Basic principles of gas exchange under Mechanical Ventilation

Lorenzo Del Sorbo, MD

Mechanical Ventilation:From physiology to clinical practice

Toronto, April 14th 2020

• gas exchange abnormalities

• physiologic principles

• clinical cases

Outline

Hypoxemia and impaired CO2 clearance are characteristics of ARDS

90 --

Radermacher et al. AJRCCM 2017

ARDS

• hypoventilation

• ↓ PIO2

• ↓ diffusion

• V/Q mismatch

• shunt

+ ↓ PvO2

mechanisms of

hypoxemia

V/Q ratio

Radermacher et al. AJRCCM 2017

V/Q ratio

Shunt

Radermacher et al. AJRCCM 2017

Higher VA/Q

Lower VA/Q

Shunt

Alveolar inhomogeneity

Lower pleural pressure

Higher pleural pressure

Shunt

Radermacher et al. AJRCCM 2017

Dead spacePaCO2=

V’CO2

V’A

= V’CO2

RR (Vt-Vd)

= V’CO2

RR (1-Vd/Vt)

Higher VA/Q

Lower VA/Q

Shunt

Alveolar inhomogeneity

Lower pleural pressure

Higher pleural pressure

Dead space

Nuckton T.J., et al. NEJM 2002

Ventilatory ratio

Pratik et al. AJRCCM 2013 & 2019

Radermacher et al. AJRCCM 2017

Clinical examples

1° case: ARDSVCV 430 ml X 25 RR, PEEP 15, Pplat 27, FiO2 40%, pH 7.31, paCO2 38, paO2 53

Brun-Buisson C, et al. ICM 2004

ARDS: hypoxemia

ARDS: means to decrease hypoxemia•Increase FiO2: not effective if hypoxemia is only from shunt

increased risk of O2 toxicity

Martin and Grocott. CCM 2013

ARDS: means to decrease hypoxemia - é FiO2

Increased risk of reabsorption atelectasis

Santos C, et al. AJRCCM 2000

Courtesy of L. Gattinoni and R. Fumagalli

Low trans-pulmonary

pressure

2° case: ARDS VCV 300 ml X 27 RR, PEEP 15

Pplat 30, FiO2 100%,pH 7.31, paCO2 38, paO2 40

Courtesy of L. Gattinoni and R. Fumagalli

intermediate trans-pulmonary

pressure

ARDS VCV 280 ml X 24 RR, PEEP 20, Pplat 30, FiO2 70%, pH 7.35, paCO2 38, paO2 74

Recruitment

Excessive trans-pulmonary

pressure

Courtesy of L. Gattinoni and R. Fumagalli

ARDS VCV 320 ml X 35 RR, PEEP 20, Pplat 32, FiO2 90%, pH 7.21, paCO2 54, paO2 74

Recruitment

and

overdistension

Courtesy of L. Gattinoni and R. Fumagalli

Excessive Trans-pulmonary

PressureVILI

non-recruiters

VCV 320 ml X 35 RR, PEEP 20, Pplat 32, FiO2 100%, pH 7.21, paCO2 54, paO2 64

CO2 clearance

ARDS: recruitment

Gattinoni L et al., NEJM 2006

Grasso S, et al. AJRCCM 2005

ARDS: recruitment

Goligher E, et al. AJRCCM 2014

ARDS: recruitment

3° case: ARDS

VCV 320 ml X 27 RR, PEEP 15Pplat 30, FiO2 100%,

pH 7.31, paCO2 51, paO2 70

Gattinoni L, et al. AJRCCM 2013

VCV 320 ml X 27 RR, PEEP 15Pplat 27, FiO2 60%,

pH 7.33, paCO2 48, paO2 70

Sud S, et al. ICM 2010

Guerin C, et al. AJRCCM 2014

Prone Position– induced Improvement in Gas Exchange Does Not Predict Improved Survival in ARDS

Prone positioning in ARDS: less VILI

Gattinoni L, et al. ICM 2013

Mutoh T, et al. Am Rev Respir Dis 1992

Guerin C, et al. ICM 2014

ARDS4° case:ARDS (post-trauma)

HR 135, BP 90/60, Hb 71, Lactate 5.1SvO2 50%VCV 480 ml X 27 RR, PEEP 16Pplat 30, FiO2 100%,pH 7.31, paCO2 38, paO2 40

• Improve hemodynamics(DO2/V’O2)

Courtesy of R. Fumagalli

ARDS (post-trauma)

HR 100, BP 110/60, Hb 95, Lactate 2SvO2 80%VCV 460 ml X 23 RR, PEEP 18Pplat 28, FiO2 80%,pH 7.37, paCO2 46, paO2 60

Post fluid resuscitation and PRBC transfusion

Courtesy of R. Fumagalli

Better DO2/V’O2

ARDS (post-trauma)

HR 100, BP 110/60, Hb 95, Lactate 2SvO2 80%VCV 460 ml X 23 RR, PEEP 18Pplat 28, FiO2 80%,pH 7.37, paCO2 46, paO2 60

Post fluid resuscitation and PRBC transfusion

Better DO2/V’O2

Improving perfusion in ventilated alveoli

Better V/Q

5° case: ARDS

Failed prone positionVCV 320 ml X 27 RR, PEEP 15

Pplat 30, FiO2 100%,pH 7.31, paCO2 38, paO2 40

Improving perfusion in ventilated alveoli

NEJM 1993

6° case: COPD exacerbation

Vt 450 mlRR 27iPEEP 18Ppeak 61FiO2 30%pH 7.25paCO2 80paO2 50

Main issue: hypoxemia/hypercapnia?Vt 450 mlRR 27iPEEP 18Ppeak 61FiO2 30%pH 7.25paCO2 80paO2 50

Effects of ñFiO2ñ PAO2ñ PaO2Change in Hypoxic pulmonary vasoconstrictionHaldane effectChange in V/QDecrease respiratory driveñ PaCO2

High V/Q

Low V/Q

Vt 450 mlRR 27iPEEP 18Ppeak 61FiO2 30%pH 7.25paCO2 80paO2 50

Expiratory flow limitation:

Main issues:

Dynamic hyperinflation:

V’exp(t) = V(t)

R*C (t)

V(trapped) = VT x t

TE

7° case: pulmonary embolism

Lower V/QñA-a difference

Dead spaceñPaCO2

V/Q=1V

V/Q=1V/Q=1V

V/Q=1

Acute pulmonary hypertension (also from high intra-thoracic pressure during MV) may aggravate hypoxemiaby low MvO2or by intra-cardiac right-to-left shunting

Petersson J, et al. Eur Respir J 2014

PaCO2 may increase or decrease according to the response of the respiratory drive to increase dead space and hypoxemia

Vt 450 ml, RR 35, FiO2 50%, pH 7.48, paCO2 30, paO2 70

Courtesy of L. Gattinoni and R. Fumagalli

Excessive Trans-pulmonary

PressureVILI

Pplat 35, FiO2 100%, pH 7.11, paCO2 64, paO2 40

8° case: refractory respiratory failure

ARDS

Courtesy of R. Fumagalli

HbO2 75%

HbO2 75%

HbO2 (75% + 75%)/2=75%

ARDS

Courtesy of R. Fumagalli

HbO2 90%

HbO2 90%

HbO2 (90% + 90%)/2=90%

VV-ECMOPre-pulmonary

-Increased SvO2-CO2 clearance

90%

60

Conclusions

Conclusions

Hemodynamics (DO2/V’O2)

Physiologic interpretation of the effects of the bedside maneuvers

ARDS is an hypoxemic syndrome, but

Carbon Dioxide elimination is the true target of ventilation (BAD CO2 ELIMINATION = risk of VILI)

Conclusions

In refractory cases…..

……think out of the BOX

Extra-Corporeal (gas exchange) Life Support

Thank you

lorenzo.delsorbo@uhn.ca