ArterialArterial BloodBlood GasesGases ArterialArterial BloodBlood GasesGases
PO2 SO2 measured oxygenation calculated pulse oximetry
PCO2 ventilation pH Acid-base BE/BD - HCO3 venous CO2 ArterialArterial PPOO22 (Pa(PaOO22)) zz Normal:Normal: 8080 –– 100100 mmmm HgHg breathingbreathing roomroom airair atat seasea levellevel inin healthyhealthy youngyoung adultsadults (103(103-- 0.50.5 xx age)age) zz PaOPaO2 affectedaffected byby
–– FFIOO2 PEEPPEEP LungLung functionfunction –– AgeAge VentilationVentilation AltitudeAltitude
PAOPAO2 == FIOFIO2(P(PB--PHPH20)0) –– PaCOPaCO2 xx 1.21.2
PAOPAO2 == FIOFIO2(700)(700) -- PaCOPaCO2 xx 1.21.2
Always interpret PaO2 in relation to FIO2 OxyhemoglobinOxyhemoglobin DissociationDissociation CurveCurve 100 90 80 70 60 50 40 30 20 10 Oxygen saturation (%) Oxygen saturation 0 0 102030405060708090100
PO2 (mm Hg) PaOPaO22/SaO/SaO22
Shifting of the Oxyhemoglobin Dissociation curve -Temperature -pH -2,3, DPG (stored blood loses 2,3, DPG) -Dyshemoglobins (carboxy, fetal, methhgb)
Shift to left facilitates Oxygen loading Shift to right facilitates Oxygen unloading PaOPaO22/SaO/SaO22
30 mm Hg = 60% saturation 60 mm Hg = 90% saturation 40 mmHg = 75% saturation
Oxygen delivery = Oxygen content x cardiac output
Oxygen content = PaO2 (0.003) + Hgb(1.34)%sat
Once PaO2 exceeds 70 mmHg further increases do not increase oxygen delivery ArterialArterial PPCOCO22 (Pa(PaCOCO22))
z Normal: 35 to 45 mm Hg
z ↑ PaCO2 = hypoventilation – Respiratory center depression – Neuromuscular disease – Pulmonary disease
z ↓ PaCO2 = hyperventilation – Central – Pain – Anxiety – Iatrogenic AcidAcid--BaseBase BalanceBalance
- HCO3 pH = 6.1 + log 0.03 × PCO2
metabolic component - HCO3 pH ~ PCO2 respiratory component
- When HCO3 is 24 mmol/L and PaCO2 is 40 mm Hg, the pH is 7.40 NormalNormal ValuesValues zz pHpH 7.357.35 –– 7.457.45 zz PaCO2PaCO2 3535--4545 mmHgmmHg zz HCO3HCO3-- 2222--2626 meq/Lmeq/L zz BE/BDBE/BD ––22 toto +2+2 zz BaseBase ExcessExcess oror BaseBase DeficitDeficit reflectsreflects thethe nonnon-- respiratoryrespiratory portionportion ofof acidacid--basebase balancebalance zz IncludesIncludes RBCRBC bufferingbuffering AcidAcid--BaseBase DisordersDisorders
zz PrimaryPrimary disturbancedisturbance –– Acidosis:Acidosis: pHpH << 7.357.35
•• Respiratory:Respiratory: ↑↑ PaPaCOCO2 - •• Metabolic:Metabolic: ↓↓ HCOHCO3 •• BE:BE: normalnormal –– Alkalosis:Alkalosis: pHpH >> 7.457.45
•• Respiratory:Respiratory: ↓↓ PaPaCOCO2 - •• Metabolic:Metabolic: ↑↑ HCOHCO3 •• BE:BE: normalnormal AcidAcid--BaseBase DisordersDisorders
RulesRules
ForFor aa 0.080.08 changechange inin pHpH –– PaCOPaCO2 changeschanges 1010 mmHgmmHg 7.407.40 4040 7.327.32 5050 7.487.48 3030
RespiratoryRespiratory compensationcompensation isis rapidrapid MetabolicMetabolic compensationcompensation isis slowslow AcidAcid--BaseBase DisordersDisorders zz CompensationCompensation
–– ChangeChange inin PaPaCOCO2 toto correctcorrect pHpH withwith metabolicmetabolic acidacid--basebase imbalanceimbalance •• e.g.,e.g., hyperventilationhyperventilation occursoccurs withwith metabolicmetabolic acidosisacidosis - –– ChangeChange inin HCOHCO3 toto correctcorrect pHpH withwith respiratoryrespiratory acidacid--basebase imbalanceimbalance - •• e.g.,e.g., HCOHCO3 increasesincreases withwith respiratoryrespiratory acidosisacidosis - HCO3 ↓↑ ↔ pH ~ ↓ ~ ↓↑ PCO2 RespiratoryRespiratory AcidosisAcidosis
zz Uncompensated:Uncompensated: ↓↓ pH,pH, ↑↑ PaPaCO2,, nlnl BE,BE, - HCOHCO3 - zz Compensated:Compensated: nlnl pH,pH, ↑↑ PaPaCO2,, ↑↑ BE,BE, HCOHCO3 zz Causes:Causes: respiratoryrespiratory centercenter depression,depression, neuromuscularneuromuscular disease,disease, lunglung diseasedisease zz Treatment:Treatment: treattreat cause,cause, mechanicalmechanical ventilation,ventilation, buffersbuffers RespiratoryRespiratory AlkalosisAlkalosis
zz Uncompensated:Uncompensated: ↑↑ pH,pH, ↓↓ PaPaCO2,, nlnl BE,BE, - HCOHCO3 - zz Compensated:Compensated: nlnl pH,pH, ↓↓ PaPaCO2,, ↓↓ΒΕ,ΒΕ, HCOHCO3 zz Causes:Causes: respiratoryrespiratory centercenter stimulation,stimulation, iatrogeniciatrogenic zz Treatment:Treatment: treattreat causecause MetabolicMetabolic AlkalosisAlkalosis
- zz Uncompensated:Uncompensated: ↑↑ pH,pH, ↑↑ HCOHCO3 ,, nlnl PaPaCO2 - zz Compensated:Compensated: nlnl pH,pH, ↑↑ HCOHCO3 ,, ↑↑ PaPaCO2 zz Causes:Causes: hypokalemia,hypokalemia, nasogastricnasogastric suctioningsuctioning oror vomiting,vomiting, contractioncontraction alkalosis,alkalosis, bicarbonatebicarbonate administration,administration, steroidsteroid therapytherapy zz Treatment:Treatment: treattreat cause,cause, KCl,KCl, volume,volume, diamox,diamox,
NHNH4Cl,Cl, argininearginine monohydrochloride,monohydrochloride, HClHCl MetabolicMetabolic AcidosisAcidosis
- zz Uncompensated:Uncompensated: ↓↓ pH,pH, ↓↓ HCOHCO3 ,, nlnl PaPaCO2 - zz Compensated:Compensated: nlnl pH,pH, ↓↓ HCOHCO3 ,, ↓↓ PaPaCO2 zz Causes:Causes: hypoxiahypoxia (lactic(lactic acidosis),acidosis), diabetesdiabetes (ketoacidosis),(ketoacidosis), renalrenal failurefailure (uremic(uremic acidosis),acidosis), - GIGI lossloss ofof HCOHCO3 (diarrhea),(diarrhea), renalrenal lossloss ofof - HCOHCO3 (renal(renal tubulartubular acidosis,acidosis, diamox),diamox), poisonspoisons (aspirin,(aspirin, methanol,methanol, ethyleneethylene glycol)glycol) zz Treatment:Treatment: treattreat cause,cause, bufferbuffer AcidAcid--BaseBase InterpretationInterpretation zz ClassifyClassify thethe disturbance:disturbance: acidosis,acidosis, alkalosis,alkalosis, metabolic,metabolic, respiratoryrespiratory zz DetermineDetermine thethe degreedegree ofof compensation:compensation: uncompensated,uncompensated, partiallypartially compensated,compensated, fullyfully compensatedcompensated zz IdentifyIdentify thethe causecause ofof thethe disturbancedisturbance zz DevelopDevelop aa treatmenttreatment planplan AcidAcid--BaseBase InterpretationInterpretation
- Disorder pH PaCO2 HCO3 Respiratory acidosis Uncompensated ↓↓ ↑↑ N Partially compensated ↓↑↑↑ Fully compensated N ↑↑ ↑↑ Respiratory alkalosis Uncompensated ↑↑ ↓↓ N Partially compensated ↑↓↓↓ Fully compensated N ↓↓ ↓↓ Metabolic acidosis Uncompensated ↓↓ N ↓↓ Partially compensated ↓↓ ↓↓ Fully compensated N ↓↓ ↓↓ Metabolic alkalosis Uncompensated ↑↑ N ↑↑ Partially compensated ↑↑ ↑↑ Fully compensated N ↑↑ ↑↑ TestTest YourYour SkillsSkills
pH = 7.25 PaCO2 = 57 - HCO3 = 24
HCO - ↔ ↓ pH ~ 3 PCO2 ↑ TestTest YourYour SkillsSkills
pH = 7.25
PaCO2 = 40 - HCO3 = 17
- HCO3 ↓ ↓ pH ~ ↔ PCO2 TestTest YourYour SkillsSkills
pH = 7.38
PaCO2 = 60 - HCO3 = 34
HCO - ↑ ↔pH ~ 3 PCO2 ↑ TestTest YourYour SkillsSkills
pH = 7.28
PaCO2 = 28 - HCO3 = 13
HCO - ↓ ↓ pH ~ 3 PCO2 ↓ MechanicalMechanical VentilationVentilation
zz VariablesVariables zz ModeMode zz FIO2FIO2 andand PEEPPEEP zz TidalTidal VolumeVolume andand frequencyfrequency zz I:EI:E ratio,ratio, inspiratoryinspiratory timetime ModesModes zz CMVCMV oror assistassist controlcontrol –– everyevery breathbreath isis thethe samesame volumevolume oror pressure,pressure, timetime zz IMVIMV –– spontaneousspontaneous breathsbreaths areare allowedallowed betweenbetween mandatorymandatory breathsbreaths zz PressurePressure supportsupport –– aa setset pressurepressure isis delivereddelivered withwith eacheach breathbreath thethe patientpatient takestakes (a(a boost)boost) zz CPAP/PEEPCPAP/PEEP –– elevatedelevated endend expiratoryexpiratory pressurepressure TidalTidal VolumeVolume && FrequencyFrequency
zz ControlControl minuteminute ventilationventilation && PaCOPaCO2
zz VVE == ff xx VVT
zz PaCOPaCO2 == VCOVCO2/V/VA
zz VVA == VVT –– VdsVds zz PostopPostop –– 88--1212 mL/kgmL/kg zz RestrictiveRestrictive –– 44--88 mL/kgmL/kg zz ObstructiveObstructive –– 88--1010 mL/kgmL/kg TidalTidal VolumeVolume –– WeightWeight && HeightHeight
TheThe majormajor determinantdeterminant ofof lunglung volumevolume isis heightheight notnot weightweight WomenWomen –– 45.545.5 ++ 2.32.3 (Ht(Ht inin inchesinches --60)60) MenMen -- 5050 ++ 2.32.3 (Ht(Ht inin inchesinches –– 60)60) ModifyModify tidaltidal volumevolume toto maintainmaintain airwayairway plateauplateau pressurepressure << 3030 cmcm H2OH2O PEEPPEEP andand FIO2FIO2 z Control oxygenation z FIO2 start at 100% and move down using SpO2 z PEEP – 5 cm H20 minimum z ARDS – 10 – 20 cm H2O z COPD – 5-10 cm H2O z PEEP is titrated to oxygenation, lung mechanics, oxygen delivery or other clinician determined endpoints WritingWriting VentilatorVentilator OrdersOrders z Mode (A/C, IMV, PSV) z Pressure or tidal volume z Frequency z FIO2 z PEEP z Goals of support z Better to write adjust FIO2 to maintain SpO2 > 92% then to write six orders to reduce FIO2 TerminologyTerminology zz WeaningWeaning impliesimplies thethe gradualgradual withdrawalwithdrawal ofof supportsupport zz LiberationLiberation fromfrom mechanicalmechanical ventilationventilation isis moremore appropriateappropriate zz LiberationLiberation maymay notnot requirerequire weaningweaning zz ExtubationExtubation isis removalremoval ofof thethe ETET tubetube zz DecannulationDecannulation isis removalremoval ofof thethe tracheostomytracheostomy tubetube WeaningWeaning FailureFailure
Minute Volume Ventilatory Drive pain, anxiety sedation, brain injury sepsis, DS, VCO2 Neuromuscular Resistive Spinal injury, Airway, secretions polyneuropathy bronchospasm Hyperinflation Load Capacity malnutrition Elastic electrolytes
Lung compliance Chest Wall chest wall compliance PEEPi flail chest, pain WHEANSWHEANS NOTNOT z Wheezes z Heart disease Ely EW, RCCNA 2000;6:303 z Electrolytes z Anxiety, airway problems, alkalosis z Neuromuscular disease z Sepsis, sedation z Nutrition (over and underfeeding) z Opiates, obesity z Thyroid disease WeaningWeaning ReadinessReadiness
DailyDaily ScreenScreen –– 55 CriteriaCriteria zz PatientPatient coughscoughs whenwhen suctionedsuctioned zz NoNo continuouscontinuous vasopressorvasopressor oror sedativesedative infusionsinfusions
zz PaOPaO2//FIO//FIO2 >> 200200
zz PEEPPEEP << 88 cmcm HH2OO
zz ff/V/VT << 105105 forfor oneone minuteminute
Ely NEJM 1996;335:1864 SpontaneousSpontaneous BreathingBreathing TrialsTrials z All pts who pass the daily screen – SBT 30 mins Termination of the SBT z Resp rate > 35 for > 5 mins z SpO2 < 90% for > 30 secs z 20% increase or decrease in heart rate for > 5 mins z SBP > 180 or < 90 for 60 secs consecutively z Agitation, anxiety, diaphoresis > baseline for > 5 minutes
Ely NEJM 1996;335:1864