Respiratory Failure and Indications of Mechanical

Ventilation

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Outline• Respiratory Failure due to ↑ Resistance• Respiratory Failure due to ↓ Compliance• Respiratory Failure due to ↑ VE • Respiratory Failure due to ↓ Neuromuscular

competence

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Gas Exchange

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4

elastance = Dpressure / Dvolume

volume

transairwaypressure

transthoracicpressure

transrespiratorypressure

Lung Mechanics resistance = Dpressure / Dflow

flow

900

600

300

00 1 2 3 4

Volu

me

(mL)

Time (sec)

RCexp

Expiratory Time Constant

900

600

300

00 1 2 3 4

Volu

me

(mL)

Time (sec)

Shorter RCexp

Variable Expiratory Time Constant

Longer RCexp

WOB Measurements

PA

B C

D

E

VWOB = ∫0

ti P x Vdt• Elasic work: ABCA• Resistive work

– Inspiratory: ADCA– Expiratory: ACEA

Work per breath is depicted as a pressure-volume areaWork per breath (Wbreath) = P x tidal volume (VT)Wmin = wbreath x respiratory rate

Pressure Pressure Pressure

Volu

me

Volu

me

Volu

me

V T

WR = resistive work

WEL = elastic work

The total work of breathing can be partitioned between an elastic and resistive work. By analogy, the pressure needed to inflate a balloon through a straw varies; one needs to overcome the resistance of the straw and the elasticity of the balloon.

Work of Breathing

Work of BreathingW

ork

of B

reat

hing

RV FRC TLC

Total Work

Elastic Work

Frictional Work

11The balance between load neuromuscular

competence strength

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Sustaining Oxygenation and Alveolar Ventilation

Load Neuromuscular Competence

The balance between load (resistive, elastic, and minute ventilation) and neuromuscular competence (drive,

transmission, and muscle strength)

Sustaining Oxygenation and Alveolar Ventilation

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Resistive Load AW Elastance Load L,CW

Minute Ventilation (VE) Load

Work of BreathTransmission Drive

(RR)

Muscle Strength (NIF)

∆V/Q

PaCO2PaO2

Respiratory Failure

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Resistive Load AW Elastance Load L,CW

Minute Ventilation (VE) Load

Work of BreathTransmission Drive

(RR)

Muscle Strength (NIF)

∆V/Q

PaCO2PaO2↓ ↑

Sustaining Oxygenation and Alveolar Ventilation

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Resistive Load AW Elastance Load L,CW

Minute Ventilation (VE) Load

Work of BreathTransmission Drive

(RR)

Muscle Strength (NIF)

∆V/Q

PaCO2PaO2

↑↑↑

Case Presentation• 6 year old male with asthma who was brought to ER

after riding on the school bus with severe respiratory distress

• RR: 32/min, tachycardic 130/min, diaphoretic, wheezes, using accessory muscles

• ABG’s: 7.47, PCO2: 30, PO2 88 O2 Sat: 95%

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Respiratory Failure due to ↑ Resistance Load

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Resistive Load AW Elastance Load L,CW

Minute Ventilation (VE) Load

Work of BreathTransmission Drive

(RR)

Muscle Strength (NIF)

∆V/Q

PaCO2PaO2

↑

↑↑

↑ Deep rapid tachypneaUse of accessory muscles

Signs of strain

↑RCExpWheezes

Prolonged expirationInflated chest

Bronchospam: asthma, COPD or bronchiolitisObstruction: croup, epiglotitis or OSA

Edema, Secretion or scarring

↑ Work of Breath

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Balanced Load and Competence

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Resistive Load

Neuromuscular Competence

ABG’s: 7.47, PCO2: 30, PO2 88 O2 , HCO3: 22, Sat: 95%

↑↑

Imbalanced Load and Competence

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Resistive Load

Neuromuscular Competence

↑

↑

ABG’s: 7.39, PCO2: 44, PO2 72 O2 , HCO3: 22, Sat: 91%

Case Presentation• A 23-year-old man is being evaluated in the emergency

room for severe pneumonia• His respiratory rate is 38/min and he is using accessory

breathing muscles, SBP 70 and HR 135/min

• FiO2: 0.9, pH 7.19, PaCO2 49 mm Hg PaO2 57 mm Hg, SaO2 86% HCO3

- 23 mEq/L• Na+ 149 mEq/L, K+ 4.1 mEq/L, Cl- 100 mEq/L, CO2 24

mEq/L, %COHb 2.1% Hb13 gm%.

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Case Presentation• Oxygenation:

– The PaO2 and SaO2 are both markedly reduced on 90% inspired oxygen

– PAO2 = FIO2 (PB – 47 mm Hg) - 1.2 (PaCO2)– PAO2 = 0.9 (760– 47 mm Hg) - (55)= 586– A-a Gradient= PAO2- PaO2= 586-57= 529– Indicating shunting process

• Ventilation:– The patient is hypoventilating despite the presence of tachypnea, most

likely indicating significant dead-pace ventilation• Acid Base:

– Combined acute respiratory acidosis, combined metabolic acidosis and metabolic alkalosis

22FiO2: 0.9, pH 7.19, PaCO2 49 mmHg PaO2 57 mmHg, SaO2 86% HCO3

- 23 mEq/LNa+ 149, K+ 4.1, Cl- 100, CO2 24 (mEq/L), %COHb 2.1% Hb13 gm%, LA: 12 mEq/L

Case Presentation• Acid Base:

– Acidosis– Acute respiratory acidosis: decrease in pH of 0.07 for each 10

PCO2: expected pH of 7.33– Actual pH 7.19 indicating combined metabolic acidosis– Metabolic acidosis of high anion gap: 149- (100+24)= 25– ∆ AG= 25-12= 13 indicating an added acid of 13 mEql/L– Lactic acid level was 12 mEq/L– ∆HCO3 = 24-23=1– ∆ AG > ∆ HCO3 indicating combined metabolic alkalosis

23FiO2: 0.9, pH 7.19, PaCO2 49 mmHg PaO2 57 mmHg, SaO2 86% HCO3

- 23 mEq/LNa+ 149, K+ 4.1, Cl- 100, CO2 24 (mEq/L), %COHb 2.1% Hb13 gm%, LA: 12 mEq/L

Respiratory Failure due to ↓ Compliance Load

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Resistive Load AW Elastance Load L

Minute Ventilation (VE) Load

Work of BreathTransmission Drive

(RR)

Muscle Strength (NIF)

∆V/Q

PaCO2PaO2

↓

↑↑↑

Rapid shallow tachypneaUse of accessory muscles

Signs of strain

↓RC Rapid shallow breathing

Alveolar edema, atelectasispneumonia, ARDS

Intrinsic PEEP

Imbalanced Load and Competence

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Elastance Load

Neuromuscular Competence

↑

↑

pH 7.29, PaCO2 55 mm Hg PaO2 77 mm Hg, SaO2 87% HCO3

- 23 mEq/L

Case PresentationA 46-year-old man has been in the hospital two days with urinary

tract infection. He was recovering but has just become diaphoretic, dyspneic, and hypotensive.

He is breathing oxygen through a nasal cannula at 3 l/min, RR 42/min, SBP 65 and HR 150/min

pH 7.40PaCO2 20 mm HgPaO2 80 mm HgSaO2 95% Hb 13.3 gm%HCO3

- 12 mEq/LNa+ 141 mEq/L, Cl- 103 mEq/L, CO2 13 mEq/L, Hb13 gm%.

Case Presentation• Oxygenation:

– The PaO2 is reduced on 32% inspired oxygen – PAO2 = FIO2 (PB – 47 mm Hg) - 1.2 (PaCO2)– PAO2 = 0.32 (760– 47 mm Hg) – 1.2 (20)= 204– A-a Gradient= PAO2- PaO2= 204-80= 124– Indicating V/Q mismatch process

• Ventilation:– The patient is hyperventilating with low PCO2– Indicating significant high minute ventilation secondary to high

metabolism• Acid Base:

– Metabolic acidosis with reparatory alkalosis indicating increased demand

27FiO2: 0.32, pH 7.40, PaCO2 20 mm Hg PaO2 80 mm Hg, SaO2 95% HCO3

- 12 mEq/LNa+ 141 mEq/L, Cl- 103 mEq/L, CO2 13 mEq/L, Hb13 gm%.

Respiratory Failure due to ↑ Minute Ventilation Load

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Resistive Load AW Elastance Load L

Minute Ventilation (VE) Load

Work of BreathTransmission Drive

(RR)

Muscle Strength (NIF)

∆V/Q

PaCO2PaO2

↑↑↑

Rapid Deep tachypneaUse of accessory muscles

Signs of strain

↑

Excessive calories, sepsis, hypovolemia, PE

VO2, VCO2, pH

↓

Balanced Load and Competence

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VE LoadNeuromuscular

Competence

pH 7.40, PaCO2 20 mm Hg PaO2 80 mm Hg, SaO2 95% HCO3

- 12 mEq/L

↑↑

Imbalanced Load and Competence

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VE Load

Neuromuscular Competence

↑

↑

pH 7.29, PaCO2 35 mm Hg PaO2 67 mm Hg, SaO2 86% HCO3

- 13 mEq/L

Case Presentation

A 27-year-old man is being evaluated in the emergency department for acute dyspnea.

FIO2 .21, pH, 7.19, PaCO2, 65 mm Hg, PaO2 65 mm Hg, HCO3

- 24 mEq/L, SaO2 90%

Case Presentation• Oxygenation:

– The PaO2 and SaO2 are reduced on 21% inspired oxygen – PAO2 = FIO2 (PB – 47 mm Hg) - 1.2 (PaCO2)– PAO2 = 0.21 (760– 47 mm Hg) – 1.2 (55)= 84– A-a Gradient= PAO2- PaO2= 84-65= 19– Indicating hypoventilating process

• Ventilation:– The patient is hypoventilating with high PCO2

• Acid Base:– Acute respiratory and metabolic acidosis

33FIO2 .21, pH, 7.19, PaCO2, 65 mm Hg, PaO2 65 mm Hg,

HCO3- 24 mEq/L, SaO2 90%

Respiratory Failure due to ↓ Neuromuscular Competence

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Resistive Load AW Elastance Load L

Minute Ventilation (VE) Load

Work of BreathTransmission Drive

(RR)

Muscle Strengh )NIF(

∆V/Q

PaCO2PaO2

↓↓↓

Low RR Low vital capacity < 15 mL/kg

Low NIF < -15 cm H2O

↑

Brain stem lesionDrugs

Hypothyroidism

Electrolytes, FatigueMyopathy, Malnutrition

ALS, AG , GB, SMGBotulism

Imbalanced Load and Competence

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Load

Neuromuscular Competence↓

pH, 7.19, PaCO2, 65, PaO2 65, HCO3- 24,

SaO2 90%

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