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Ventilation: Physiology Made Simple (although may still be a little boring) Dr M Bloch Anaesthetics Consultant NHSG.

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Presentation on theme: "Ventilation: Physiology Made Simple (although may still be a little boring) Dr M Bloch Anaesthetics Consultant NHSG."— Presentation transcript:

1 Ventilation: Physiology Made Simple (although may still be a little boring) Dr M Bloch Anaesthetics Consultant NHSG

2 Overview Why? Basic anatomy, breathing basics & basic physiology Oxygen delivery

3 Why? To develop a foundation of knowledge of the physiology of the respiratory system that will enable systematic and considered approach to the care of the critically ill patient. To understand the impact of acute and chronic pathological processes on their presentation and treatment.



6 DDANGER (scene, self, patient) 4 DIMENSIONS: up/done, front/back, left/right & time RRESPONSE (Patient, Help) RELATION (MOI, SAMPLE) Reassure Reassess  Hc AcAIRWAY with C-SPINE  O 2  ‘do airway think spine’ Safe, maintainable / at risk (position / manoeuvres / adjuncts), unmaintainable (secure / definitive) Effort (signs of  WOB esp. RR with  TV, recession), Efficacy (depth / AE / O 2 / CO 2 ), Effect. (HR / LOC / colour / hyponia /  FiO 2 ), Exhaustion ‘twelve FLAPS’ – open / expanding PTx, flail, HTx. NGT A&B on scene – good 1 st principles BBREATHING CCIRCULATIONAssess: clinical vs. measured (cough, movement, LOC, pulse, CFT, RR, temp. difference [creeping proximally], colour [pink, pale, mottled], urine vs. SaO 2, BP, ETCO 2, BD, lactate) ‘On the floor and four more’. Manage: Controllable vs. uncontrollable haemorrhage: arrest & fluids. C en-route DDISABILITYDRUGS:  analgesia, & sedation  appropriate antibiotics DON’T EVER FORGET GLUCOSE AVPU (GCS + modifications), PEARL, focal, recurrent seizures, ‘protecting airway’ (prevent 2 0 injury) E EVALUATE EMOBILISE EVACUATE EXPOSURE 1 0  2 0 survey vs. ‘time-critical’ Hypothermia F ‘FEELING’Pain, anxiety and fear G GUIDANCE‘ask for help’ H HARMe.g. NAI I IMMUNE / INFECTION

7 Oxygen demand and supply:  Supply of O 2 to the tissues requires the integration & regulation of 3 systems:  the lungs  the blood  the circulation A B C

8 Myocardial O 2 Supply vs. demand Decrease O 2 delivery  Decreased coronary flow Tachycardia Diastolic hypotension Hypocapnia (vasoconstriction) Coronary artery spasm Increased preload / afterload (wall tension)  Decreased O 2 content Anaemia Arterial hypoxaemia Shift of ODC to left Increased O 2 demand  Sympathetic stimulation Tachycardia Increased myocardial contractility Increased preload / afterload (wall tension)

9 Some ANATOMICAL differences are more difficult to deal with than others…

10 Anatomy (contd.)

11 Anatomy Trachea Right & Left main bronchus Lobar, segmental and terminal bronchii Respiratory bronchioles Alveolar ducts Alveolar sacs Conducting airways Deadspace of ~150ml Gas moves by inspiration Respiratory airways. Just a few millimeters thick Contain 2-3L at rest = most of the lung volume Gas movement is by diffusion, Occurs within a second 1 16 23

12 Structure and innervation Thoracic structure –12 pairs of ribs. –Articulate with vertebrae posterior. –Drop inferior at anterior aspect at rest in expiration. –Articulate at the front with the sternum via costal cartilage. –Inferior aspect of thoracic cage bordered by diaphragm.

13 Innervation –Diaphragm: phrenic nerve C3-5 used in quiet respiration. –Intercostal nerves on inferior aspect of rib supplies the intercostal muscles which are used in forceful respiration. –Direct lung innervation limited because of paucity of fibres. –Sensitive to humoral influences.

14 Compliance: Volume and pressure relationship

15 respiratory centres in medulla chemoreceptors on aorta and carotid artery heart brain intercostal nerve to external intercostal muscles phrenic nerve to diaphragm diaphragm ribs Control of Respiration

16 The breathing basics

17 Magill video

18 Lung volumes VT (tidal volume) 500ml Anatomic deadspace 150ml Alveolar gas volume 3000ml Ventilatory rate ~15 Total ventilation 7.5L/min Alveolar ventilation = (500-150).15 = approx 5L/min = Cardiac output.


20 E ffort E fficacy E ffect

21 V A /Q mismatch is an inequality in either ventilation or perfusion. Ratio of ventilation to perfusion varies Range from zero to infinity! Ideal ratio is 0.8 (0.5-2.0) Ventilation-perfusion mismatch and the alveolar-arterial pO 2 difference.

22 V/Q mismatch Ventilation increases gradually as you move from apex to base Perfusion poor in apices better at bases V/Q ratio decrease as you move down from apices A) Well-ventilated poorly perfused (V>Q) B) Underventilated well perfused (V<Q) C) Optimally ventilated + perfused (V=Q)


24 Gas exchange

25 Ideal Alveolar Gas Equation. Clinically Useful Form: Complete Form:

26 A-a Difference P A O 2 - P a O 2  A-a = FiO2 – PaCO2 – PaO2 Normally up to +/- 25 mmHg –Because of normal anatomical shunt –Ventilation/Perfusion mismatching. A-a difference increases with pulmonary disease. 21 13 516 50 5 12 45 80 575 18 33357

27 a/A ratio Normally averages just over 0.8 a/A ratio falls with pulmonary disease. Lower limit normal: –young (room air) :0.74 –older (room air) :0.78 –Both groups (100% O 2 ):0.82


29 Oxygen carriage Is either carried bound to Hb or dissolved  Oxygen dissociation curve Henry’s Law for dissolved blood –Amount of oxygen dissolved in the blood is equal to the partial pressure of oxygen –For each mmHg pO2, there is 0.003ml 02/100ml –Therefore at 100mgHg (alveolar pO2), there is 0.3ml O2/100ml blood.

30 Oxygen flux = cardiac output x arterial oxygen content Oxygen flux = C.O.   (1,39  Hb  SaO2) + (0,003  PaO2)  C.O. = cardiac output = stroke vol. X heart-rate where SV depends on preload, contractility and afterload Hb = haemaglobin SaO2 = saturation PaO2 = arterial partial pressure O2


32 Oxygen dissociation curve

33 The perfect lung – oxygen cascade showing V/Q Inequality Hypoventilation Shunt Diffusion 20 15 10 5 0 kPa Hypoxic mixture


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