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HyPoxia the basics

HyPoxia the basics. By deborah dewaay md Acknowledgment: antine stenbit md. Objectives. Knowledge: Understand the difference between hypoxia and hypoxemia Understand physiologic adaptation to hypoxia Understand how hypoxia causes cell death

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HyPoxia the basics

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  1. HyPoxiathe basics By deborahdewaay md Acknowledgment: antinestenbit md

  2. Objectives • Knowledge: • Understand the difference between hypoxia and hypoxemia • Understand physiologic adaptation to hypoxia • Understand how hypoxia causes cell death • Review the different modalities of providing oxygen to a patient • Know the differential diagnosis of acute hypoxia in the adult patient • Skills: • Use the algorithm to determine cause of hypoxia in a particular patient • Attitude: • Understand the importance of keeping the patient comfortable when they are hypoxic • Understand the importance of communicating compassionately with an acutely ill patient

  3. Key messages • In acute hypoxic respiratory failure, the patient should be put on a non-rebreather. • An ABG is crucial to the work up of hypoxia. • It is important to continue good communication with a patient during emergent situations like acute respiratory failure.

  4. Definitions • Hypoxia: a reduction of oxygen supply to a tissue below physiological levels despite adequate perfusion of the tissue by blood. • Hypoxemia: a decreased partial pressure of oxygen in blood less than 60mmHg on room air or less than 200mmHg on 100% oxygen.

  5. How O2 gets to tissues • Inhale oxygen • Enters alveoli • Crosses alveoli/capillary membranes • Diffuses into blood • Binds to hemoglobin • Is carried to tissues • Unbinds from hemoglobin • Tissues use oxygen

  6. definitions • Hypoxic hypoxia: arterial blood Po2is reduced • Pneumonia • Anemic hypoxia: arterial blood Po2is normal, but the amount of hemoglobin is too low to meet the tissues demands • Sickle cell disease • Ischemic hypoxia: arterial blood Po2is normal, but the blood flow is too impaired to meet the tissues demands • STEMI, compartment syndrome • Histotoxic hypoxia: arterial blood Po2is normal but a toxin is preventing the cells to utilize the O2 • Cyanide poisoning

  7. How hypoxia results in cell death • When there is diminished oxygen availability, the tissue changes to inhibit oxidative phosphorylation and increases anaerobic glycolysis • As a result less ATP is produced • Less ATP leads to in adequate energy to maintain ionic and osmotic equilibrium • The cell swells • Result: cell death

  8. Adaptations to hypoxia • Hypoxia causes: • Systemic arteriole dilatation • Pulmonary vascular constriction • If there is little oxygen in the alveoli  the vascular bed in that area will constrict and send blood to better ventilated areas • Results in better ventilation/perfusion matching • Results in increased pulmonary vascular resistance • Results in increased right ventricular afterload

  9. So they have acute hypoxia: now what? • Assess the patient – ABCs • Vital signs • Is the oxygen saturation monitor accurate? Wave form? • Put on at a facemask (Non-rebreather) on at least 10L • Can the patient talk? If so, get a history • Physical Exam: • Are they wet or dry? • Lungs: Crackles? Where is air moving? • Extremities: edema (palpate sacrum too) • Neck: JVD • You will have to make an initial decision without the CXR. • Learn to trust your exam. • Other signs of chronic hypoxia: clubbing • Get an ABG, basic labs, CXR, EKG stat

  10. How to give the patient oxygen: The nose • Nasal Cannula: • Regular: can go up to 6L (39%). After 4L you need add humidity. • Oxymizer: gives a more accurate FIO2.Cannot give with humidity. Can give up to 15L (66%)

  11. How to give the patient oxygen:the mouth • Ventimask: this is a “high flow” mask. • Good for “air hunger” and mouth breather • Very precise amount of FIO2 • 24%-50% = 1L – 10L • Non-rebreather: • Have flow high enough to keep bag open • All or nothing: 50 – 66% = 10-15L

  12. Non-Invasive Ventilationand Invasive Ventilation • Non-invasive Ventilation: • CPAP: Continuous Positive Airway Pressure, does not initiate breaths • BiPAP: Bilevel Positive Airway Pressure, gives different pressures (high for inhalation, low for exhalation), can time breaths. Chronic use – OSA. Acute use: Acute pulmonary edema (CHF, HTN emergency), COPD exacerbation. • Invasive Ventilation: the machine breaths for the patient or supports the patients breath via a tube that is placed through the mouth into the in the trachea.

  13. Hypoxia If wet: Stop the fluids! Give nitroglycerin 1st to venodilate (SL or paste). Lasix: Dilate now, pee later. ESRD: give it anyway (dilate now, dialyze later)

  14. Hypoxia If dry: stabilize with oxygen When in doubt get a spiral CT (once stable enough). Throughout all of this mess, don’t forget your ABC’s, ask RTs help with CPAP/BiPAP…

  15. The Art of Multitasking So you know they are hypoxic/hypoxemic: but you need to know WHY

  16. Lung Anatomy

  17. V/Q - normal • Normal physiology: • V = ventilation (How well O2 gets into alveoli) • Q = perfusion (How well Blood gets to capillaries) • Blood vessels and alveoli are preferential to the bases, BUT the blood vessels > alveoli • V/Q is highest in the apices • V/Q is lowest in the bases

  18. 5 Causes of Hypoxia • Reduced inspired oxygen tension • not enough O2 is in the air the patient is breathing, for example: high altitude • Hypoventilation • broken pump • Ventilation-Perfusion Mismatch • V/Q mismatch • Shunt • Really bad V/Q mismatch • Diffusion impairment

  19. On the ABG • “A-a O2 Gradient = [ (FiO2) * (Atmospheric Pressure - H2O Pressure) - (PaCO2/0.8) ] - PaO2 from ABG] • DON’T MEMORIZE THIS – • Use the online calculators • Can also estimate. • If you put a “normal” person on 10L their PaO2 should be around 300. • A normal A-a gradient = 4 +age/4. 

  20. Reduced inspired oxygen tension • Normal A-a gradient • Altitude: • Bad air - breathing air that has a low FIO2

  21. hypercarbic Respiratory Failure~ Broken Pump ~ • There is NO difference between the Alveolar O2 and the arterial O2 [No A-a gradient] & increase PCO2 then the problem is a matter of HYPOVENTILATION. The air isn’t moving = Pump failure. • CNS depression: drugs, CNS infection, metabolic alkalosis, stroke, hypothyroidism. • Myopathies: diaphragm, myositis, dystrophies, electrolytes (phosphorus). • Neuropathies: cervical spine, phrenic nerve, GBS, ALS, polio • Neuro-muscular junction: Myasthenia, botulism

  22. If there is an A-a Gradient • If there is an A-a gradient  Hypoxic respiratory failure  V/Q problem. • What is a V/Q mismatch? All it means is the blood and the oxygen are not going to the same places. • If you put the patient on oxygen and they get better… • “V/Q mismatch” • DDx: airway problem (asthma, COPD), alveolar problem (PNA, CHF), Vascular problem (PE).

  23. If there is an A-a Gradient • If there is an A-a gradient and you give the patient O2 and the hypoxemia doesn’t improve = “Shunt”. • This is confusing – just remember  Shunt = Really Bad V/Q mismatch • DDx Shunt: Alveolar collapes (atalectasis), Alveolar filling (CHF, PNA), RL intracardiac shunt (VSD), intrapulmonary shunt (AVM).

  24. Diffusion limitation • Usually characterized by exercised-induced or exacerbated hypoxemia • During exercise  less time for diffusion. Healthy lungs will have capillary dilation to increase the surface area available so oxygenation is not affected • Lungs with alveolar or interstitial inflammation/fibrosis (ILD) can’t recruit additional surface area so hypoxia occurs • If causing acute hypoxia it is usually occurring concurrently with V/Q mismatch • Need PFTs to diagnose

  25. Hypoxemia - recap Low FIO2 (altitude) No A-a gradient Hypoventilation Yes (CNS↓, MM, Nerve, NMJ) Corrects w/ O2? No Yes Shunt V/Q mismatch (atalectasis, CHF, PNA (Asthma, COPD, PNA, CHF,PE) PE, intracard or intrapul shunt) ***if diffusion limitation is suspected in addition to the above, get PFTs after patient is stable

  26. Don’t forget the patient • Patients feel like they are drowning • Give reassurance • Don’t forget to talk to them about what is happening and what you are going to do for them as you try to stabilize them • Assume they can hear you • Give low dose IV morphine (1-2mg) if the patient is awake and suffering to help with the “drowning” feeling

  27. References • Lung anatomy picture: http://www.nhlbi.nih.gov/health/dci/images/lung_anatomy.jpg • Pocket Medicine, Sabatine, Lippincott 2008 • Uptodate.com: oxygenation and mechanisms of hypoxemia • Harrison’s Online: chap 35 • Ganong’s Review of Medical Physiology: chap 36

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