(Patho-)Physiology of Acute Respiratory Failure

CT1 Education Series (Intro)

Respiratory Physiology Curriculum

Gas exchange - O2 & CO2 transport, hypoxia & hypercapnia, hyper- and hypo-baric pressuresFunctions of Hb in O2 carriage & acid-base equilibriumPulmonary ventilation: volumes, flows, dead spaceEffects of IPPV on lungsMechanics of ventilation: V/Q abnormalitiesControl of breathing; acute & chronic ventilatory failure; effects of O2 therapyNon-respiratory functions of the lungs

Respiratory FailurepO2 < 8 kPa (60 mmHg) and/orpCO2 > 6 kPa (45 mmHg)

Type I = hypoxiaType II = hypercapniaType III = Perioperative (atelectasis)Type IV = Shock (Hypoperfusion)

Case scenarioJohn is a (slightly weird) 23 year old with mild asthma. He is admitted to hospital with 3 day history of myalgia and fever, and increasing shortness of breath and cough over the last 36 hours.His room air O2 sats are 84% and his respiratory rate is 37/minute and shallow. He is using accessory muscles and there is active exhalationABG: pH 7.34, pCO2 6.1 pO2 7.8 on airHe is sweaty and tiring rapidly. You can detect creps at his right base and widespread wheeze.

Key IssuesMechanisms of hypoxiaWork of breathingDead space and alveolar ventilationHypoxic pulmonary vasoconstrictionAlveolar gas composition & A-aDO2

ShuntEffects of anaesthesia on all of theseEffects of mechanical ventilation

Why is John hypoxic?

Why is John hypoxic?Mechanisms:

Ventilation/perfusion (V/Q) mismatchShuntDiffusion impairmentAlveolar hypoventilation (normal A-aDO2)

What about John’s breathing pattern?

What about John’s breathing pattern?Increased work = increased O2 consumption

Rapid shallow breathing = increased VD/VT ratioRapid shallow breathing = turbulent flowTurbulent flow = increased work (vicious O)Bronchospasm = gas trapping

Work of breathing - f/VT

Work of breathing

Dead-spaceAlveolar minute ventilation = MVA

MVA = (tidal volume - deadspace) x RRAnatomical = volume of air in conducting airwaysAlveolar = gas volume in unperfused alveoliPhysiological = anatomical + alveolar

Bohr EquationVolume of CO2 removed from ideal alveolar gas = volume exhaled in mixed expired gas%CO2Alv [=PaCO2] x VAlv [=VT-VD] = %CO2Exp x VExp [=VT]

VD/VT = (PaCO2-PECO2)/PaCO2

Normal VD/VT = 0.2-0.3Higher deadspace = bigger gap between PaCO2 and ETCO2

John’s still alive....just!You got worried and intubated John before he collapsed. After 15 minutes on the ventilator (PEEP 5, VT 550, rate 14, FiO2 80%) his pH is 7.34 pCO2 5.8 pO2 8.6

What is John’s A-aDO2?

What does that mean?

Alveolar gas compositionPAO2 = PIO2 - PaCO2/R

PAO2 = Alveolar partial pressure of O2

PIO2 = FiO2 x (PB -PH2O)R = resp. quotient = 0.8 (ish)

A-aDO2 = PAO2 - PaO2

Shunt & venous admixtureDeoxygenated blood returning to left heart

e.g. Bronchial veins (< 1% of CO) Thebesian veins from LV walls (0.3% of CO)Congenital heart diseaseAlveolar collapse

Venous admixture = amount of mixed venous blood required to mix with pulmonary capillary blood to produce observed A-aDO2

Shunt equation (!)Total O2 delivery = cardiac output O2 content = Shunt O2 content + Pulmonary blood flow O2 contentCardiac output = total flow = QtShunt flow = Qs, Pulmonary flow = Qt-QsCaO2 = arterial O2 contentCvO2 = mixed venous O2 contentCcO2 = pulmonary end capillary O2 content (calculated from alveolar gas equation & O2 dissociation curve)

Qs/Qt = (CcO2 - CaO2)/(CcO2-CvO2)

How are you going to fix John?

What is wrong with his lung compliance?How can the ventilator help?How will you know it is helping?How is it going to help his oxygenation?

Compliance

Lung Volumes

Closing capacity & volume

Dependent airways close during expirationThis occurs at closing capacity (CC)The closing volume = CC - residual volumeCC < FRC in young adultsCC is independent of position but FRC isn’t!CC = FRC at 44 years in supine positionCC = FRC at 66 years when upright