Decreased Partial Pressure Of Oxygen Hypoxemia

HypoxemiaTroy Schaffernocker, MDHypoxemic Respiratory FailureDecreased PartialPressure of Oxygen Occurs at altitude Barometric pressure and altitude have addramaticti effectff t on oxygen tensionti Oxygen tension of inspired air:9 Sea level 150 mm Hg9 Denver 130 mm Hg9 Mt. Everest 43 mm HgEffect of Barometric Pressure andAltitude on Oxygen Mechanisms of hypoxemia:9 Decreased partial pressure of oxygen9 Impaired diffusion9 Ventilation/perfusion mismatch9 Shunt9 Hypoventilation1

Impaired Diffusion Interstitial lung disease9 Thickened interstitium impedes diffusion of oxygenfrom the alveolus to the capillary EarlEarly in cocourserse of ILDILD, hhypoxemiapo emia usuallys all notsignificant except during states of increasedoxygen demand (exercise) Combination of impaired diffusion and increasedtransit time of blood through alveolar capillaries(due to increased cardiac output from exercise)results in hypoxemiaVentilation/Perfusion The adequacy of gas exchange in the lungsis determined by the balance betweenpulmonary ventilation and capillary bloodflow. Expressed as the ventilation-perfusion(V/Q) ratio.Clinical Situation of Low V/Q(Shunt) V/Q 0 is represented by true right to leftshunting (intracardiac defect) with venousadmixture of blood.9 Alveoli completely bypassed Any situation where alveoli are filled (notventilated):9 Blood, pus, water9 Alveolar hemorrhage, pneumonia, CHF, ARDS Atelectasis of lungClinical Situations of High V/Q(Increase Deadspace) Pulmonary embolism PhysiologicPh i l i ddeadd space as seen iin COPD9 Normal response is to increase minuteventilation2

Hypoventilation Results in hypoxemia that is alwaysassociated with hypercapnia (by definition) Normal physiologic response to PaCO2 isto increase minute ventilation and thusalveolar ventilationNormal Arterial Blood 4-47344734-4734-4734-4734-47A – a PO2(mm Hg)4-177-2110-24102414-2717-3121-3425-38All values related to FIO2 21% at sea levelAdapted from Intermountain Thoracic SocietyManual, 1984 44-45Hypoxemia:Diagnosis and Monitoring Arterial Blood Gas (ABG)AlAlveolarl OxygenOTensionTiAlveolar-arterial (A-a) Oxygen GradientOxygen Content and DeliveryPulse OximetryVariation in ABGsVariationPaO2(mm Hg)PaCo2(mmHg)Mean132.595 Percentileth /- 18 /- 4Range2 – 370 - 12Represents variation over a 1-hour period in 26clinically stable ventilator dependent patientsFrom Hess D, Agarwal NN. J Clin Monitor 19923

Alveolar Oxygen Tension(A-a) Gradient Determined by the alveolar gas equation: (Barometric pressure – H2O vaporpressure)FiOp)py2 – PaCO2/Respiratoryquotient9 (BP – WVP)FiO2 – PaCO2/0.8 (760 – 47)0.21 – 40/0.8 150 – 50 100 mm Hg Normal if hypoxemia is due tohypoventilation (e.g. narcotic overdose) orlow atmospheric O2 (e.g. high altitude).(A-a) Gradient Partial pressure of oxygen in the alveolusminus partial pressure of oxygen in an artery.9 [FIO2 * (Barometric pressure - water vapor) (1 25*PCO2)] - PaO2(1.25*PCO2)] At Room Air9 [150 - (1.2 * PCO2)] - PaO2 A-a Gradient– Normal 8 - 12 mmHg– Increases with age - Age/4 4 High if hypoxemia is due to V/Q mismatch(e.g. Pulmonary Embolus), ImpairedDiffusion (e.g. ILD), or Shunt (e.g. ASD)Oxygen Saturation &Oxygen Delivery Remember oxygen content (CaO2) is amore important management measurethan PaO29 ([Hb] * %Sat * 1.34 ml/g) (PaO2 *0.003) Oxygen delivery the key parameter9 CaO2 * Cardiac output (CO)4

Pulse OximetryNasal Cannula Uses the differential absorbance of light byoxyhemoglobin and deoxyhemoglobin to estimate theoxygen saturation Caveats:1-6 LPM*1L 24%*2L 28%*3L 32%*4L 36%*5L 40%*6L 44%9 Detection of Acute Hypoxemia may be slow9 Does not measure ventilation9 Ambient light9 Electromagnetic Radiation9 Severe Anemia9 Hypoperfusion9 Hypothermia9 Venous Congestion9 Nail PolishAdvantages and Disadvantages ofthe Nasal CannulaOxygen DeliveryDevicesAdvantages: Comfortable Able to communicate Patient can eat andtake oral medications. Easy to use at home.Disadvantages: Nasal obstructionmay impede gasflow. May cause nasalmucosal drying (canbe humidified withsterile water)5

Simple MaskLOW FLOW DeviceNon-Rebreather MaskLOW FLOW Device5-8 LPM* 5-6L 40%15 LPM*6-7L 50%g should remain 1/3-1/2 fullBagafter the patient takes a deepbreath7-8L 60%*7-8L 60% Flow should be set at 5 L/minor more in order to avoidrebreathing exhaled carbondioxide (CO2)Least used mask do tounpredictable FI02 percentage(easier to use Venti Mask)Partial Rebreather Mask15 LPMNO Valvesg should remain 1/3-1/2 full afterBagthe patient takes a deep breathDelivers 60%-80% oxygen Must have all values removedto be considered a PartialRebeather MaskValvesDelivers 90%-100% oxygen Must have all 3 valvesFace TentAdvantages: Designed forpatients with facialtrauma or surgerythat cannot wear aregular mask ornasal cannula.Disadvantages: Clumsy and uncomfortable Variable FIo2 due poor mask seal6

Low-Flow Oxygen DeliveryDeviceNasal CannulaResevoirCapacity(mL)50Simple Face mask150 -250Mask -resevoir bag750 -1250PartialR ebreatherNonrebreatherOxygenFlow(L/min)1Venturi Mask (Venti Mask)ApproximateFIO20.21 -0.2420.24 -0.2830.28 -0.3440.34 -0.380.3850.38 -0.4260.42 -0.465-100.40 -0.605-70.35 -0.755-100.40 -1.03-15 LPM24%-50% (set on base of mask)Set FIo2 with percentagemarkings on the base ofmask and adjust the oxygenflowmeter the the appropriateLPMEstimated based on tidal volume of 500 mL, RR of 20 and I:E of 1:2From Shapiro BA, et al 1991Venturi Mask and Bernoulli’sPrincipleCool Aerosol Mask High Flow Oxygen Delivery with HighParticulate Humidity9 Hydration of airways for tenacioussecretions9 Treatment of Airway Edema9 Accommodate HighLiter Flow of HighFlow Systems Bernoulli’s Principle : Pressure isleast where the velocity of flow isthe greatest. As FI02 and entrained room aircombine and flow through theconstricted opening of the Venturidevice the flow velocity to thepatient increases greatly. By changing the opening size andoxygen flow the FIo2 can be varied.7

HFNCHFNC (High Flow Nasal Cannula) Principle: In the past O2Delivery by nasal routewas limited by the abilityto humidify and warm theinspired gas. Provides adequatelywarmed and humidifiedgas Provides more “washout” of thenasopharyngeald ddeadspace. Greater flow matches thepatient’s naturalinspiratory flow. High flow can be titratedto potentially providepositive distendingpressure for lungrecruitment. CONTRAINDICATIONS9999Unable to protect their airwayInability to adequatelyventilateFacial traumaSignificant epistaxis (Nosebleed) or patients with nasalcomplications COMPLICATIONS/PRECAUTIONS9999Nasal dryness, edema orbleedingDrying mucous, mucousplugging or airwayinflammationSinusitisInappropriate or interruptedoxygen flow may causehypoxemia and orhypercapniaRoca et al Respiratory Care 2010Dewan et al Chest 1994Spence et al Journal of Perinatology 2007O2 DeliveryNon-invasive Ventilation CPAP9 Continuous Positive Airway Pressure BiPAP/Bilevel9 Bilevel Positive Airway Pressure Best Evidence9 COPD Exacerbations9 CHF (Congestive Heart Failure) withPulmonary EdemaWettstein et al Respiratory Care 20058

Non-invasive Ventilation Supportive Evidence9 Facilitation of weaning and extubation inCOPD9 Immunosuppressed Patients9 Extubation Failure in COPD or CHF9 Prevention of Respiratory Failure inAsthma9 PalliativeCPAP vs BiPAP/Bilevel A trial of BiPAP may be worthwhile in patientswho do not tolerate CPAP. This is particularlytrue for patients who seem likely to benefit from alow expiratory pressure:9 patients with discomfort caused by exhaling againstthe CPAP9 patients with mouth leaks despite optimization of theinterface9 and patients with musculoskeletal chest pain due tobreathing at a higher functional residual capacityBiPAP/Bilevel Bilevel positive airway pressure (BiPAP) is amode that delivers an inspiratory positive airwaypressure (IPAP) and expiratory positive airwaypressure (EPAP) The magnitude of the difference between IPAPand EPAP is directly proportional to the amountof tidal volume augmentation and the alveolarventilation. If using a ventilator9 Pressure Support PEEP IPAP9 PEEP EPAPNasal vs Full Face Mask Nasal9 Smaller9 Easy to fit9 CanC hhave significanti ifit leakl k throughthh mouthth Full Face9 Bulkier9 Aspiration Risk9 Claustrophobia9

Non-invasive VentilationHypoxemia Contraindications9 Respiratory Arrest9 Anatomically unable to fit mask9 Inability to protect airway9 Inability to manage secretions9 Inability to cooperate with therapy –poor mental status, agitation, etc9 Aspiration Risk9 Recent upper airway or upper GI surgeryOutpatientUse of supplemental oxygenRuthann Kennedy, RNMechanical Ventilation #1 Indication “If you think about it” Elective intubation is much safer thanemergent intubation Airway control in an unstable patient isbetter for the patient Being on the ventilator does not createventilator dependence – Severe illnesscreates ventilator dependenceIndications for OxygenResting room air saturation 88%PaO2 55 mmHgDesaturation SaO2 88% with exertionDuring sleep: desaturation PaO2 55mmHgor SaO2 88% PaO2 59 mmHg or SaO2 89% in thepresence of cor pulmonale, right heartfailure, hematocrit 55%. Marino ICU book10

Ordering OxygenMedicare Requirements1. Qualification of oxygen need2. Select DMEpp3. Written prescription Flow rate, instructions & length of therapy Example: Oxygen 2L/m with rest, and4L/m with exertion and sleep, length oftherapy -lifetime. Provide home andportable equipment1. Resting, Room Air Saturation 88%2 Desaturation 88% with exertion or sleep2.3. Improved saturation with the addition ofsupplemental oxygenExampleDocumentation requirements The desaturation must be obtained within 2days of hospital discharge or within 30days of outpatient testing Oxygen saturation 88%9 AT REST on room air9 Requires no further testing 56 year old patient with interstitial lungdisease presents with an initial room airSaO2 90%.90% While walking in the hallhall, thesaturation drops to 84%. With the additionof supplemental oxygen at 2 L/m, thesaturation increases to 95%.11

Pulse OximetryOxygen-Hgb Dissocation Curve Standard of care for the assessment ofoxygen saturation “Fifth” vitalit l signi Easily accessible Available from DME for patient useLimitations of Pulse Oximetry Digital injury, especially in conjunction withvasopressors Delay in the detection of acute hypoxemia Does not assess ventilation A significant drop in the PaO2 must occurbefore the saturation decreases (oxygenhemoglobin dissociation curve)Standard OxygenConcentrator Non-portable, compressed air (gas) Oxygen delivery customarily up to 5L/m Special concentrators can deliver 10L/m Requires electricity12

Compact Oxygen ConcentratorE cylinder Compressed air EclipseWeight is approximately18 pounds9 4 hour battery life or AC capable9 Continuous 0.5 – 3.0 L/m9 Pulse 0.5 – 6.0 L/mE tank(Compressed Oxygen) Tank Duration 5.7 hours with flow rate of 2L/mcontinuous. Commonly used in conjunction with wheeledcart “Portable” but not necessarily convenient fortransportation Tank itself weighs 4-5 lbs and is 25 inches tallConserving Device Reservoir Cannulas – Partial rebreatherdevices that store oxygen from exhalation9 Oximizer9 Oximizer Pendant Transtracheal oxygen Pulsed Delivery13

Oximizer Advantage: comfort and easy to use Disadvantage: cosmetic EExtendst d largerlthanth a nasall cannulal andd isiclearly visibleOximizer Pendant Appears like a standard nasal cannula There is a pendant that sits on the upperchestTranstracheal Oxygen Advantages:9 Increased mobility and exercise9 Lower oxygenyg flow requirementsq Disadvantages9 Invasive9 Dislodgement9 Infection14