Mechanical Ventilation & Strategies for Oxygenation

1. Mechanical Ventilation & Strategies for Oxygenation Dawn Oddie

2. What are we going to talk about? • Physiology • Ventilation classifications • Types of Ventilation • Optimising Oxygenation • Complications of Ventilation • Weaning from Ventilation

3. Physiology

4. Where it all happens! 300 million alveoli

5. Physiology of normal breathing I:E ratio times How do we breathe? Negative pressure Active inspiration Passive expiration Respiratory rate High lung volume (Inhalation) Low lung volume (Exhalation) / Functional residual capacity Tidal volumes

6. How is normal breathing controlled How do you know, • Rate - How fast / slow to breathe? • Tidal volume - How big a breathe to take in? • I:E ratio – How long to breath in / out for? • When to cough / sneeze?

7. Nervous Control / Chemical • Respiratory centre. Reticular formation – brain stem • Medullary rhythmicity area • Pneumotaxic area / Apneustic area (transition from I to E) • Inflation (Hering-Breuer) reflex - Stretch receptors • Cortical influences – cerebral cortex giving some voluntary control eg hold breath underwater • Central chemosensitive area (pH / H+) – Medulla • Peripheral chemoreceptors (CO2 / O2 / H+) – carotid bodies • Proprioceptors – joints / muscles • Other influences – Baroreceptors / Temp / Pain / stretching the anal sphincter muscle / Irritation of the air passages

8. Lung Volumes

9. Respiratory Mechanics- Compliance • Compliance is ΔV/ΔP (Change in Volume / change in pressure) • Total lung is made up of thoracic and lung compliance • Pulmonary compliance (or lung compliance) is the ability of the lungs to stretch during a change in volume relative to an applied change in pressure. • Compliance is greatest at moderate lung volumes, and much lower at volumes which are very low or very high. LIP and UIP can be good guides • Pulmonary Surfactant increases compliance by decreasing the surface tension of water. The internal surface of the alveolus is covered with a thin coat of fluid. The water in this fluid has a high surface tension, and provides a force that could collapse the alveolus. The presence of surfactant in this fluid breaks up the surface tension of water, making it less likely that the alveolus can collapse inward. If the alveolus were to collapse, a great force would be required to open it, meaning that compliance would decrease drastically.

10. Respiratory Mechanics- Compliance • Low compliance indicates a stiff lung and means extra work is required to bring in a normal volume of air. This occurs as the lungs in this case become fibrotic, lose their distensibility and become stiffer. • In a highly compliant lung, as in emphysema, the elastic tissue has been damaged, usually due to their being overstretched by chronic coughing. Patients with emphysema have a very high lung compliance due to the poor elastic recoil, they have no problem inflating the lungs but have extreme difficulty exhaling air. In this condition extra work is required to get air out of the lungs.

11. Causes of Decreased Intrathoracic Compliance

12. Some Important Physiology • V/Q Mismatch • Oxygen Cascade • Oxyhaemoglobin Dissociation Curve • Spirometry Trace

13. Supply and demand • V/Q mismatch • V = Ventilation • P = Perfusion • Hypoxic Pulmonary Vasoconstriction • Functional alveoli • Permeable membranes • Circulating volume – with • Adequate haemoglobin levels • Oxygen saturation of haemoglobin (affinity) • Oxygen dissociation • Perfusion pressure

14. When room air just isn’t enough….. • Increased metabolic demand • V/Q mismatch • Damaged alveoli / airways • Blocked alveoli • Inadequate circulation

15. Some indications to increase O2 • Acute respiratory failure eg pneumonia, asthma, pulmonary oedema, pulmonary embolus • Acute myocardial infarction • Cardiac Failure • Shock • Hypermetabolic states eg major trauma, sepsis, burns • Anaemia • Carbon monoxide poisoning • Cardio respiratory resuscitation • During / post anaesthesia • Pre-suction • Suppressant drug eg narcotics • Pyrexia (Oxygen consumption increases by 10% for each degree rise)

16. Effect of insufficient oxygen • Reduced oxygen supply leads to cellular shift from aerobic to anaerobic metabolism • Production of lactic acid • Increasing metabolic acidosis • Low pH • Low HCO3 • Negative base excess • Cell death / system wide failure

17. What is oxygen • What percentage of oxygen is in atmospheric air? In normal circumstances with a average respiratory rate sufficient to meet metabolic demands Oxygen delivery (mls O2/min) = Cardiac output (litres/min) x Hb concentration (g/litre) x 1.31 (mls O2/g Hb) x % saturation • Oxygen Consumption = 200 - 250 mls / min

18. Haemoglobin

19. Haemoglobin • Intracellular protein contained within erythrocytes (red blood cells) • Made up of 2 pairs of polypeptide chains (2Alpha, 2 Beta), each bound to a haem group that contains iron. Each molecule of haemoglobin can combine with 4 molecules of oxygen • Primary vehicle for oxygen transportation in the blood (small amount in plasma Approx 1.5-3%) • Each haemoglobin molecule has a limited capacity for holding oxygen molecules. How much of that capacity that is filled by oxygen bound to the haemoglobin at any time is the oxygen saturation (SaO2)

20. Haemoglobin • Average 70Kg Adult = 900g of circulating haemoglobin (Hb 14-18g/dl) • 1g Haemoglobin can carry 1.34ml oxygen Example, 10g/dl with an average 5l circulating volume = 500g total body haemoglobin If fully saturated 500 x 1.34 = 670ml of oxygen (Only approx 25% unloads leaving venous sats (SvO2) 70-75% - useful in times of higher metabolic demand etc

21. The transfusion debate… Risks of vs Reduced oxygen transfusion carrying capacity

22. Factors affecting carriage • Timing of haemoglobin uptake and release of oxygen affected by, • Partial pressure of oxygen (PaO2) • Temperature • Blood pH • Partial Pressure of Carbon dioxide (PaCO2)

23. Partial Pressure - effect of Altitude • At sea level we live under a layer of air that is several miles deep – the atmosphere. The pressure on our bodies is about the same as 10 metres of sea water pressing down on us all the time. At sea level, because air is compressible, the weight of the air around us compresses making it denser. As you go up a mountain, the air becomes less compressed and therefore thinner.

24. Partial Pressure - effect of Altitude The important effect of this decrease in pressure is: in a given volume of air, there are fewer molecules present. The percentage of those molecules that are oxygen is exactly the same: 21%. The problem is that there are fewer molecules of everything present, including oxygen. So why is this an issue?

25. Partial Pressure of gases • In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture. Dalton's law (also called Dalton's law of partial pressures) states that the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture.

26. Partial Pressure • Partial pressure (PP) is a way of describing how much of a gas is present. All gases exert pressure on the walls of their container as gas molecules bounce constantly of the walls • PP is also used to describe dissolved gases. In this case, the PP of a gas dissolved in blood is the PP that the gas would have, if the blood were allowed to equilibrate with a volume of gas. When blood is exposed to fresh air in the lungs, it equilibrates almost completely so that the PP of oxygen in the air spaces in the lungs is equal to the partial pressure of oxygen in the blood. • PP of arterial blood is slightly less than PP of oxygen in lungs – due to physiological shunt (some blood passing through lungs without encountering an air space)

27. Partial Pressure of gases • The partial pressure of a gas dissolved in a liquid is the partial pressure of that gas which would be generated in a gas phase in equilibrium with the liquid at the same temperature. The partial pressure of a gas is a measure of thermodynamic activity of the gas's molecules. Gases will always flow from a region of higher partial pressure to one of lower pressure; the larger this difference, the faster the flow. • Gases dissolve, diffuse, and react according to their partial pressures, and not necessarily according to their concentrations in a gas mixture.

28. Oxygen dissociation curve Dissociation curve relates oxygen saturation of Haemoglobin (Y axis) and partial pressure of arterial oxygen (X axis) in the blood

29. Dissociation curve explained • Extent of oxygen binding to haemoglobin depends on PaO2 of blood, but relationship not precisely linear • Slope steeply progressive between 1.5 – 7kPa (area of most rapid uptake and delivery of oxygen to and from haemoglobin), then plateaus out between 9 – 13.5kPa • Haemoglobin almost completely saturated at 9kPa – further increases in partial pressure of oxygen will result in only slight rises in oxygen binding

30. Oxygen dissociation curve • The partial pressure of oxygen in the blood at which haemoglobin is 50% saturated (26.6mmHg) is known as the P50 • P50 is conventional measure of haemoglobin affinity for oxygen • Increased P50 indicates a right shift of the standard curve – meaning larger partial pressure necessary to maintain a 50% oxygen saturation

31. Oxygen dissociation curve Increased affinity Reduced Affinity

32. To the right, Hyperthermia Acidosis (pH) Increased pCO2 Endocrine disorders Curve shifts to left, Hypothermia Alkalosis Decreased pCO2 Carbon monoxide Factors influencing the position of oxygen dissociation curve Generally a shift to the, Right will favour unloading of oxygen to the tissues Leftwill favour reduced tissue oxygenation

33. To the right As pH declines (acidosis) the affinity of haemoglobin for oxygen reduces. Result – less oxygen is bound while more oxygen is unloaded Bohr effect To the left Temperature – as body temp falls the affinity of haemoglobin for oxygen increases. Result – more oxygen is bound while less oxygen is unloaded Factors influencing the position of oxygen dissociation curve - explained

34. mmHg vs. kPa • Both measures commonly in use • The kiloPascal: A pressure of one thousand pascals (1 kPa) is about 10.2 cm H2O or about 7.75 mmHg. • Atmospheric pressure is about 1034 cmH2O or 101.9 kPa. The useful approximations are 1000 cm H2O or 100 kPa. • mmHg to kPa: To convert pressure in mmHg to kPa, divide the value in mmHg by 7.5. Eg. • 60mmHg = 8.0kPa • 30mmHg = 4.0kPa

35. The oxygen cascade • Transport has three stages (steps), • By gas exchange in the lungs • Partial pressure gradient of oxygen (PaO2) in alveoli 13.7kPa • Partial pressure gradient of oxygen (PaO2) in pulmonary capillaries 5.3kPa • Transport of gases in the blood • Partial pressure gradient of oxygen (PaO2) in arterial blood 13.3kPa • Movement from blood into the tissues • Partial pressure gradient of oxygen (PaO2) in tissues 2.7kPa • Mitochondrial pressure 0.13-1.3kPa

36. Oxygen delivery to tissues…. • The amount of oxygen bound to the haemoglobin at any time is related to the partial pressure of oxygen to which the haemoglobin is exposed. • Eg in lungs at the alveolar-capillary interface, partial pressure of oxygen is high so oxygen readily binds. As the blood circulates to other body tissue in which the partial pressure of oxygen is less the haemoglobin releases the oxygen into the tissues. • Haemoglobin cannot maintain its full bound capacity in the presence of lower oxygen partial pressures.

37. Supplementing Oxygen • Nasal cannula • Fixed performance mask • Variable performance mask • Non rebreathe reservoir • Tracheostomy mask • Tents / head boxes • Bag valve mask • CPAP – nasal / facial or hood • BiPAP – IPAP / EPAP • Intubation and mechanical ventilation

38. Indicators for initiating mechanical ventilation?

39. Types of positive pressure ventilation • Non invasive • Invasive

40. CPAP / PEEP / EPAP • Pressure applied at end of expiration to maintain alveolar recruitment • Airway pressure kept positive Beware of gas trapping (autoPEEP) in non compliant lungs

41. CPAP

42. NIV - BiPAP • IPAP / PS / ASB • Inspiratory assistance with each spontaneous breath • EPAP • Expiratory resistance

43. The science of mechanical ventilation is to optimise pulmonary gas exchange; the art is to achieve this without damaging the lungs

44. What is a Mechanical Ventilator? • Generates a controlled flow of gas in and out of a patient • Inhalation replenishes alveolar gas • Balance needed between O2 replenishment and CO2 removal

45. Ventilators – What do they need to do… • Mechanical ventilators are flow generators • Must be able to, • Control • Cycling • Triggering • Breaths • Flow pattern • Mode or breath pattern

46. Ventilator strategy • Aim to achieve adequate minute volume with the lowest possible airway pressure

47. Ventilator Classification Control • How the ventilator knows how much flow to deliver • Can be, • Volume controlled (volume limited, volume targeted) & pressure variable • Pressure controlled (pressure limited, pressure targeted) & volume variable • Dual controlled (volume targeted (guaranteed) pressure limited

48. Ventilator Classification Cycling • How the ventilator switches from inspiration to expiration (the flow has been delivered – how long does it stay there?) • Time cycled e.g. pressure controlled ventilation • Flow cycled e.g. pressure support • Volume cycled. The ventilator cycles to expiration once a set tidal volume has been delivered.

49. Ventilator Classification Triggering • What causes the ventilator to cycle to inspiration. Ventilators may be…… • Time triggered • Cycles at set frequency as determined by the rate • Pressure triggered • Ventilator senses the patients inspiratory effort by sensing a decrease in baseline pressure • Flow triggered • Constant flow through circuit – flow-by. Ventilator detects a deflection or change in this flow. Requires less work from the patient than pressure triggered

50. Ventilator Classification Breaths • Mandatory • (controlled) – determined by the respiratory rate • Assisted • E.g. synchronised intermittent mandatory ventilation (SIMV) • Spontaneous • No additional assistance during inspiration e.g. CPAP