Prone positioning is commonly used to treat ARDS patients. This study evaluated 13 ARDS patients undergoing prone positioning for up to 15 hours. The study found that changes in PaCO2 (partial pressure of carbon dioxide in arterial blood) and alveolar dead space were more relevant indicators of the respiratory response to prone positioning than changes in the PaO2/FiO2 ratio. Prone positioning reduced alveolar dead space, which correlated with improved respiratory mechanics. Estimating physiological dead space using predictive equations underestimated directly measured values. Defining response to prone positioning based on PaCO2 changes may be more useful than using changes in PaO2/FiO2.

1. PaCO2 and alveolar dead space are more relevant
than PF ratio in monitoring the respiratory response
to prone position in ARDS patients
學生: 黃菁鳳
指導老師: 林鳳卿 老師 施玫如 老師
日期: 2014/03/12

2. Outline
 Introduction
 Material and Methods
 Results and Discussion
 Conclusion
 Limitation
 Q&A

3. Introduction

4. Acute respiratory distress syndrome
(ARDS)
 Baby lung :
Since its first description in 1967, it has been accepted that ARDS
includes a number of lung injuries of various origins whose
consequences are decreased lung capacity and thus limit the
ventilation ability.
 Positive end-expiratory pressure (PEEP) is to restore the
functional residual capacity.
 A Considerable progress has been made over the past decade in
the ventilatory management of patients with ARDS.

5. Prone Position
 Improve oxygenation by relieving atelectasis and improving
perfusion.. Recruitment and restore FRC , and improve
diaphragm movement.
 Reduction of alveolar dead space.
 Improve V/Q Matching
 Improve survival in severe ARDS (P/F Ratio <100
mmHg) .Define the outcome by increasing P/F Ratio> 20 mmHg
after 1 to 6 hours. (2010 Sud et al)

6.  Anatomical dead space
(In Conducting airways: about 150mL in an average adult or 2.2mLs/kg )
 Physiological dead space
 Alveolar dead space
 Apparatus dead space
Dead-space ventilation (VD)

7. Physiologic dead space
 Part of the tidal volume which does not participate in gas
exchange.
 The quantity of CO2 exhaled from the healthy alveoli will be
diluted by anatomical dead space and alveoli with no perfusion.
 Measure expiratory mixed gas
By collecting the exhaled breath in a gas impermeant bag ( Douglas
bag)

8. Alveolar dead space
 When the poorly perfused alveoli empty at the same rate as the
normal alveoli, it is possible to measure the alveolar dead space.
 ETCO2 represents the partial pressure or maximal concentration
of CO2 at the end of exhalation

9. ETCO2 Monitoring
 The end-tidal sample of gas (measured by capnography)
 Determinants of ETCO2 are:
(1) alveolar ventilation,
(2) pulmonary perfusion (cardiac output) and
(3) CO2 production.
Reference: Paramedicine.com

10. Reference: Paramedicine.com

11. Ventilated area which do not participate in gas
exchange.
Reference: Paramedicine.com

12. Bohr's equation
(Based on :all expired CO2 comes from alveolar gas)
PECO2
PetCO2

13. Objective 1
 Prone Position allows recruitment of a slow compartment
previously excluded from ventilations.
 Associated with a decrease in PaCO2→ an indirect reflection of
the reduction of the alveolar dead space (VDalv).
 VDalv appears to be an independent risk factor for mortality in
patients with ARDS .
(Nuckton TJ et al. Pulmonary dead-space fraction as a risk factor for death in the acute respiratory
distress syndrome. N Engl J Med 2002. )

14. Objective 2
 Siddiki et al used to evaluate he physiological dead space fraction
(VDphysiol/VT) by using a rearranged alveolar gas equation for
PaCO2 without any expired CO2 measurement.(2010 ).

15. Purpose
1) Changes in PaCO2 and VDalv might be more relevant
than changes in PaO2 in defining the respiratory response
to Prone Postion.
2) To validate the method of evaluation of the estimate
VDphysiol/VT .

16. Material and Methods

17. Inclusion
 15 patients.
 January 2008 to March2010
 Criteria of Acute Respiratory Distress Syndrome Network.
 PaO2/FiO2 ratio < 100mmHg after 24 to 48 hours of mechanical
ventilation.
 Hemodynamic stability: systolic blood pressure> 90 mmHg with
norepinephrine < 0.5 μg/kg/minute.
Exclusion
 COPD ( 2 patients will be excluded )
Material & Methods

18. Material & Methods
 Ventilated in volume-controlled mode (Servo-i)
 Ventilator setting :
• VT of 6 to 8 mL/kg IBW
• I: E =1:2 (Hypercapnia without generating intrinsic PEEP)
• End inspiratory pause of 0.5 seconds. (Pplat < 30 cmH2O)
 First session of Prone Position.
 Blood gas analysis, Pplat, total PEEP, end-tidal CO2(PetCO2) ,mixed
expired CO2 (PECO2) just before turning the patient to the PP, every
3 hours in the PP until 15 hours.

19.  Expired CO2 was measured by capnometer positioned between
the proximal end of the endotracheal tube and the Y piece of the
ventilator circuit(COSMO; Novametrix, Wallingford, CT, USA).
Material & Methods
Reference: Egan’s Ch18

20. Material & Methods
 Bohr's method
1. VDalv/VT = 1 - PetCO2/PaCO2
2. VDphysiol/VT = 1 - PECO2/PaCO2.
 The estimated VDphysiol/VT :
1 -[(0.86 × VCO2est)/(VE × PaCO2)]
(VCO2est :CO2 production calculated by Harris-Benedict equation.)

21.  Responders to Prone Position were defined in
1. An increase in PaO2/FiO2 ratio >20 mmHg after 15 hours of
Prone Position.
2. A decrease in PaCO2 > 2 mmHg after 15 hours of Prone Position.
Material & Methods

22. Result and Discussion

23. Table 1
Respiratory parameters and blood gas analysis at inclusion.
15Patients
13Patients
APACH II:62
SOFA: 11
H1N1/Pneumonia
2COPD

24. Table 2
Changes in respiratory mechanics, blood gas analysis and VDalv in PP

25. Mancebo et al 2006 Charron C et al
2011
This study
76/138 prone patients 57/218 prone patients
Over 20 hours/day
decrease mortality
rate.
Over 18 hours / day
increase survival rate.
The maximum effect
of VDalv, PaCO2,
Pplat show after 6 to
9 hours.
Compare to previous study

26. Table 3
Changes in respiratory mechanics, blood gas analysis and VDalv in
PaO2 responders (n = 7) and PaO2 nonresponders (n = 6)

27. Figure 1
Alterations during PP in PaO2/FiO2, PaCO2, plateau pressure (Pplat)
and alveolar dead space (VDalv/VT) in responders and
nonresponders (dotted lines) according to PaO2/FiO2 changes.

28. Table 4
Changes in respiratory mechanics, blood gas analysis and VDalv in
PaCO2 responders (n = 7) and PaCO2 nonresponders (n = 6)
Compare to
PO2
response
group

29. Figure 2
Alterations during PP in PaO2/FiO2, PaCO2, plateau pressure (Pplat)
and alveolar dead space (VDalv/VT) in responders and
nonresponders according to PaCO2 changes.

30.  Correlation between
(ΔVDalv/VT) and (ΔCrs)
 Correlation between
(ΔVDalv/VT) and (ΔPaO2/FiO2)
R=o.oo6
p=o.95
R=o.29
p=o.03

31. Compare to previous study
Gattinoni et al 2003 Protti A et al 2009
225Patients 32Patients
Result • ↓PaCO2≥1 mmHg is predictive of the patient’s
prognosis then ↑P/F Ratio > 20 mmHg .
• Study show an increased survival at 28 days.
• Reason: Improved the efficiency of alveolar
ventilation (decreased physiologic deadspace
ratio)
PaCO2 variation is
associated with lung
recruitability instead
of PaO2/FiO2. (by CT)

32. Figure 4: Comparison between measured VDphysiol/VT and
estimated VDphysiol/VT
Comparison for each paired data set
(n = 78) in the supine position and
after 3, 6, 9, 12 and 15 hours in the
prone position

33. Figure 4: Comparison between measured VDphysiol/VT and
estimated VDphysiol/VT

34. Based on linear correlation
R~0.435 R~0.529
 Estimated VDphysiol/VT using the Harris-Benedict equation
systematically underestimated measured VDphysiol/VT (B).

35. Based on Bland and Altman representation
 Estimated VDphysiol/VT using the Harris-Benedict equation
systematically underestimated measured VDphysiol/VT(B).

36. Compare to previous study
 Pelosi st al ,found not significant decrease in Vdphysiol .
Pelosi st al 1998 This study
Patients 16 13
Duration 120mins 15hrs.
PEEP 12.3 cmH2O 6 cmH2O
PaCO2 ↓ ↓
VD physiol x ↓
Did not compare
with the measure
VDphysicol
Did Compare with
the measute
VDphysicol
Protti A et al 2009
PEEP:
13 cmH2O

37. Conclusion

38. 2.
Defining the respiratory response to prone appeared more
relevant when using PaCO2 changes rather than PaO2/FiO2
changes.
3.
Estimated VDphysiol/VT ratios systematically underestimated
measured VDphysiol/VT ratios.
1.
Prone position induced a decrease in VDalv/VT, which was
correlated with an improvement in respiratory mechanics.

39. Limitation
 Patients numbers.
 The prone duration and initial time relate to the consequence
 Is the measurement necessary ? How this may affect management
at bedside still remain to be studied.
 Define the sensitivity of PaCO2 and PaO2 .
 Limited treatments.
 The limitation of the sensor: secretions and humidity effect the
clearance and the accuracy of the measure value.

40. Q&A

41. Thank you.