and SEM are presented in Table 4.
A spirometry examination performed before the surgical treatment of idiopathic scoliosis can detect pulmonary impairment even if it is not clinically evident . Body height is one of the variables considered when calculating the predicted values, LLN and ULN of pulmonary parameters according to GLI 2012 reference values . In patients with severe idiopathic scoliosis, the measured body height is decreased as a consequence of spine deformation. The loss of body height was calculated based on the Stokes formula, which analysed Cobb angles, the spinal length, and the spinal height on 387 standard standing radiographs of patients with juvenile or adolescent scoliosis (182 single curves, 205 double curves) aged 9-20 years old. For patients with a single curve and a double curve, separate formulas were proposed to calculate body height loss . Tyrakowski et al.  concluded that none of the four compared methods- Bjure, Kono, Ylikowski, Stokes- could be recommended as most valid. However, according to Gardner et al. , who compared 5 methods- Bjure, Kono, Ylikowski, Stokes, and Hwang, Stokes’s formula is one of the two most valid methods of calculating the loss of height. A significant difference was found between the measured versus the corrected body height (p < 0.01), the results are in line with our earlier findings [23-25]. Also, the use of corrected body height instead of the measured one may affect the interpretation of spirometry results, as well as change the classification of pulmonary impairment severity [23,25]. Both the z-score value and the percentage of the predicted FEV1 and FVC value revealed significantly lower results when calculated based on the corrected body height comparing to the meaured body height
(p < 0.01). Furthermore, in our study, in 30 out of 92 patients (32.6%), spirometry results suggested a restrictive pattern when the measured body height was used, while this number increased to 39 patients (42.4%), if the corrected body height was taken for reference value calculations. Although a spirometry examination alone is not the gold standard to diagnose restriction that is often observed in patients with idiopathic scoliosis, our study helped to assess the pulmonary status of patients and determine if further pulmonary investigation is needed, or identify the patients who may require more intensive rehabilitation before or after surgery.
On the other hand, spirometry is recommended as
a tool for assessing the pattern of airway obstruction . Out of 92 patients, 10 patients (10.9%) were diagnosed with airway obstruction when the measured body height was used versus 8 patients (8.7%) when the corrected body height was used. Farell and Garridos’  preoperatively analysed the morphology of large airways in patients with right thoracic idiopathic scoliosis and demonstrated that right-sided airways become narrowed as a result of loss of thoracic kyphosis. Furthermore, the authors concluded that FEV1/FVC correlates negatively with airway narrowing, suggesting that obstruction contributes to lung function impairment in patients with thoracic scoliosis and hypokyphosis. Borowitz et al.  reported the flattening of the initial portion of the expiratory loop, suggesting the obstruction of a large airway, which improved after the surgical correction of scoliosis. According to Boyer et al.  lower airway obstruction may develop as spine deformity increases and is often reversible with a bronchodilator. The airway reversibility test may help to assess whether aiway obstruction may be a consequence of spinal deformity. It may also help to assess if a patient needs further investigation of the cause of the airway obstruction. However, in the presented case of a 16-year-old girl, the test results were negative suggesting that airway obstruction might be secondary to spine deformity .
Weinstein et al.  suggested that pulmonary impairment may occur in patients with idiopathic scoliosis presenting a thoracic Cobb angle of more than 100-120°. However, studies showed that pulmonary impairment may be observed in patients with smaller curves. Johnston et al.  concluded that in untreated adult patients with idiopathic scoliosis, a thoracic curve magnitude greater than 40° correlated negatively with the FVC value (p < 0.008). Also, our study revealed a negative correlation between thoracic curve magnitude and pulmonary parameters (FVC, FEV1). These observations are consistent with other authors [4-7]. Moreover, our study showed a negative correlation between the loss of body height versus the FVC or FEV1 absolute value (r = -0.2441, p = 0.019; r = -0.2489,
p = 0.017, respectively).
As previous studies showed, preschool children can perform reproducible and technically acceptable spirometry maneuvers [34-36]. However, prior to the spirometry examination, it is of critical importance to explain the procedure using age appriopriate language and describe the technique step by step so that the patient understands and cooperates . However, no difference was described in FEV1 and FVC values in children with and without a nose clip . Spirometry can be conducted in both a sitting and a standing position. Due to safety reasons and to avoid falling down as a syncope result, we decided to use a straightened-up sitting position, both feet on the ground . Patients who perform spirometry examinations may need several attempts to learn the proper technique and achieve acceptable repeatability. Collaboration and commitment of both the patient and the technician are needed to achieve credible spirometry results.
Pulmonary impairment can occur in adolescent patients with severe thoracic idiopathic scoliosis. In patients with pulmonary impairment, more cases present a restrictive pattern, however, airway obstruction is also observed. The correction for body height loss may affect the predicted values of pulmonary parameters and the interpretation of spirometry examination results.
Due to obstruction pattern, the patient performed airway obstruction reversibility test. The FEV1 and FVC improvement after bronchodilator did not reach the threshold of 200 ml or 12% change. Furthermore, the FEV1/FVC value remains abnormal suggesting that the bronchodilatation test is negative with irreversible airway obstruction .
1. Graham BL, Steenbruggen I, Miller MR, Barjaktarevic IZ, Cooper BG, Hall GL, et al.: Standardization of Spirometry 2019 Update. An Official American Thoracic Society and European Respiratory Society Technical Statement. Am J Respir Crit Care Med. 2019 Oct 15;200(8):70–88.
2. Kearon C, Viviani GR, Kirkley A, Killian KJ: Factors Determining Pulmonary Function in Adolescent Idiopathic Thoracic Scoliosis. Am Rev Respir Dis. 1993 Aug;148(2):288–94.
3. Jackson RP, Simons EH, Stripinis D: Coronal and Sagittal Plane Spinal Deformities Correlating with Back Pain and Pulmonary Function in Adult Idiopathic Scoliosis. Spine. 1989 Dec;14(12):1391–7.
4. Newton PO, Faro FD, Gollogly S, Betz RR, Lenke LG, Lowe TG: Results of preoperative pulmonary function testing of adolescents with idiopathic scoliosis. A study of six hundred and thirty-one patients. J Bone Joint Surg Am. 2005 Sep;87(9):1937–46.
5. Upadhyay SS, Mullaji AB, Luk KD, Leong JC: Relation of spinal and thoracic cage deformities and their flexibilities with altered pulmonary functions in adolescent idiopathic scoliosis. Spine (Phila Pa 1976). 1995 Nov 15;20(22):2415–20.
6. Johnston CE, Richards BS, Sucato DJ, Bridwell KH, Lenke LG, Erickson M, et al.: Correlation of preoperative deformity magnitude and pulmonary function tests in adolescent idiopathic scoliosis. Spine (Phila Pa 1976). 2011 Jun 15;36(14):1096–102.
7. Politarczyk K, Janusz P, Stępniak Ł, Kozinoga M, Kotwicki T: Relationship between radiological parameters and preoperative pulmonary function in adolescents with idiopathic scoliosis-preliminary study. Issues of Rehabilitation, Orthopaedics, Neurophysiology and Sport Promotion – IRONS. 2019 Sep 15;
8. Campbell RM, Smith MD, Mayes TC, Mangos JA, Willey-Courand DB, Kose N, et al.: The effect of opening wedge thoracostomy on thoracic insufficiency syndrome associated with fused ribs and congenital scoliosis. J Bone Joint Surg Am. 2004 Aug;86(8):1659–74.
9. Kotani T, Minami S, Takahashi K, Isobe K, Nakata Y, Takaso M, et al.: An analysis of chest wall and diaphragm motions in patients with idiopathic scoliosis using dynamic breathing MRI. Spine (Phila Pa 1976). 2004 Feb 1;29(3):298–302.
10. Tsiligiannis T, Grivas T: Pulmonary function in children with idiopathic scoliosis. Scoliosis. 2012 Mar 23;7(1):7.
11. McPhail GL, Ehsan Z, Howells SA, Boesch RP, Fenchel MC, Szczesniak R, et al.: Obstructive lung disease in children with idiopathic scoliosis. J Pediatr. 2015 Apr;166(4):1018–21.
12. Bartlett W, Garrido E, Wallis C, Tucker SK, Noordeen H: Lordoscoliosis and large intrathoracic airway obstruction. Spine (Phila Pa 1976). 2009 Jan 1;34(1):59-65.
13. Borowitz D, Armstrong D, Cerny F: Relief of central airways obstruction following spinal release in a patient with idiopathic scoliosis. Pediatr Pulmonol. 2001 Jan;31(1):86–8.
14. Cobb, JR: Outline for the study of scoliosis. Instr. Course Lect. 1948, 5, 261–275.
15. Lenke LG, Betz RR, Harms J, Bridwell KH, Clements DH, Lowe TG, Blanke K: Adolescent idiopathic scoliosis: a new classification to determine extent of spinal arthrodesis. J Bone Joint Surg Am. 2001 Aug;83(8):1169-81.
16. Risser JC: The Iliac apophysis; an invaluable sign in the management of scoliosis. Clin Orthop. 1958; 11:111–9.
17. Stokes IAF: Stature and growth compensation for spinal curvature. Stud Health Technol Inform. 2008; 140:48–51.
18. Quanjer PH, Stanojevic S, Cole TJ, Baur X, Hall GL, Culver BH, et al.: Multi-ethnic reference values for spirometry for the 3-95-yr age range: the global lung function 2012 equations. Eur Respir J. 2012 Dec;40(6):1324–43.
19. Miller MR, Crapo R, Hankinson J, Brusasco V, Burgos F, Casaburi R, et al.: General considerations for lung function testing. Eur Respir J. 2005 Jul;26(1):153–61.
20. Jat KR. Spirometry in children. Prim Care Respir J: 2013 Jun;22(2):221–9.
21. Kim YJ, Lenke LG, Bridwell KH, Kim KL, Steger-May K: Pulmonary function in adolescent idiopathic scoliosis relative to the surgical procedure. J Bone Joint Surg Am. 2005 Jul;87(7):1534–41.
22. Kim YJ, Lenke LG, Bridwell KH, Cheh G, Whorton J, Sides B: Prospective pulmonary function comparison following posterior segmental spinal instrumentation and fusion of adolescent idiopathic scoliosis: is there a relationship between major thoracic curve correction and pulmonary function test improvement? Spine (Phila Pa 1976). 2007 Nov 15;32(24):2685–93.
23. Politarczyk K, Stepniak Ł, Kozinoga M, Czaprowski D, Kotwicki T: Loss of body height due to severe thoracic curvature does impact pulmonary testing results in adolescents with idiopathic scoliosis. Stud Health Technol Inform. 2021 Jun 28; 280:231–4.
24. Politarczyk K, Kozinoga M, Stępniak Ł, Panieński P, Kotwicki T: Spirometry Examination of Adolescents with Thoracic Idiopathic Scoliosis: Is Correction for Height Loss Useful? J Clin Med. 2021 Oct 22;10(21):4877.
25. Politarczyk K, Popowicz-Mieloch W, Kotwicki T: Pulmonary Parameters in Adolescents with Severe Thoracic Idiopathic Scoliosis: Comparison Girls versus Boys. Healthcare (Basel). 2022 Aug 19;10(8):1574.
26. Vedantam R, Lenke LG, Bridwell KH, Haas J, Linville DA: A prospective evaluation of pulmonary function in patients with adolescent idiopathic scoliosis relative to the surgical approach used for spinal arthrodesis. Spine (Phila Pa 1976). 2000 Jan;25(1):82–90.
27. Tyrakowski M, Kotwicki T, Czubak J, Siemionow K: Calculation of corrected body height in idiopathic scoliosis: comparison of four methods. Eur Spine J. 2014 Jun;23(6):1244-50.
28. Gardner A, Price A, Berryman F, Pynsent P: The use of growth standards and corrective formulae to calculate the height loss caused by idiopathic scoliosis. Scoliosis Spinal Disord. 2016; Feb 26; 11:6.
29. Quanjer PH, Brazzale DJ, Boros PW, Pretto JJ: Implications of adopting the Global Lungs Initiative 2012 all-age reference equations for spirometry. Eur Respir J. 2013 Oct;42(4):1046–54.
30. Farrell J, Garrido E: Effect of idiopathic thoracic scoliosis on the tracheobronchial tree. BMJ Open Respir Res. 2018;5(1): e000264.
31. Boyer J, Amin N, Taddonio R, Dozor AJ: Evidence of airway obstruction in children with idiopathic scoliosis. Chest. 1996 Jun;109(6):1532–5.
32. Boros PW, Martusewicz-Boros MM: Reversibility of airway obstruction vs bronchodilatation: do we speak the same language? COPD. 2012 Jun;9(3):213–5.
33. Weinstein SL, Zavala DC, Ponseti IV: Idiopathic scoliosis: long-term follow-up and prognosis in untreated patients. J Bone Joint Surg Am. 1981 Jun;63(5):702–12.
34. Crenesse D, Berlioz M, Bourrier T, Albertini M: Spirometry in children aged 3 to 5 years: reliability of forced expiratory maneuvers. Pediatr Pulmonol. 2001 Jul;32(1):56–61.
35. Eigen H, Bieler H, Grant D, Christoph K, Terrill D, Heilman DK, et al.: Spirometric pulmonary function in healthy preschool children. Am J Respir Crit Care Med. 2001 Mar;163(3 Pt 1):619–23.
36. Aurora P, Stocks J, Oliver C, Saunders C, Castle R, Chaziparasidis G, et al.: Quality control for spirometry in preschool children with and without lung disease. Am J Respir Crit Care Med. 2004 May 15;169(10):1152–1159.
37. Chavasse R, Johnson P, Francis J, Balfour-Lynn I, Rosenthal M, Bush A: To clip or not to clip? Noseclips for spirometry. European Respiratory Journal. 2003 May 1;21(5):876–8.