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Contribution of respiratory muscle blood flow to exercise-induced

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       Contribution of respiratory muscle blood flow to
         exercise-induced diaphragmatic fatigue in
                        trained cyclists


    I. Vogiatzis1,, D. Athanasopoulos1,, R. Boushel2, J. Guenette3,
      M. Koskolou1, H. Wagner4, C. Roussos1, S. Zakynthinos1,
                              PD Wagner4


1Universityof Athens, Department of Critical Care Medicine & Pulmonary Services
2Department of Exercise Science, Concordia University Montreal, Quebec, Canada

  3School of Human Kinetics, University of British Columbia, Vancouver, Canada

          4University of San Diego, Department of Medicine, California
    Diaphragmatic fatigue in healthy subjects
• High-intensity endurance exercise (≥85% VO2max) induces
  diaphragm fatigue in subjects with varying degrees of
  fitness


                                      Johnson et al., J. Physiol, 1993
                                     Vogiatzis et al., J. Physiol, 2006



•   Magnitude of diaphragmatic fatigue depends on:
- amount of diaphragmatic work
- degree of arterial hypoxaemia
- competition for blood flow between respiratory and
   locomotor muscles
                                            Babcock et al., JAP, 2002
Hypoxaemia exaggerates diaphragmatic
  fatigue at same work of breathing




                             VE (L/min)
     125   133   129   135




                        Vogiatzis et al., J. Physiol, 2007
                        Initial Expectation
              Lower leg work rate in hypoxia (at same ventilation)


                             Lower leg blood flow


               Greater respiratory muscle blood flow availability


              Lower diaphragmatic fatigue compared to normoxia



                            Prior Findings
Greater diaphragm fatigue in hypoxia attributed to uncompensated hypoxaemia

                                                 Vogiatzis et al., J. Physiol, 2007
Quantification of absolute changes in respiratory muscle blood
    flow by near-infrared spectroscopy & ICG green dye




                                           Guenette et al., JAP, 2008
                                    Aim
To investigate:
•   Whether greater diaphragmatic fatigue in hypoxia compared to normoxia
    could have a contribution from limited respiratory muscle blood flow


                                 Hypothesis
    •   If respiratory muscle blood flow in hypoxia was greater than in
        normoxia, greater diaphragm fatigue would be due to
        uncompensated arterial hypoxaemia

    •   If respiratory muscle blood flow in hypoxia was similar or lower than
        in normoxia, greater fatigue would be due to hypoxaemia & limited
        blood flow
                 Methodological Design
 • 7 healthy competitive Greek male cyclists


            WRmax (Watts) in room air          373 ± 10
            VO2max (ml.kg-1.min-1)             63.7 ± 4.2




           Exercise at different FIO2 in balanced order


90% WRmax                     80% WRmax               65% WRmax
5-min exercise               5-min exercise           5-min exercise
in 100% FIO2                   in room air             in 13% FIO2


  Producing similar ventilation, tidal volume & breathing frequency
   Assessment of Diaphragmatic Fatigue
Bilateral phrenic nerve stimulation was used to determine reductions
    in twitch transdiaphragmatic pressure (Pdi,tw) during recovery
           Measurement of Cardiac Output and
                   Leg Blood Flow
• Cardiac Output


Dye dilution using known volumes of Indocyanine
green (ICG) injected into the right femoral vein
and withdrawn from the right femoral artery


• Leg Blood Flow

Dye dilution method using known volumes of ICG
injected into the right femoral artery and
withdrawn from the right femoral vein


                                                   Dow, Physiol Rev, 1956
                                                   Boushel et al., JAP, 2000
Respiratory Muscle Blood Flow


         Determined by near-infrared spectroscopy using
         optodes placed on the skin over the left 7th
         intercostal space.


         Optodes were connected to a spectrophotometer
         measuring the ICG concentration injected into the
         right femoral vein


                                  Boushel et al., JAP, 2000;
                         Duncan et al., Phys Med Biol, 1995
                                  Guenette et al., JAP, 2008
                                                          Breathing Pattern
A                     140                                        D                  400
V E (L.min-1)



                                                                                    350




                                                                  WR (W)
                      120
                                                                                    300
                                                                                                                        †
                      100
                                                                                    250

                      80                                                            200


 B                    3.4                                        E                  6.5


                                                                                                                            •




                                                                 VO 2 (L.min-1)
                      3.0                                                           5.0
                                                                                                                                Hyperoxia

                                                                                                                            ◦
  V T (L)




                                                                                                                        †
                                                                                                                                Normoxia
                      2.6                                                           3.5
                                                                                                                            □   Hypoxia

                      2.2                                                           2.0

 C                     45                                        F                  195
                                                                 HR (beats.min-1)
  f (breaths.min-1)




                       40                                                           180


                       35                                                           165


                       30                                                           150

                            1    2      3      4      5                                   1    2      3      4      5

                                Exercise time (min)                                           Exercise time (min)
                                                 Work of Breathing
A
                                          1600




Pressure-time product
                        (cmH2O s min-1)
                                          1200




                                          800




                                          400
                                                                                       •   Hyperoxia

B
                                                                                       ◦   Normoxia

                                           500
                                                                                       □   Hypoxia

                                           400
   Work of Breathing
                          (J min-1)




                                           300



                                           200



                                           100



                                             0

                                                 1   2            3            4   5

                                                         Exercise time (min)
Diaphragmatic Fatigue: greater in hypoxia


                          110
    ΔPdi,tw (%Baseline)



                          100


                          90
                                       †
                                                               •   Hyperoxia

                                                               ◦
                          80
                                                                   Normoxia

                          70                                   □   Hypoxia


                          60


                                B     10           30     60
                                    Recovery time (min)
                                      Haemodynamics
                             35.0

                             30.0



 Cardiac Output (L/min)
                             25.0

                             20.0

                             15.0

                             10.0

                              5.0


                                    NORMOXIA   HYPOXIA   HYPEROXIA
                             20.0
Two Leg Blood Flow (L/min)




                             15.0




                             10.0




                              5.0


                                    NORMOXIA   HYPOXIA   HYPEROXIA
No effect of FIO2 on respiratory muscle blood flow




                                                  Hyperoxia
                                                  Normoxia
                                                  Hypoxia




                                  Guenette et al., JAP, 2008
                        Conclusion
  When respiratory muscle load is similar during heavy but
        submaximal normoxic and hypoxic exercise:


       Greater diaphragmatic fatigue occurs in hypoxia




• Respiratory muscle blood flow is not increased in hypoxia to
                    counteract hypoxaemia


• Greater fatigue in hypoxia due to hypoxaemia and Limited
                Respiratory Muscle Blood Flow
Pressure Swings during Isocapnic
        Hyperventilation




                         Guenette et al., JAP, 2008

				
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