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

Outline

Introduction

Material and Methods

Results and Discussion

Conclusion

Limitation

Q&A

Introduction

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.

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)

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)

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)

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

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

Reference: Paramedicine.com

Ventilated area which do not participate in gas exchange.

Reference: Paramedicine.com

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

PECO2

PetCO2

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. )

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 ).

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 .

Material and Methods

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

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.

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

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.)

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

Result and Discussion

Table 1Respiratory parameters and blood gas analysis at inclusion.

15Patients

13Patients

APACH II:62

SOFA: 11

H1N1/Pneumonia

2COPD

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

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

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

Figure 1Alterations 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.

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

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.

Correlation between (ΔVDalv/VT) and (ΔCrs)

Correlation between (ΔVDalv/VT) and (ΔPaO2/FiO2)

R=o.oo6p=o.95

R=o.29p=o.03

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)

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

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

Based on linear correlation

R~0.435 R~0.529

Estimated VDphysiol/VT using the Harris-Benedict equation systematically underestimated measured VDphysiol/VT (B).

Based on Bland and Altman representation

Estimated VDphysiol/VT using the Harris-Benedict equation systematically underestimated measured VDphysiol/VT(B).

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 2009PEEP: 13 cmH2O

Conclusion

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.

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.

Q&A

Thank you.