A urodynamic study (UDS) can be a valuable tool in the investigation of patients with lower urinary tract symptoms (LUTS). In this type of examination, pressures in the urinary bladder and rectum (or vagina) are measured with the help of catheter-based systems. Traditionally, conventional urodynamic studies have been performed using water filled catheters (WFC) with external pressure transducers. This technique has been used for the existing normal values regarding urodynamic findings and is still what is recommended by the International Continence Society (ICS) [1,2]. One alternative type of catheter is air filled catheters (AFC) which are being used more and more in urodynamic laboratories around the world. However, there are only a few available studies regarding comparison of WFC and AFC in urodynamic examinations and those that exist are all based on rather small number of patients. The aim of our study was thus to evaluate if there are any significant differences in pressure measurements using air filled technique compared to water filled technique during urodynamic studies.

Men and women who were referred to our unit for a conventional urodynamic study during a period of 2 years were randomly recruited. Patients were included if they accepted to participate in the study according to the study plan. The research schedule at the department allowed a few time slots per week among our clinical studies. Men who had a history of any kind of prostatic operation were excluded from dual pressure measurement of the urethral pressure profile. A total number of 68 patients were included (59 men and 9 women). Apart from using air filled catheters, the cystometry and pressure-flow studies were performed in accordance with the ICS Good Urodynamic Practices (GUP) [1,2]. We compared pressure measurements of intravesical pressure during urodynamic pressure-flow studies (Pves@Qmax) and urethral pressure during urethral pressure profile measurements (Pura) using the technique of water filled catheters and air filled catheters simultaneously with only one inserted catheter. The water-filling channel served first as the bladder filler and then as the water pressure reader. 

Out of the 68 recruited patients, 5 patients were excluded due to inadequate quality of the dual pressure recordings. For the rest of the patients either pressure flow study, urethral pressure profile or both were obtained as follows:
-	Pves@Qmax and Pura from two consecutive urethral pressures were obtained for 17 patients
-	Pves@Qmax and Pura from one urethral pressure profile were obtained for 19 patients
-	Pves@Qmax only were obtained for 14 patients
-	Pura from two consecutive urethral pressure profiles only were obtained for 5 patients
-	Pura from one urethral pressure profile only were obtained for 8 patients.

In total we obtained 50 separate dual measurements of Pves@Qmax and 71 separate dual measurements of Pura (48 from the first urethral pressure profile and 23 from a repeated urethral pressure profile).

The pressure measurements performed with the air filled technique showed overall significantly higher pressures than measurements performed with the water filled technique (table 1), both regarding intravesical pressures (difference in means 12 ± 14 cm H2O) as well as urethral pressures (difference in means 21 ± 14 cm H2O and 21 ± 10 cm H2O). In only 5 out of 50 measurements of Pves during voiding did the WFC show higher pressures than the AFC. In the urethral pressure measurements, only 2 out of 71 Pura measurements showed higher pressure readings with WFC compared to AFC. The comparison between the two consecutive measurements of Pura with the water filled and the air filled technique respectively showed no significant difference.
The pressures obtained with the air filled technique had a high positive correlation with pressures obtained with the water filled technique (Fig.1). The pressures were obtained during pressure-flow studies and during the two urethral pressure profile measurements.

Both intravesical voiding pressures as well as urethral pressures measured with the air filled technique are significantly higher compared with the water filled technique. This could be a clinical problem since air filled catheters are being used more and more in urodynamic laboratories even though the normal values are based upon the water filled technique. As our results indicate, the intravesical voiding pressure could be up to 26 cmH2O (12 ± 14 cmH2O) higher with AFC than with WFC. If this difference is transferable to the detrusor pressure it could affect the clinical interpretation of the urodynamic study as the difference for example is large enough to change a weak detrusor pressure to a normal detrusor pressure or to move a patients BOOI (Bladder Outlet Obstruction Index) from the non-obstructed to the obstructed interval.

Our results are in concordance with previous studies. There is one review article on the subject [3] concluding that more systematic research is warranted before AFC can be recommended for routine clinical use. A statement that we strongly support.
Since there is strong correlation between the pressures from the two types of catheters, it may be possible to use a correction factor to convert pressures from AFC to WFC, thereby being able to use the same normal values as with WFC. However, the clinically most relevant pressure for this comparison is the detrusor pressure (Pdet) which was not obtained during this study. Further studies are therefore needed in order to obtain the correction factor.

Urodynamic pressure measurements with air filled and water filled technique respectively differ significantly and cannot be used interchangeably. We agree with ICS that water filled catheters are the preferred catheter type until normal values for urodynamic pressures measured with other catheter types have been developed. This is especially true for patients where the exact level of pressure is important e.g. in the assessment of intravesical obstruction in men. However, the strong positive correlation between water filled and air filled catheters may be used to introduce a correction factor. The same normal values could then be used independently of catheters used and would facilitate the work at the department.

Schäfer W, Abrams P, Liao L, Mattiasson A, Pesce F, Spangberg A, Sterling AM, Zinner NR, van Kerrebroeck P; International Continence Society. Good urodynamic practices: uroflowmetry, filling cystometry, and pressure-flow studies. Neurourol Urodyn. 2002;21(3):261-74
Rosier PFWM, Schaefer W, Lose G, Goldman HB, Guralnick M, Eustice S, Dickinson T, Hashim H. International Continence Society Good Urodynamic Practices and Terms 2016: Urodynamics, uroflowmetry, cystometry, and pressure-flow study. Neurourol Urodyn. 2017 Jun;36(5):1243-1260
Abrams P, Damaser MS, Niblett P, Rosier PFWM, Toozs-Hobson P, Hosker G, Kightley R, Gammie A. Air filled, including "air-charged," catheters in urodynamic studies: does the evidence justify their use? Neurourol Urodyn. 2017 Jun;36(5):1234-1242