using nasal pressure to diagnose hypopnea

1. Using Nasal Pressureto Diagnose Hypopnea

3. The Challenge:Locate and Identify Hypopneas Early sleep research focused on apnea. Later we recognized the reduced airflow/effort pattern we call hypopnea Hypopnea may be just as physiologically disruptive as apnea Defined as a 30-50% drop in airflow Hypopneas have often been undetected – RERA’s, UARS

4. Monitoring Airflow Airflow has been traditionally monitored by using thermocouples and thermistors. Thermistors and thermocouples are still the most common technique used in labs These sensors respond to changes in temperature They are very effective in detecting apneas – where there is no flow. HOWEVER…..

5. Limitations of Thermistors Thermistors and thermocouples Have inherently slow response times from the thermal mass of the sensor beads Have a curvilinear response to temperature change that flattens as airflow increases

6. Actual Flow vs. Thermistor Thermistors have very poor correlation against measures of flow or minute ventilation Qualitative, but not quantitative

7. Searching for a Better Way Requirements: Relatively inexpensive Easy to use Tolerated by patients Sensitive to changes in airflow Specific to real reductions in airflow

8. Research to the Rescue! 1997 - Berg, et al., Comparison of therm, nasal pressure, and RIP to measured ventilation (body box). Show that NP is much better than therm though not perfect 1998 – Hosselet,et al comparing therm, NP and pneumotach during sleep Show that NP directly relates to flow and the contour of inspiratory signal is useful in detecting UARS

9. How are Airflow and Pressure Related? Bernouli Principle Flow is related to pressure, temperature and gas composition p + ½?V2 + ?gh = constant

10. The Pitot Tube One of the most immediate applications of Bernoulli's equation is in the measurement of velocity with a Pitot-tube. The Pitot tube (named after the French scientist Pitot) is one of the simplest and most useful instruments ever devised. It simply consists of a tube bent at right angles

11. The Pitot Tube By pointing the tube directly upstream into the flow and measuring the difference between the pressure sensed by the Pitot tube and the pressure of the surrounding air flow, it can give a very accurate measure of air velocity.

12. Nasal Pressure Response

13. Nasal Cannula as Pitot Tube The nasal cannula, when placed in the nares and terminated at a pressure transducer, acts similar to a Pitot tube Variations due to gas composition, temperature and placement of cannula ports in the nares affect signal But the overwhelming result is a reading directly related to the airflow measurement.

14. Making a NP Measurement Using a standard or custom nasal cannula: Attach to patient Attach other end to Pressure Transducer

15. NP vs Measured Airflow Linear relationship as flow increases, so does NP NP is not calibrated and relationship will vary from patient to patient

16. NP vs. Measured Flow NP to flow is not entirely linear It is nearly linear for the majority of the flow rates of normal breathing

17. AASM Recommendations AASM recommendations published in 1999 indicated that the thermistors and thermocouples are very poor at detecting reduced airflow (though still excellent for apnea) Pneumotach is best, but not practical Nasal pressure is somewhere in between AASM Task Force. Sleep-related breathing disorders in adults: recommendations for syndrome definition and measurement techniques in clinical research. Sleep 1999;22:667-689

18. Clinical Significance of Nasal Pressure

19. Normal Breathing Note “sinusoidal” pattern to inspiratory portion of nasal pressure trace

20. Apnea DetectionNP vs Therm NP signal tends to disappear, while thermocouple will show some fluctuations Thermocouple can respond to extremely low flow better where NP signal is lost

21. Sensitivity of NP NP flow compared with driving pressure via a glottic or esophageal catheter.

22. Hypopnea by NP vs Therm Hypopneas are more evident in the NP trace The morphology of the trace is indicative

23. Hypopnea by NP vs Therm Note the clear drop in amplitude and change in shape on the NP trace

24. Morphology of NP Signals Detection of Flow Limitation with a Nasal Cannula/Pressure Transducer System, Hosselet JJ, Norman RG, Ayappa I, Rapoport DM; AM J Respir Crit Care Med 1998; 157:1461-1467

25. Comparison of AHI NP vs. Therm.

26.  Nasal Pressure = More Events Study shows that RDI is at least 5 to 25 events per hour higher with Nasal Pressure COMPARISON OF NASAL PRESSURE AND THERMISTOR RECORDINGS IN THE DETECTION OF SLEEP-DISORDERED BREATHING EVENTS, Cunninham SL, Shea SA, White DP; SLEEP Vol. 21, Supplement, pp 62

27. Detecting Snore with NP Snore can be detected in NP signal if filtered appropriately (Low Pass = 70Hz) Usually used as second trace to keep NP flow trace clean

28. Snoring Sample

29. Problems with NP Over scoring of AHI with NP Mouth breathing - #1 concern Requires appropriate pressure transducer and amplifier settings Fluid blockage from condensation and “patient sources” Minimal issue, Remedied with replacement cannula Increased nasal resistance from presence of nasal prongs

30. Linearized Flow from NP Square root of pressure is flow Processed signal is closer to pneumotach flow Improved linear response compared to straight NP Farre, Monteserat, 2001

31. Clinical Effect from Derived Flow from NP Hosselet, et al (1998) suggested it made little clinical difference and that hypopnea events were more exaggerated in the NP trace than the derived flow trace. But since the relationship between nasal pressure and nasal flow is a square function, a 50% drop in NP reflects a 30% drop in flow; a 75% drop in NP reflects a 50% drop in flow (approximately)

32. Affect on Hypopnea Detection Farre, et al, 2001, concluded that NP can over-detect hypopneas, due to the square response and that when using NP a 75% drop in signal from baseline could be used to score hypopneas.

33. Adjusted AHI for Nasal Flow Thurnheer, et al, 2001, found that using the derived flow from the NP signal (square root of pressure) resulted in a lower AHI. They suggested using a factor of .84 to correct the overestimation of the AHI from NP.

34. Mouth Breathing An example of exaggerated mouth breathing Increased signal at mouth transition – from additional sensor at mouth Note the NP signal does not disappear

35. Mouth Breathing In most cases, mouth breathing will not eliminate flow through the nose. Only 1-2% of cases will lose nasal airflow completely (Monteserrat, et al)

36. Technical Considerations of NP Pressure transducer range - ±2 to ±10cm H20 since signal rarely exceeds 4cm H2O If using AC Amplifier, HPF (LFF) should be set to 0.05 Hz or less To see snoring, LPF (HFF) must be at 30Hz or higher (preferably closer to 100Hz) Digital sampling rate must be at least twice LPF setting. (10Hz to 256 Hz)

37. NP & ? Nasal Resistance Lorino AM, et al noted a significant increase in nasal resistance from the nasal prongs from a standard oxygen cannula Depends on size of nares Affected by shape of nose and soft tissue Also affected by nasal congestion from allergies and viral infections Care should be taken to use a cannula with small nasal prongs

38. Does NP Meet the Criteria? Easy to use – Yes Relatively inexpensive – Yes Tolerated by patients – Yes Sensitivity - Yes Specificity – Much better than thermal sensors but may over detect apneas

39. With Nasal Pressure have we Found Hypopnea? NP detects more hypopneas than thermal with consistently higher AHI’s, but it can over score if rules are not adjusted. NP provides both amplitude and morphology change with events that help detect UARS NP is easy to use (assuming transducer and recording system are setup correctly) NP has limitations but they are outweighed by the advantages.

40. Should the Rules Change? Hypopnea guidelines for flow have been developed and followed using thermal sensors. If NP is used, should the rule be changed to use a 50% drop in signal (indicating a 30% reduction in flow) to indicate hypopnea? Should the AHI be adjusted?

41. References Measurement of Human Nasal Ventilation Using an Oxygen Cannula as a Pitot Tube. Guyatt AR, Parker SP, McBride MJ; ARRD 126:434-438, 1984 Detection of Respiatory Events During NPSG: Nasal Cannula Pressure Sensor Versus Thermistor. Norman RG, Ahmed MM, Walsleben JA, Rapoport DM; Sleep 20(12):1175-1184, 1997 Comparison of Direct and Indirect Measurements of Respiratory Airflow: Implications for Hypopneas. Berg S, Haight JS, Yap V, Hoffstein V, Cole P; Sleep 20:60-64, 1997 Evaluation of Nasal Prongs for Estimating Nasal Flow. Montserrat JM, Farre R, Ballester E, Felez MA, Pasto M, Navajas D; AJCCM 155:211-215, 1997 Detection of Flow Limitation with a Nasal Cannula Pressure Transducer System. Hosselet JJ, Norman RG, Ayappa I, Rapoport DM; AJRCCM 157:1461-1467, 1998 Comparison of Nasal Pressure and Thermistor Recordings in the Detection of Sleep-Disordered Breathing Events. Cunninham SL, Shea SA, White DP; Sleep Vol. 21, Supplement, pp 62, 1999 Nasal Pressure Recording in the Diagnosis of Sleep Apnoea Hypopnea Syndrome. Series F, Marc I; Thorax 54:506-510, 1999 AASM Task Force Report. Sleep-related breathing disorders in adults: recommendations for syndrome definition and measurement techniques in clinical research. Sleep 22:667-689, 1999

42. References (cont.) Effects of Nasal Prongs on Nasal Airflow Resistance. Lorino AM, Lorino H, Dahan E, d’Ortho MP, Coste A, Harf A, Lofaso F; Chest 11892):366-371, 2000 Classification of Sleep-disordered Breathing. Hosselet JJ, Ayappa I, Norman RG, Rapoport DM; AJRCCM 163(2):398-405, 2001 Acuracy of Nasal Cannula Pressure Recordings fo Assessment of Ventilation during Sleep. Thurnheer R, Xiaobin X, Bloch K; AJRCCM Vol 164:1914-1919, 2001 Relevance of Linearizing Nasal Prongs for Assessing Hypopneas and Flow Limitation During Sleep. Farre R, Rigau J, Montserrat J, Ballester E, Navajas D; AJRCCM 163:494-497, 2001 Performance o Nasal Prongs in Sleep Studies: Spectrum of Flow-Related Events. Hernandez L, Bellester E, Farre R, Badia JR, Lobelo R, Navajas D, Montserrat JM; Chest 119:442-450, 2001 Assessment of Inspiratory Flow Limitation in Children with Sleep-disordered Breathing by a Nasal Cannula Pressure Transducer System. Sererisky D, Cordero R, Mandeli J, Kattan M, Lamm C; Pediatric Pulmonology 33:380-387, 2002