Oxygen, carbon dioxide, breathing and hypoxia J Albrett House surgeon teaching 2016

Aims To develop an appreciation of problems with oxygenation and ventilation. A stepwise approach to diagnosis and treatment of respiratory failure. To recognise severe abnormalities and when to call for help.

Firstly the basic physiology Oxygenation is primarily determined by FiO2 and airway pressure Failure to oxygenate = Type 1 respiratory failure PaO2 < 60 mmHg when FiO2 21% (air) & Barometric pressure 760mmHg (sea level) No intracardiac shunt Failure to ventilate = Type 2 respiratory failure PaCO2 > 50 mmHg unless primary metabolic alkalosis

Oxygen

Oxygen saturation

ATSP 65 year old lady with pneumonia On 6L/min oxygen via hudson mask. What is her FiO2? What is the airway pressure?

Delivery of oxygen Class Device Oxygen flow (L/min) Approx FiO2 Comments Variable delivery Nasal prongs 2 4 6 28% 35% 45% Stable patients only Semi-rigid (Hudson mask) 5 8 10 12 50% 55% 60% 65% Low cost Frequently used Not accurate at controlling FiO2 Reservoir plastic mask 6-15 FiO2 21% + 4% per L/min 6L/min 15L/min 81%

Delivery of oxygen Class Device Oxygen flow (L/min) Approx FiO2 Comments Fixed delivery Venturi mask 2-8 24-50% Reliable fixed delivery but more expensive CPAP/BiPAP High flow or turbine flow > peak inspiratory flow Reasonably accurate control of FiO2

You decide to put her on an airvo system? What is the FiO2 possible? How accurate? What is the airway pressure?

AIRVO system on wards

OPTIFLO system in HDU/ICU

Airway pressure in optiflo

You are asked to review a 22 year old man post appendix surgery 12 hours ago. He is hypoxic! What is hypoxia?

Hypoxia

Introduction Definition Mechanisms Measurement Treatment

Hypoxia Inadequate oxygen supply for tissues Local General Sat 95%, fall >5% with exercise. In context

Outline a system for assessing and treating hypoxia.

Mechanisms I start at the wall... Inadequate supply, check pressure, flow, connection, tubing Airway Patency Suction ETT blockage, valves etc Coma Aspiration

Blocked airway

Jaw thrust

Pharyngeal airway

OPA

Mechanisms Air to alveolus Bronchospasm Foreign body Hypoventilation Nitrous oxide

Asthma Can get air in but can’t get it out

Mechanisms Alveolar/capillary Pulmonary oedema Fibrosis Consolidation Atelectasis

Pulmonary capillaries Pulmonary embolus Shunt (Intra-cardiac, V/Q mismatch)

Tissues Compartment syndrome / limb ischaemia Dysoxia Sepsis Cyanide toxicity

Measurement of oxygen Clinical exam Pulse oximetry Blood gas

Clinical exam Cyanosis Associated with very low oxygen saturations Need 5g/dL ie oxygen saturation of 67% !!

Pulse oximetry Rapid Easy to use Non-invasive But not always reliable We need to know and anticipate sources of error

Pulse oximetry Looks at the pulsatile component of tissue Measures change in quantity of absorbed light. Assumes any change must be due to arterial blood (which pulses)

Non-patient error Mal-position / off Interference from external light sources Motion artefact

Patient sources of error Hypoperfusion Hypothermia Venous pulsation Nail polish Methylene blue Abnormal haemoglobin Carboxyhaemaglobin (false high) Methaemoglobin (false low)

Arterial blood gas Invasive (made easier with arterial line) Measures pO2 therefore can indicate too much oxygen Measures pCO2 so can indicate adequacy of ventilation Measures Hb, HbO2, MetHb, Hb-CO

Oxygenation Vs Ventilation Ventilation: Process of inspiration and expiration exchanging gas from the lungs with the environment Oxygenation: Process on increasing oxygen to a tissue

Time critical processes Consider: The pulse oximeter takes an average of the last six pulsations. Ventilation can fall but increasing inspired oxygen results in no change to measured saturation. With preoxygenation we can maintain oxygen saturation for many minutes with no ventilation

FRC

FRC 70kg person FRC = 30mLx70kg FRC = 2.1 L With pre-oxygenation 80% oxygen in lungs = 1.6L oxygen If oxygen consumption is 200mL/minute MEANS 7 minutes before patient desaturates.

Special patients Emphysema Obesity Sepsis Larger lung volumes therefore more time but slower to achieve preoxygenation Obesity Smaller lung volumes therefore less time Sepsis Higher oxygen consumption therefore less time

Ventilation Driven by CO2 Simple measures Sit the patient up Check LOC +/- jaw thrust Apply oxygen

NIV CPAP, BiPAP Indications COPD with acute CO2 retention Heart failure OSA ?Hypoxia (not really)

NIV Higher airway pressures improves oxygenation It makes sense to apply BPAP in order to increase oxygenation BUT patient still unstable, may only delay respiratory arrest Patient effectively sicker at point of arrest Better to involve expert team. Never initiate on ward.

Compression only CPR As good as/maybe better than conventional CPR Layperson Sole rescuer NOT Children Arrest due to hypoxia

DRSABC D Dangers? R Responsive? S Send for help A Open Airway B Normal Breathing? C Start CPR 30 compressions : 2 breaths if unwilling / unable to perform rescue breaths continue chest compressions C December 2010