Speaker : Dr. S Sai Janani ASSESSMENT OF LUNG AS AN OXYGENATOR CONCEPT OF VENTILATION – PERFUSION RATIO, DEAD SPACE, SHUNT Speaker : Dr. S Sai Janani University College of Medical Sciences & GTB Hospital, Delhi

CONTENTS Ventilation – perfusion ratio Regional differences in ventilation Regional differences in perfusion Ventilation – perfusion ratio and anesthetic implications Dead space Shunt Anesthesia and dead space, shunt and V/Q mismatch Assessment of lung as an oxygenator

VENTILATION- PERFUSION RATIO Defined as the ratio between ventilation and perfusion in a lung unit. Ventilation(V) = 4 L/ min Perfusion(Q) = 5 L / min V/Q = 0.8 (young adult) Differs in different lung regions due to regional differences in ventilation and perfusion.

Regional differences in ventilation: Compliance of the alveolus MORE LUNG VOLUME AT THE BASE PRESSURE SUPPORT NEEDED AT BASE > APEX

Regional differences in ventilation Lung is suspended from the hilum The weight of the lung needs more pressure support from below than above Pressure at the base > apex Intra pleural pressure is less negative at the base and more negative at the apex

Regional differences in Ventilation IN AWAKE ERECT POSITION AT APEX: INTRAPLEURAL PRESSURE IS MORE NEGATIVE DURING INSPIRATION, WHEN-VE THORACIC PRESSURE IS CREATED ALVEOLI NEAR APEX MAXIMALLY INFLATED ∆PRESSURE BASE > ∆PRESSURE APEX Capacity to inflate> base than the apex. Thus due to the larger pressure gradient at the base than at the apex, the ventilation is greater at the base. VENTILATION BASE>APEX

Compliance……. Thus ventilation is always better in the dependent portion than in the non dependent portion in the awake individual. In the erect position, the base is the dependent portion. Hence in the erect position, ventilation at the base is > apex

Regional differences in ventilation SUPINE POSITION: Dorsal region is dependent. Hence, ventilation is better dorsally than ventrally. Thus, the diaphragm is DESIGNED TO HAVE GREATER EXCURSIONS POSTERIORLY than anteriorly. However, the bulk of the lung volume lies posteriorly, and this has to bear the weight of the entire anterior ans middle mediastinum too.

Ventilation in anesthetised pt. FRC < CV. THE ALVEOLI IN THE DEPENDENT REGION COLLAPSE 2. THE PREFENTIAL DORSAL EXCURSION OF DIAPHRAGM IS LOST IN PARALYSIS

Atelectasis after anesthesia in the dependent parts Augmented when higher FiO2 is used or during pre – oxygenation ABSORPTION ATELECTASIS

Ventilation in anesthetised pt THE VENTILATION OF NON DEPENDENT PART > DEPENDENT PART IN SUPINE : ANTERIOR > POSTERIOR IN LATERAL DECUBITUS: NONDEPENDENT LUNG > DEPENDENT LUNG

REGIONAL DIFFERENCES IN PERFUSION Primary determinant – GRAVITY Healthy adult – erect position: Distance from apex to base = 30 cm Assuming that pulm. Artery enters the lung midway b/w apex and base - 15 cm H20 ( overcome) PASCAL’S LAW P = hῥg + 15 cm H20(gain) PERFUSION AT BASE > APEX

Regional differences in perfusion

Regional differences in perfusion Based on the differences in perfusion, the lung is divided onto West’s zones: Alveolar pressure is constant from apex to base If the pulm. Artery system is considered a continuous column of blood, the pressure exerted by the column above is greater at the base than at the apex (by 30cmH2O)

Regional differences in perfusion: The West’s Zones ZONE I : MINIMAL BLOOD FLOW DEPENDS ON a-v DIFFERENCE

Regional differences in perfusion: The West’s Zones ZONE I : MINIMAL BLOOD FLOW DEPENDS ON a-v DIFFERENCE INDUCTION OF ANESTHESIA. ↓QT , “WASTED VENTILATION” ,ALVEOLAR DEAD SPACE

Regional differences in perfusion: The West’s Zones ZONE II: WATERFALL EFFECT DEPENDS ON a-A DIFFERENCE

Regional differences in perfusion: The West’s Zones ZONE III: HIGH PERFUSION DEPENDS ON a-v DIFFERENCE

Regional differences in perfusion Supine position: West’s zones

VENTILATION – PERFUSION RATIO ↑ PERFUSION > ↑ VENTILATION FROM APEX TO BASE V/ Q AT APEX > V/Q AT BASE

Types of alveoli based on V/Q SHUNT DEAD SPACE

Ventilation –Perfusion ratio SHUNT DEAD SPACE Due to the presence of a spectrum, the v/q of the lung is non uniform. This has a profound influence on the arterial blood gases. PHYSIOLOGICALLY THESE UNITS ARE NOT ABSOLUTE AND OCCUR AS A SPECTRUM

Arterial blood gases and V/Q SHUNT 02 = 103 Co2 = 0 O2=40 Co2 = 45 DEAD SPACE 02 =100 Co2=40 02 = 103 Co2 = 0 Thus the arterial oxygenation depends on the number of normal alveoli, v/q mismatch , amount of shunt and dead space.thus inc. in shunt or deadspace will reduce arterial oxygenation.this is important for us because anesthesia changes the balnce between the

Regional differences in gas exchange

MEASUREMENT OF V/Q Regional differences- Measured by radioactive gases: Xe Single and multiple N2 breath tests By measuring (A-a)PO2 PAO2measured by alveolar gas equation PAO2 = PIO2 – PACO2 + F R PACO2 = PaCO2

MULTIPLE BREATH TEST After this we now go to 1 end of the spectrum – that is dead space

Hypoxic Pulmonary Vasoconstriction Adaptation of pulmonary blood flow to ventilation. Constriction of pulm. Blood vessels in areas Of reduced ventilation GA attenuates HPV > inhalational agents Less attenuation when Thiopentone is used for induction

DEAD SPACE That portion of the respiratory tree in which no gas exchange takes place V / Q = ∞ Volume occupied by the conducting system. Represents “wasted ventilation”

DEAD SPACE Types: Anatomical dead space (Vd anat) Physiological dead space (Vd phys) Apparatus dead space

Dead space Anatomical dead space: The volume of the respiratory passages, extending from nostrils and mouth down to (but not including) the respiratory bronchioles. Varies with age and sex Normal = 150 mL Young women = 100 mL Old men = 200 mL Normally = 1 mL / lb body weight or 2.2 mL / kg body weight

Dead space….. Anesthesia and Anatomical dead space: Head tilt- chin lift (+40 mL) Depression of jaw with flexion of head (Airway obstruction) = - 30 mL Tracheostomy / Pneumonectomy = ↓

Measurement of anatomical dead space Modification of FOWLER’S Single breath N2 wash out method

Dead space Physiological dead space (Vd phys): Fraction of tidal volume not available for gas exchange. Anatomical + Alveolar dead space Alveolar dead space = ↑ V/Q regions = wasted ventilation

Physiological dead space Normally, Anatomical dead space = Physiological dead space = 1/3 tidal volume Expressed as that fraction of tidal volume that contributes to “wasted ventilation” (Vd / Vt) Vd / Vt = 0.25 – 0.4

Vd phys Factors affecting Vd phys: Old age Upright position ↑ Tv ↑ RR Atropine T inspi < 0.5 secs during CMV ↑V/Q states Hypotension ( esp. with head up position)

Measurement of Physiological Dead space PHYSIOLOGICAL DEAD SPACE AND ANESTHESIA: Roughly estimated by Cooper’s formula: Measurement of Vd phys: (Vt – Vd)PACO2 = PeCO2 X Vt OR (subtract apparatus dead space) Vd/ Vt = 33 + age /3 % Vd phys = PaCO2 – PeCO2 PaCO2 ENGHOFF’S MODIFICATIONOF BOHR EQUATION

Bohr Equation

Dead space… Vd phys and the capnogram: (Non invasive measurement) (PaCO2 – ETCO2) gradient increases when there is significant Vd alveolar thus increasing vd phys Hence at that time, ETCO2 is an unreliable monitor for PaCO2 This is seen in conditions like COPD esp. emphysema where there is inc. in alveolar dead space due to air trapping.

PaCO2 PeCO2 PaCO2 PeCO2 PaCO2 PeCO2

Dead space APPARATUS DEAD SPACE: Volume of gas contained in any anesthetic apparatus between the patient and that point in the system where rebreathing of exhaled carbon dioxide ceases to occur

Dead space Anatomical face mask = ↑ Vd / Vt= 0.68 COPA or LMA = Vd/Vt = 0.3 – 0.4 ETT or Tracheostomy = ↓Vd/ Vt Breathing circuits – Length Diameter Compressible volume ( 2 -3 mL gas for every 1 cmH2O inspiratory pressure)

Dead space Pathologies producing dead space : Anatomic: Rapid shallow breathing Alveolar dead space: Acute pulmonary embolus Redistribution of pulmonary perfusion: ↓cardiac output Acute pulm. Hypertension Ventilation > Perfusion PPV Alveolar septal destruction

SHUNT Defined as blood that enters the arterial system without going through ventilated areas of the lung.

Shunt….. Types: Anatomical shunt: “True shunt” due to area in which there is absolutely no ventilation, but perfusion exists. Physiological shunt: Normal degree of venous admixture due to true shunt + ↓V/Q ratio Pathological shunt: Those forms of anatomical shunt which are not present in a normal subject. Atelectatic shunt: Blood which has passed through collapsed regions of the lung.

Anatomical shunt NORMAL ABNORMAL Extra pulmonary Thebesian veins CnHD with R – L shunt Pulmonary Bronchial veins Atelectasis Pulm infection Pulm AV shunts Pulm neoplasm Circn in edematous lung

Physiological shunt PAO2 = 101 PaO2 = 97 (A- a) O2 = 5- 25 mmHg

SHUNT PAO2 = 101 AMOUNT OF MIXED VENOUS BLOOD ADDED INORDER TO PRODUCED AN OBSERVED (A-a) – VENOUS ADMIXTURE PaO2 = 97 (A – a) PO2 = 5 – 25 mmHg

Shunt……….. Sources of venous admixture (5% CO) ↓V/Q ratio Shunt

Shunt………. OXYGENATION IN ↓V/Q AREAS – IMPROVES WITH ↑FiO2 ↓V/Q RATIO SHUNT OXYGENATION IN ↓V/Q AREAS – IMPROVES WITH ↑FiO2 SHUNT IS REFRACTORY TO ↑FiO2

Calculating the shunt fraction….. The shunt equation is derived as: Pulmonary capillary blood flow (Qc) + blood flow through shunt(Qs) = cardiac output(Qt) In terms of O2 content: ( CcO2 X Qc)+(CvO2 X Qs) = (CaO2 XQt) Qc + Qs = Qt

The shunt equation Since, Qc = Qt – Qs, (CcO2 X Qt) – (CcO2 X Qs)+ (CvO2 X Qs) = (CaO2 X Qt) Dividing both sides by Qt, Qs = CcO2 – CaO2 Qt CcO2 - Cvo2

Shunt equation Inorder to remove CvO2 from the equation, We add and subtract CaO2 in the denominator. Qs = (CcO2 – CaO2) Qt (CcO2 – CvO2 + CaO2 –Cao2) Since CaO2 – CvO2 = 5 in non critical patients, And 3.5 in critical patients, we substitute in the equation to get the Estimated shunt Equation

Estimated physiologic shunt equation…… Does not need a pulm. Artery sample. Non critical patients: (Spontaneously breathing,mod. FiO2, mod. CPAP) Estimated Qs= (CcO2 – CaO2) Qt [5+(CcO2-CaO2)]

Estimated shunt equation….. For critical patient: (Mechanical ventilation, High FiO2, high level of positive end-expiratory pressure) Estimated Qsp = (CcO2 –CaO2) Qt [3.5 + (CcO2 – caO2)]

Shunt Equation…. <10% 10 -20% 20 -30% >30% PHYSIOLOGIC SHUNT INTERPRETATION <10% NORMAL 10 -20% MILD 20 -30% MODERATE >30% SEVERE

Shunt…..

Shunt producing pathology… Anatomic: 1.Congenital heart disease 2.Pulmonary fistula 3.Vascular lung tumors Capillary shunting: 1.Acute atelectasis 2.Alveolar fluid 3.Consolidation Perfusion > Ventilation 1.Hypoventilation 2.Uneven distribution of ventilation 3.Diffusion defect

Anesthesia and V/Q mismatch, shunt and dead space BEFORE INDUCTION: AWAKE SUPINE SUBJECT HPV Dead space = Anatomical dead Space Minimal shunt Ventilation perfusion relationships during anesthesia. Hedenstierna G, Thorax 1995; 50: 85 -91

After induction, with anatomical face mask and inhalational agent ANESTHESIA INDUCED ANATOMICAL FACE MASK INHALATIONAL AGENT: HPV attenuated Absorption atelectasis- ↑ shunt Hypotension by induction agent- ↑ alv Dead space Face mask – ↑ apparatus dead space ↑ dead space, shunt and V/Q mismatch areas Arterial oxygenation compromised

After endotracheal intubation and mechanical ventilation TRACHEA INTUBATED ON MECHANICAL VENTILATION Dead space reduced to awake levels Slight widening of V/Q distribution Shunt almost the same

Shunt and V/Q mismatch in anesthetised and awake individuals

ASSESSMENT OF LUNG AS AN OXYGENATOR Clinical assessment: Level of consciousness ~ adequate cerebral oxygenation Vital signs: RR, HR, Blood pressure, temp. General physical examination: Head and neck: Inspection of face – any nasal flaring cyanosis pursed lip breathing Neck: position of trachea Shifted in upper lobe collapse pneumothorax, pleural effusion lung tumor

Inspection: Shape of the chest Kyphosis Scoliosis Flattening Over inflation Measurement of chest: Rate of respiration Rhythm Chest expansion Symmetry Percussion: Resonance/ dullness Auscultation: Breath sounds, adventitious sounds

Lung as an oxygenator…….. ABG: PaO2 = 80 – 100 mmHg paCO2 = 35 -45 mmHg P(A-a)O2 = 5- 25 mmHg (↑ V/Q mismatch, shunt) PaO2/ FiO2 = 100 /0.2 = 500 O2 content = 19.8mL/dl blood SpO2 ETCO2 V/Q mismatch shunt

Lung as oxygenator: Oxygenation indices: PaO2 (A – a) PO2 PaO2 / FiO2 PaO2 / FiO2 x Paw Pao2 / PAO2 PaO2 / PvO2

Bibliography 1.Respiratory Physiology- John.B.West 7th ed 2.Egan’s Respiratory Care- 4 th ed 3.Miller’s Anesthesia- 5th ed 4. Textbook of Anesthesiology- Morgan 5 th ed 5. Applied Respiratory Physiology- J.F.Nunn 6. Textbook of Anesthesia- gray and Nunn 7.Mechanical ventilation- Chang- 4th ed 8.Clinical Application of Blood Gases- Shapiro- 5thed. 9.understanding Anesthesia equipment- dorsch & dorsch- 7th ed. 10.A practice of Anesthesia-Wiley 5 th ed. 11.review of Medical Physiology – W.F. Ganong 12.V/Q distribution and correlation of atelectasis in anesthetised paralysed humans- JAP 13. ventilation – perfusion ratio inAnesthesia –hedenstierna et al – Thorax- 1995 ; 50: 85- 91

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