pulmonary function tutorial,bhutani

Tutorial: Pulmonary Function--Dr. Bhutani

Clinical Case695 g male neonate with RDS, treated with surfactant and on ventilatory support @ 18 hours age:

Settings: 18 / 5 cm H20 x 45 breaths / min; FiO2 = 0.65

Driving Pressure: 18-5 = 13 cm H20

Arterial Gas: pH = 7.3; PaO2 = 74; PaCO2 = 55 mmHg

Is this baby a candidate for permissive hypercapnia?

Normocapnia varies?

• Maternal: 32 - 34 mmHg (progesterone effect)

• Fetal: 40 - 42 mmHg

• At birthing: 45 - 65 mmHg

• First day : 34 mmHg (progesterone effect)

• After first week : 35 - 42 mmHg

• Term PCA: 35 - 42 mmHg (RR = 40-60/min)

• 52 wks PCA: 35 - 42 mmHg (RR = 20-30/min)

Strategies to Prevent Non-Permissive Hypercapnia

• Define optimum PaCO2 levels

• Level to at which you will intervene• Level to at which you will wean

• Ventilate at optimal FRC

• Continuous monitoring of tidal volume

• Use least pressure for maximal tidal volume

• Facilitate spontaneous breathing

• Consider “dual wean” of driving pressures

• Consider pseudo-adaptive strategies

Alveolar Algebra: Ventilation

• Alveolar Ventilation inversely correlated to PaCO2

• Alveolar Ventilation is (VT-VD) x rate• Provided dead space is constant: proportional

changes in VT or rate will lead to proportional changes in alveolar ventilation and PaCO2.

Based on Pulmonary Gas Law: in a steady state, when inspired CO2 is negligible, then PACO2 = 863 (VCO2 / VA); where, VCO2 is CO2 production; PACO2 is partial pressure of carbon dioxide; 863 is body temp. x std. pressure / standard temperature ( 310 X 760/273 = 863 )

Clinical Case695 g male neonate with RDS, treated with surfactant and on ventilatory support and is now about 18 hours age and has stable vital signs, normotensive and is normoglycemic:

Settings: PIP:18 cm H20; PEEP: 5 cm H20; SIMV: 45 br / min;


Is the Alveolar Ventilation compromised?

If so, by how much?




PaCO2 = 55 mmHg is 35% higher than 40 mmHg

Alveolar Ventilation is decreased by 35%




PaCO2 = 54 mmHg is 35% higher than 40 mmHg

Questions: Do we wean? Which option?

Clinical Case

OPTIONS:

1. No Change

2. Reduce PIP by I cm H20 (17/5) :

Driving Pressure = 12 cm H20

3. Reduce PEEP by I cm H20 (18/4):


4. Reduce SIMV: Decrease minute ventilation

Clinical Case: Use of Tidal Volume

695 g male neonate with RDS, treated with surfactant and on ventilatory support:

Settings: 18 / 5 cm H20 x 35 breaths / min


Tidal Volume (measured): = 4 ml (5.7ml/kg)

Effective Compliance: = V / P ; 5.7 / 13 ml/cm H20/kg


Clinical Case




Tidal Volume (measured): = 4.0 ml (5.7 ml/kg)

Effective Compliance: = V / P ; 5.7 /13

= 0.44 ml / cm H20/kg


Relationship to FRC

Least Intervention or Barotrauma

• Increase Lung Volume : CPAP

• Adequate Lung Inflation : T insp.

• Adequate Lung Deflation : T exp.

• Adjust T insp. / T exp. : by estimating Time Constants

Ventilate at Optimal FRC

Clinical Case




Tidal Volume (measured): = 4.0 ml (5.7 ml/kg)

Effective Compliance: = V / P ; 5.7 /13

= 0.44 ml / cm H20


(PEEP) 5 18 (PIP)

P = 13 cm H20

V = 5.7 ml/kg

Graphic Representation of Ventilator Settings

(PEEP) 5 18 (PIP)

P = 13 cm H20

V = 5.7 ml/kg

V / P = 5.7 / 13 = 0.44 ml/cm H20 /kg

Graphic Representation of Ventilator Settings

5

TLC

RV

19

P = Driving pressure

V

PRESSURE (cmH20)

VO

LU

ME

Peak Inflating Pressure

PEEP

1817

Linear Change in V/ P

V / P = 5.7 / 13 = 0.44 ml/cm H20

5

TLC

RV

19

P = Driving pressure

V

PRESSURE (cmH20)

VO

LU

ME


PEEP

1817

Imaginary P-V relationship

Clinical Case

OPTIONS:

1. No Change






5

17

18

TLC

RV

19

16

P = 12

V

PRESSURE (cmH20)

VO

LU

ME


PEEP

Wean Peak Inflating Pressure

Tidal Volume decreases

with weaning

Driving pressure decreased

Clinical Case

OPTIONS:

1. No Change






5

18

TLC

RV

1917

P = 14

V

PRESSURE (cmH20)

VO

LU

ME


PEEP4

Wean PEEP

Tidal Volume increases with

weaning

Driving pressure increased

Circuit Airflow

• Airflow that is set, at operator’s discretion, to flow from inspiratory to expiratory circuits.

• Magnitude dependent on patient’s minute ventilation

• Excessive circuit airflow can lead to turbulence and may impede expiratory flow from the patient

• Low flow may limit inspiratory airflow

• Optimal circuit flow is about 5 to 8 fold minute ventilation

Clinical Case

OPTIONS:

1. Dual Wean: Reduce both PIP/PEEP

Dual Wean: 18/5 to 17/4 cm H20

= Driving Pressure

= 13 cm H20

5

17

18

TLC

RV

19

16

P = 13

V

PRESSURE (cmH20)

VO

LU

ME


PEEP

4

DUAL WEAN: concurrent wean of both PIP and PEEP


weaning


weaning

Driving pressure unchanged

5

18TLC

RV

14

P = 10

V

PRESSURE (cmH20)

VO

LU

ME


PEEP4

DUAL WEAN that allows for permissive

hypercapnia

Tidal Volume unchanged

with weaning

Driving pressure decreased

Clinical Case • PaCO2 = 55 torr at 18/5:

• IATROGENIC HYPERCAPNIA secondary to ventilation at “flattened” portion of the P-V relationship

• After Dual Wean, PaCO2 will be lowered provided ventilation is occurring closer to total lung capacity.

• Subsequent weaning to 14/4 may result in a PaCO2 = 55 torr

• PERMISSIVE HYPERCAPNIA

45

17

18

TLC

RV

19

16

P

V

PRESSURE (cmH20)

VO

LU

ME


PEEP

Iatrogenic hypercapnia with inappropriate Dual

Wean

Tidal Volume decreases

with weaning

pulmonary function tutorial,bhutani

Health & Medicine