Pathophysiology of hypoxic respiratory failure

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PATHOPHYSIOLOGY OF ACUTE RESPIRATORY FAILURE:

PATHOPHYSIOLOGY OF ACUTE RESPIRATORY FAILURE Dr. Andrew Ferguson Consultant in Anaesthetics &  Intensive Care Medicine Craigavon Area Hospital, U.K. http://www.slideshare.net/fergua

INTRODUCTION:

This is a tutorial covering the pathophysiology of hypoxaemic respiratory failure, and reviewing how this information can be used to support a rational management plan. Although this tutorial is primarily aimed at anaesthesia residents in the early part of their career, it should be useful to medical students and to doctors from other specialties who encounter these patients or are interested in understanding some of the underlying physiology. This tutorial is an introduction to the topic, and although detailed in some areas it does not attempt to be an exhaustive reference. You should refer to standard texts and to the literature for further information. INTRODUCTION

RESPIRATORY PHYSIOLOGY CURRICULUM:

Gas exchange O 2 & CO 2 transport Hypoxia & hypercapnia Hyper- and hypobaric scenarios Functions of Hb In O 2 and CO 2 carriage In acid-base equilibrium Pulmonary ventilation: volumes, flows, dead-space Effects of IPPV on the lungs (and the heart) Mechanics of ventilation & V/Q abnormalities Control of breathing Acute & chronic ventilatory failure Effects of O 2 therapy Non-respiratory functions of the lungs RESPIRATORY PHYSIOLOGY CURRICULUM

OVERVIEW:

Definition of respiratory failure Case scenario (running through the tutorial) Mechanisms of hypoxia Respiratory patterns and work of breathing Definitions and calculation of dead-space Alveolar-arterial oxygen difference and the alveolar gas equation Venous admixture, V/Q mismatch, shunt and the shunt equation Lung volume, compliance, and functional residual capacity The importance of mean airway pressure Recruitment as a component of the ventilatory strategy OVERVIEW

WHAT IS RESPIRATORY FAILURE?:

pO 2 < 8 kPa (60 mmHg) on room air and/or pCO 2 > 6 kPa (45 mmHg) Type I = primarily hypoxaemia Type II = primarily hypercapnia Type III = perioperative (atelectasis) Type IV = shock (hypoperfusion) WHAT IS RESPIRATORY FAILURE?

CASE SCENARIO:

John is a 43 year old with mild asthma. He attends the Emergency Department with a 72 hour history of myalgias and fever, with increasing dyspnoea and a productive cough. His room air SpO 2 is 84% and his respiratory rate is 37/minute and shallow. He is using his accessory muscles and there is evidence of muscle use on exhalation. ABG shows pH 7.34, pCO 2 6.1 kPa, pO 2 7.8 kPa on room air He is sweaty and tiring rapidly. You detect crepitations at his right lung base and widespread wheeze. CXR confirms a right lower zone infiltrate. CASE SCENARIO

KEY PHYSIOLOGY RELEVANT TO THIS CASE:

Mechanisms of hypoxia Work of breathing Dead space and alveolar ventilation Hypoxic pulmonary vasoconstriction Alveolar gas composition & alveolar-arterial O 2 difference Shunt and V/Q mismatch Effects of anaesthesia on all of these Effects of mechanical ventilation on all of these KEY PHYSIOLOGY RELEVANT TO THIS CASE

WHY IS JOHN HYPOXAEMIC?:

Gather your thoughts then proceed… WHY IS JOHN HYPOXAEMIC?

MECHANISMS OF HYPOXAEMIA:

Ventilation-perfusion (V/Q) mismatch E.g. secretions or bronchial constriction, emphysema, PE, pulmonary arterial vasospasm At least partially O 2 responsive Shunt E.g. collapsed or flooded alveoli Typically poorly responsive to O 2 Alveolar hypoventilation See alveolar gas equation in subsequent slides Reduced inspired partial pressure of O 2 e.g. altitude, industrial accident, fire Diffusion impairment Rarely a major contributor MECHANISMS OF HYPOXAEMIA Many of these are associated with a fall in FRC (functional residual capacity)

RESPONSE TO INCREASED FIO2 IN SHUNT:

RESPONSE TO INCREASED FIO 2 IN SHUNT Shunt fraction (%) Alveolar pO 2 As shunt fraction increases, higher inspired (and alveolar) pO 2 has less and less effect on arterial pO 2

WHAT ABOUT JOHN’S BREATHING PATTERN?:

Gather your thoughts then proceed… WHAT ABOUT JOHN’S BREATHING PATTERN?

IMPACT OF RESPIRATORY PATTERNS:

Increased respiratory muscle work Increases O 2 consumption (can be dramatic) May exceed ability to deliver and so precipitate cardiac ischaemia Rapid shallow breathing Increased dead-space/tidal volume (VD/VT) ratio Turbulent flow In rapid shallow breathing & bronchospasm Requires greater work for same flow Bronchospasm Gas trapping and auto-PEEP Requires increased work of breathing including expiratory IMPACT OF RESPIRATORY PATTERNS

RAPID SHALLOW BREATHING: F/VT RATIO:

f/VT = respiratory rate divided by tidal volume (in litres) Significant increase in work of breathing above f/VT = 60-70 Corresponds to rate of 30 with tidal volume 0.5L Hence common use of rate > 30 as predictor of severity RAPID SHALLOW BREATHING: F/VT RATIO

WORK OF BREATHING: P-V CURVE:

WORK OF BREATHING: P-V CURVE

HOW DO YOU WORK OUT DEAD-SPACE?:

Gather your thoughts then proceed… HOW DO YOU WORK OUT DEAD-SPACE?

RESPIRATORY DEAD-SPACE:

Anatomical Conducting airway volume = around 150 ml (2ml/kg ideal BW) Increased by: Rapid shallow breathing (as above) Very high VT (pull on trachea and bronchi increases volume) Decreased by: Head-down position (compresses conducting airways) Measured by Fowler’s method Physiological Alveolar dead-space = alveoli ventilated but not perfused Physiological dead-space = alveolar + anatomical = 170 ml Measured by Bohr’s method using Bohr equation RESPIRATORY DEAD-SPACE

IMPLICATIONS OF DEAD-SPACE VOLUME:

Impacts on effective alveolar ventilation Alveolar minute ventilation = (VT – VD) x resp. rate Increased dead-space Decreases effective alveolar ventilation Increases pCO 2 Increases work of breathing IMPLICATIONS OF DEAD-SPACE VOLUME

FOWLER’S (1948) METHOD (ANATOMICAL D-S):

FOWLER’S (1948) METHOD (ANATOMICAL D-S)

FOWLER’S METHOD:

FOWLER’S METHOD

FOWLER’S METHOD:

Area under the exhalation curve = C exp x (V exp – V d ) Because the upstroke is not vertical the separation of Vexp and Vd is found by dropping a perpendicular so that area A = area B Where this hits the volume axis = anatomical dead-space FOWLER’S METHOD

BOHR EQUATION:

You might want to get coffee….we are going to derive the Bohr equation and there are some mathematics and mental gymnastics involved! You have been warned! And yes…it does have some clinical relevance…read on! BOHR EQUATION

BOHR EQUATION (PHYSIOLOGICAL D-S):

Let’s start with the fact that exhaled tidal volume is made up of dead-space volume plus some alveolar volume VT = V d + V alv [re-arranged to V alv = VT – V d ] The amount of a gas exhaled will be the sum of the amount in the alveolar portion plus the amount in dead-space (we use CO 2 ) The amount of gas = volume x concentration SO… VT x C exp = V d x C d + V alv x C alv & for CO 2 C d is near zero SO… VT x C exp = V alv x C alv and V alv = VT – V d SO… VT x C exp = (VT - V d ) x C alv SO… VT x C exp = (VT x C alv ) – (V d x C alv ) Rearranging gives (V d x C alv ) = (VT x C alv ) – (VT x C exp ) BOHR EQUATION (PHYSIOLOGICAL D-S)

BOHR EQUATION:

SO V d x C alv = VT (C alv – C exp ) leading to VD/VT = (C alv -C exp )/C alv Using partial pressures, VD/VT = (P Alv CO 2 -P E CO 2 )/(P Alv CO 2 ) It is then assumed that P Alv CO 2 = PaCO 2 (arterial) SO finally VD/VT = (PaCO 2 -P E CO 2 )/PaCO 2 (norm = 0.2-0.3) and all are measurable so you can now work out the physiological dead-space of your patients! REMEMBER the application of this: The larger the gap between PaCO 2 and ETCO 2 the greater the physiological dead-space BOHR EQUATION

JOHN IS STILL ALIVE…JUST!:

You got pretty worried and intubated John before he collapsed. After 10 minutes on the ventilator with PEEP 5, VT 550 and FiO 2 0.8 John’s ABG shows: pH 7.30, pCO 2 6.6 kPa, pO 2 8.6 kPa A nurse has just done the FCCS course and asks you 2 questions: how much of the hypoxia is due to the CO 2 retention? What is John’s A-aDO 2 ? Gather your thoughts then proceed… JOHN IS STILL ALIVE…JUST!

ALVEOLAR-ARTERIAL OXYGEN DIFFERENCE:

The easy bit… The difference is normally quite small (<15 mmHg or 2 kPa) It increases with age by 1 mmHg for every decade above 18 years If a patient has hypoxia with a normal A-aDO 2 the cause is either: Hypoventilation (hypercapnia), or Breathing a gas with low pO 2 If the A-aDO 2 is elevated the hypoxia is pulmonary in origin i.e. pulmonary system (vessels, alveoli etc.) The harder bit… Assumes that we know the O 2 concentration in the alveoli ALVEOLAR-ARTERIAL OXYGEN DIFFERENCE

THE ALVEOLAR GAS EQUATION (SIMPLE):

Assumptions Inspired gas has no CO 2 P A CO 2 = P a CO 2 Alveolar gas is saturated with water Simple equation: P A O 2 = P I O 2 – PaCO 2 /R where P I O 2 =FiO 2 (P atm -P H2O ) R = respiratory quotient = 0.8 P H2O = SVP of water at 37 o C = 47 mmHg or 6.25 kPa P atm = atmospheric pressure assumed to be 760 mmHg (101 kPa) Substituting gives: P A O 2 = 94.75 x FiO 2 – 1.25 x PaCO 2 THE ALVEOLAR GAS EQUATION (SIMPLE)

THE ALVEOLAR GAS EQUATION (COMPLEX):

If R < 1 (and especially if FiO 2 is high) then more O 2 is taken up from the alveolus than CO 2 is released into it, and alveolar volume would fall if more gas did not move in passively from the airway to replace it. This gas moving in can increase the P A O 2 very slightly and if we want to correct for this we need to use this larger equation. OUCH! Luckily the effect is so small that in the real world we can pretty much ignore it. THE ALVEOLAR GAS EQUATION (COMPLEX) P A O 2 = (P atm – P H2O ) x FiO 2 – PaCO 2 /R + (FiO 2 x P a CO 2 x (1-R)/R))

Back to the A-aDO2:

A-aDO 2 = P A O 2 -P a O 2 So John, assuming R is 0.8 and breathing 80% O 2 should have an alveolar O 2 of: 94.75 x 0.8 – 1.25 x 6.6 = 75.8 – 8.25 = 67.6 kPa John’s A-aDO 2 is 67.6 – 8.6 = 59 kPa Which is WAY above the normal of 2 kPa, so his hypoxia is plainly not due to his hypercapnia!!! Back to the A-aDO 2

So back to john’s hypoxia…:

The helpful staff nurse wants to understand what’s going on in John’s lungs, since the CO 2 isn’t the main problem. You tell her that he is “shunting” which draws a puzzled look and inevitably leads to her asking you to explain. You walked into it, now you have to get yourself out! Gather your thoughts then proceed… So back to john’s hypoxia…

SHUNT & VENOUS ADMIXTURE:

Shunt = perfusion without ventilation i.e. V/Q = 0 Mixing of deoxygenated blood back into the arterial circulation V/Q mismatch = imbalance of ventilation and perfusion Venous admixture is a construct representing the amount of (deoxygenated) mixed venous blood that would have to be added to (oxygenated) pulmonary end-capillary blood to produce the A-aDO 2 that you observe in your patient, so it reflects the degree of shunting and V/Q mismatch SHUNT & VENOUS ADMIXTURE

SHUNT:

Anatomical shunts Physiological Thebesian veins draining from the LV walls (0.3% of cardiac output) Bronchial veins (< 1% of cardiac output) This blood is NOT mixed venous blood Pathological Congenital heart disease with right to left shunt Pulmonary A-V shunts e.g. haemangioma Perfusion of non-ventilated alveoli (atelectasis/flooded) Protective mechanism = hypoxic pulmonary vasoconstriction Diverts blood away from diseased alveoli Not 100% effective so some flow remains Effect diminished by disease processes and drugs (anaesthetics) SHUNT

THE SHUNT EQUATION:

Don’t panic….you have seen the principle before (Bohr’s)! Let’s take it step by step: The blood leaving the lungs (Qt) is made up of the blood that went through working lung (Qc) and the blood that shunted (Qs), so Qt (cardiac output) = Qs + Qc and so Qc = Qt – Qs We want to know about blood flow and oxygen content, so: The oxygen content of Qt (cardiac output) is CaO 2 (arterial) The oxygen content of Qs (the pure shunt) is CvO 2 (mixed venous) The oxygen content of Qc (working capillaries) is CcO 2 (pulm. capillary) Substituting equation 1 we get Qt = Qs + (Qt - Qs) So far so go I hope! Think that through then proceed… THE SHUNT EQUATION

THE SHUNT EQUATION:

OK, so we have Qt = Qs + (Qt – Qs) , but we are interested in the total O 2 running in this system, which equals flow x content Let’s add in the content and go from there: Qt.CaO 2 = Qs.CvO 2 + (Qt – Qs).CcO 2 (since Qt – Qs = Qc) Qt.CaO 2 = Qs.CvO 2 + Qt.CcO 2 – Qs.CcO 2 Arrange the equation so Qs and Qt are on opposite sides: Qs.CcO 2 – Qs.CvO 2 = Qt.CcO 2 – Qt.CaO 2 now factorise… Qs(CcO 2 – CvO 2 ) = Qt(CcO 2 – CaO 2 ) and rearrange to get Qs/Qt Shunt fraction Qs/Qt = (CcO 2 – CaO 2 ) / (CcO 2 – CvO 2 ) THE SHUNT EQUATION

THE REALITY:

CaO 2 results from a mixture of blood from different lung units Some with pure shunt Some with varying degrees of V/Q mismatch over or under-ventilated relative to blood flow Some with normal blood and gas flow The impact of increasing FiO 2 on your patient will depend on their individual mix of V/Q ratios. It’s important to realise that this is dynamic and that you can have an impact on it to improve the arterial pO 2 . THE REALITY

EFFECTS OF MIXED V/Q RATIOS:

EFFECTS OF MIXED V/Q RATIOS

SO BACK TO FIXING JOHN’S OXYGENATION:

We’ve established John has altered V/Q and shunt He has thick purulent lung secretions in his right lower lobe He has bronchial oedema related to his asthma He has been rapid shallow breathing which will have reduced his FRC and induced atelectasis, adding to his problems What we want to do is to: improve the V/Q mismatch Convert areas of shunt (O 2 resistant) to V/Q mismatch (O 2 responsive) Reverse atelectasis And we are going to do this by improving functional residual capacity (FRC) RECRUITING collapsed or flooded alveoli SO BACK TO FIXING JOHN’S OXYGENATION

LUNG VOLUMES AND COMPLIANCE:

Remember blowing up a balloon! At low volume it’s stiff and really hard to inflate In the middle all is great! Before it bursts it gets difficult to inflate again We want John’s lungs on the steep (compliant) part of the curve LUNG VOLUMES AND COMPLIANCE Stiff overdistended lung Stiff atelectatic lung Pressure PEEP Peak Ventilator pressures FRC

VENTILATION TO IMPROVE OXYGENATION:

Remember! Recruitment manoeuvres can reduce BP severely Marked fall in RV preload and increase in RV afterload More severe in hypovolaemic patients Recruitment manoeuvre on initiation of ventilation Manual Inflate to given pressure (30 or 40 cmH 2 O) Hold for 30 seconds if BP/SpO 2 maintained Connect to ventilator On ventilator Ensure safe peak inspiratory pressure < 30 cmH 2 O Ensure safe tidal volume < 6-8 ml/kg predicted body weight if lung injury Press inspiratory hold button on ventilator (some need to be held in) Repeat to hold breath for 30 seconds Assumes adequate PEEP afterwards to maintain “open lung” Recheck pressures and volumes on ventilator Compliance may have changed VENTILATION TO IMPROVE OXYGENATION

MAINTAINING FRC, AVOIDING OVERDISTENSION:

All of the ventilation manoeuvres you might perform to improve oxygenation rely on an increase in MEAN AIRWAY PRESSURE which is the driver for maintenance of FRC via increased alveolar pressure The options include: Increasing PEEP (producing a higher baseline pressure) Increasing peak pressure (not used often) Not beyond 30 cmH 2 O Maintaining safe tidal volumes (6 ml/kg PBW in lung injury) Increasing inspiratory time Increases inspiration:expiration time ratio (I:E) ratio Usually in 1:2 range, increased towards 1:1 or sometimes even higher Very prolonged inspiratory times may lead to harmful gas trapping Increasing respiratory rate Provided inspiratory time is not shortened too much MAINTAINING FRC, AVOIDING OVERDISTENSION

MEAN AIRWAY PRESSURE:

O 2 is a band-aid, not a good primary therapy Increasing FiO 2 makes the ABG look better, but does not correct the pathophysiology Recruitment and PEEP improve V/Q and convert some of the shunt to V/Q mismatch This allows a lower FiO 2 to achieve same pO 2 target MEAN AIRWAY PRESSURE

SO BACK TO JOHN…:

John has been ventilated for 2 hours. It looks like you have stabilised him!!! SO BACK TO JOHN… Initial + 1 hour Current PEEP (cmH 2 O) 8 8 10 PC (cmH 2 O) 20 16 18 Peak (cmH 2 O) 28 28 28 Mean AP (cmH 2 O) 12.5 12 14 RR / min 15 16 16 I:E ratio 1:2 1:1 1:1 FiO 2 80% 60% 60% pO 2 (kPa) 9.9 9.1 9.7 pCO 2 (kPa) 4.9 5.3 5.2

REVIEW:

You have looked at a case of severe hypoxaemia You have reviewed the mechanisms of hypoxaemia and the concepts of dead-space, alveolar gas composition, V/Q mismatch, and shunt fraction (along with their mathematics) You have seen this information used as the physiological basis for a clinical approach to this disorder You have seen the importance of lung recruitment and the maintenance of FRC REVIEW

THANK YOU FOR TAKING THIS TUTORIAL!:

Please let me know if you found it helpful, or if you have other areas you would like to see covered. You can email me at: fergua at gmail.com THANK YOU FOR TAKING THIS TUTORIAL!

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