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U

PRINCIPLES OF MECHANICAL PRINCIPLES OF MECHANICAL VENTILATION andVENTILATION andBLOOD GAS INTERPRETATIONBLOOD GAS INTERPRETATION

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UDefinitionsDefinitions

• Tidal Volume (TV):Tidal Volume (TV): volume of each breath. volume of each breath.• Rate:Rate: breaths per minute. breaths per minute.• Minute Ventilation (MV):Minute Ventilation (MV): total ventilation per total ventilation per

minute. MV = TV x Rate.minute. MV = TV x Rate.• Flow:Flow: volume of gas per time. volume of gas per time.• Compliance:Compliance: the distensibility of a system. The the distensibility of a system. The

higher the compliance, the easier it is to inflate higher the compliance, the easier it is to inflate the lungs.the lungs.

• Resistance:Resistance: impediment to airflow. impediment to airflow.• SIMVSIMV: patient breathes spontaneously between : patient breathes spontaneously between

ventilator breaths. Allows patient-ventilator ventilator breaths. Allows patient-ventilator synchrony, making for a more comfortable synchrony, making for a more comfortable experience.experience.

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UDefinitionsDefinitions

• PIP:PIP: maximum pressure measured by the ventilator maximum pressure measured by the ventilator during inspiration. during inspiration.

• PEEP:PEEP: pressure present in the airways at the end of pressure present in the airways at the end of expiration.expiration.

• CPAP:CPAP: amount of pressure applied to the airway amount of pressure applied to the airway during all phases of the respiratory cycle.during all phases of the respiratory cycle.

• PS:PS: amount of pressure applied to the airway amount of pressure applied to the airway during spontaneous inspiration by the patient.during spontaneous inspiration by the patient.

• I-time:I-time: amount of time delegated to inspiration. amount of time delegated to inspiration.

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UTypes of VentilationTypes of Ventilation

• Volume ControlVolume Control• Pressure ControlPressure Control• Pressure Support-CPAPPressure Support-CPAP• Pressure-Regulated Volume ControlPressure-Regulated Volume Control

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UVolume ControlVolume Control

• The patient is given a specific volume of air The patient is given a specific volume of air during inspiration.during inspiration.

• The ventilator uses a set flow for a set period of The ventilator uses a set flow for a set period of time to deliver the volume: TV (cc) = Flow time to deliver the volume: TV (cc) = Flow (cc/sec) x i-time (sec)(cc/sec) x i-time (sec)

• The PIP observed is a product of the lung The PIP observed is a product of the lung compliance, airway resistance and flow rate. The compliance, airway resistance and flow rate. The ventilator does not react to the PIP unless the ventilator does not react to the PIP unless the alarm limits are violated.alarm limits are violated.

• The PIP tends to be higher than during pressure The PIP tends to be higher than during pressure control ventilation to deliver the same volume of control ventilation to deliver the same volume of air.air.

• With SIMV, the patient can breath spontaneously With SIMV, the patient can breath spontaneously between vent breaths. This mode is often between vent breaths. This mode is often combined with PS.combined with PS.

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UTriggering the VentilatorTriggering the Ventilator

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UPressure ControlPressure Control

• Patient receives a breath at a fixed airway Patient receives a breath at a fixed airway pressure.pressure.

• The ventilator adjusts the flow to maintain the The ventilator adjusts the flow to maintain the pressure.pressure.

• Flow decreases throughout the inspiratory cycle.Flow decreases throughout the inspiratory cycle.• The pressure is constant throughout inspiration.The pressure is constant throughout inspiration.• Volume delivered depends upon the inspiratory Volume delivered depends upon the inspiratory

pressure, I-time, pulmonary compliance and pressure, I-time, pulmonary compliance and airway resistance.airway resistance.

• The delivered volume can vary from breath-to-The delivered volume can vary from breath-to-breath depending upon the above factors. MV not breath depending upon the above factors. MV not assured.assured.

• Good mode to use if patient has large air leak, Good mode to use if patient has large air leak, because the ventilator will increase the flow to because the ventilator will increase the flow to compensate it.compensate it.

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UVolume vs. PressureVolume vs. Pressure

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UChanges in ARDSChanges in ARDS

Volume Control Pressure Volume Control Pressure Control Control

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UCPAP-Pressure SupportCPAP-Pressure Support

• No mandatory breathsNo mandatory breaths• Patient sets the rate, I-time, and respiratory effort.Patient sets the rate, I-time, and respiratory effort.• CPAP performs the same function as PEEP, except CPAP performs the same function as PEEP, except

that it is constant throughout the inspiratory and that it is constant throughout the inspiratory and expiratory cycle.expiratory cycle.

• Pressure Support (PS) helps to overcome airway Pressure Support (PS) helps to overcome airway resistance and inadequate pulmonary effort and is resistance and inadequate pulmonary effort and is added on top of the CPAP during inspiration.added on top of the CPAP during inspiration.

• The ventilator increases the flow during inspiration The ventilator increases the flow during inspiration to reach the target pressure and make it easier for to reach the target pressure and make it easier for the patient to take a breath.the patient to take a breath.

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USIMV + PSSIMV + PS

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UPressure-Regulated Volume ControlPressure-Regulated Volume Control

• In this mode, a target minute ventilation is set. In this mode, a target minute ventilation is set. • The ventilator will adjust the flow to deliver the The ventilator will adjust the flow to deliver the

volume without exceeding a target inspiratory volume without exceeding a target inspiratory pressure. pressure.

• Decelerating flow pattern.Decelerating flow pattern.• No change in minute ventilation if pulmonary No change in minute ventilation if pulmonary

conditions change.conditions change.• Can ventilate at a lower PIP than in regular volume Can ventilate at a lower PIP than in regular volume

control.control.• Hard to use on a spontaneously breathing patient Hard to use on a spontaneously breathing patient

or one with a large air leak.or one with a large air leak.• Not a “weaning” mode.Not a “weaning” mode.

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UInitial Ventilator SettingsInitial Ventilator Settings

• Rate: 20-24 for infants and preschoolersRate: 20-24 for infants and preschoolers 16-20 for grade school kids 16-20 for grade school kids

12-16 for adolescents 12-16 for adolescents• TV: 10-15ml/kgTV: 10-15ml/kg

• PEEP: 3-5cm HPEEP: 3-5cm H22OO

• FiOFiO22: 100%: 100%

• I-time: 0.7 sec for higher rates, 1sec for lower ratesI-time: 0.7 sec for higher rates, 1sec for lower rates

• PIP (for pressure control): about 24cm HPIP (for pressure control): about 24cm H22O. O.

• Pressure Support: 5-10cm HPressure Support: 5-10cm H22O.O.

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UAdjusting The VentilatorAdjusting The Ventilator

• pCOpCO22 too high too high

• pCOpCO22 too low too low

• pOpO22 too high too high

• pOpO22 too low too low

• PIP too highPIP too high

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UpCOpCO22 Too High Too High

• Patient’s minute ventilation is too low.Patient’s minute ventilation is too low.• Increase rate or TV or both.Increase rate or TV or both.• If using PC ventilation, increase PIP.If using PC ventilation, increase PIP.• If PIP too high, increase the rate instead.If PIP too high, increase the rate instead.• If air-trapping is occurring, decrease the rate If air-trapping is occurring, decrease the rate

and the I-time and increase the TV to allow and the I-time and increase the TV to allow complete exhalation.complete exhalation.

• Sometimes, you have to live with the high pCOSometimes, you have to live with the high pCO22, , so use THAM or bicarbonate to increase the pH so use THAM or bicarbonate to increase the pH to >7.20.to >7.20.

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UpCOpCO22 Too Low Too Low

• Minute ventilation is too high.Minute ventilation is too high.• Lower either the rate or TV.Lower either the rate or TV.• Don’t need to lower the TV if the PIP is <20.Don’t need to lower the TV if the PIP is <20.• PIP <24 is fine unless delivered TV is still PIP <24 is fine unless delivered TV is still

>15ml/kg.>15ml/kg.• TV needs to be 8ml/kg or higher to prevent TV needs to be 8ml/kg or higher to prevent

progressive atelectasisprogressive atelectasis• If patient is spontaneously breathing, consider If patient is spontaneously breathing, consider

lowering the pressure support if spontaneous lowering the pressure support if spontaneous TV >7ml/kg.TV >7ml/kg.

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UpOpO22 Too High Too High

• Decrease the FiODecrease the FiO22. .

• When FiOWhen FiO22 is less than 40%, decrease the PEEP is less than 40%, decrease the PEEP to 3-5 cm Hto 3-5 cm H22O.O.

• Wean the PEEP no faster than about 1 every 8-Wean the PEEP no faster than about 1 every 8-12 hours.12 hours.

• While patient is on ventilator, don’t wean FiOWhile patient is on ventilator, don’t wean FiO22 to <25% to give the patient a margin of safety to <25% to give the patient a margin of safety in case the ventilator quits.in case the ventilator quits.

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UpOpO22 Too Low Too Low

• Increase either the FiOIncrease either the FiO22 or the mean airway or the mean airway pressure (MAP).pressure (MAP).

• Try to avoid FiOTry to avoid FiO22 >70%. >70%.• Increasing the PEEP is the most efficient way of Increasing the PEEP is the most efficient way of

increasing the MAP in the PICU.increasing the MAP in the PICU.• Can also increase the I-time to increase the MAP Can also increase the I-time to increase the MAP

(PC).(PC).• Can increase the PIP in Pressure Control to Can increase the PIP in Pressure Control to

increase the MAP, but this generally doesn’t add increase the MAP, but this generally doesn’t add much at rates <30bpm.much at rates <30bpm.

• May need to increase the PEEP to over 10, but May need to increase the PEEP to over 10, but try to stay <15 if possible.try to stay <15 if possible.

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UPIP Too HighPIP Too High

• Decrease the PIP (PC) or the TV (VC).Decrease the PIP (PC) or the TV (VC).• Increase the I-time (VC).Increase the I-time (VC).• Change to another mode of ventilation. Change to another mode of ventilation.

Generally, pressure control achieves the same Generally, pressure control achieves the same TV at a lower PIP than volume control.TV at a lower PIP than volume control.

• If the high PIP is due to high airway resistance, If the high PIP is due to high airway resistance, generally the lung is protected from barotrauma generally the lung is protected from barotrauma unless air-trapping occurs.unless air-trapping occurs.

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UWeaning PrioritiesWeaning Priorities

• Wean PIP to <35cm HWean PIP to <35cm H22OO

• Wean FiOWean FiO22 to <60% to <60%

• Wean I-time to <50%Wean I-time to <50%

• Wean PEEP to <8cm HWean PEEP to <8cm H22OO

• Wean FiOWean FiO22 to <40% to <40%

• Wean PEEP, PIP, I-time, and rate towards Wean PEEP, PIP, I-time, and rate towards extubation settings.extubation settings.

• Can consider changing to volume control Can consider changing to volume control ventilation when PIP <35cm Hventilation when PIP <35cm H22O.O.

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UComplicationsComplications

• PulmonaryPulmonary– BarotraumaBarotrauma– Ventilator-induced lung Ventilator-induced lung

injuryinjury– Nosocomial pneumoniaNosocomial pneumonia– Tracheal stenosisTracheal stenosis– TracheomalaciaTracheomalacia– PneumothoraxPneumothorax

• CardiacCardiac– Myocardial ischemiaMyocardial ischemia– Reduced cardiac outputReduced cardiac output

• GastrointestinalGastrointestinal– IleusIleus– HemorrhageHemorrhage– PneumoperiteneumPneumoperiteneum

• RenalRenal– Fluid retentionFluid retention

• NutritionalNutritional– MalnutritionMalnutrition– OverfeedingOverfeeding

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UAcute DeteriorationAcute Deterioration

• DIFFERENTIAL DIAGNOSESDIFFERENTIAL DIAGNOSES– PneumothoraxPneumothorax– Right mainstem intubationRight mainstem intubation– PneumoniaPneumonia– Pulmonary edemaPulmonary edema– Loss of airwayLoss of airway– Airway occlusionAirway occlusion– Ventilator malfunctionVentilator malfunction– Mucus pluggingMucus plugging– Air leakAir leak

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UPhysical ExamPhysical Exam

• Tracheal shiftTracheal shift– PneumothoraxPneumothorax

• WheezingWheezing– BronchospasmBronchospasm– Mucus pluggingMucus plugging– Pulmonary edemaPulmonary edema– Pulmonary Pulmonary

thromboembolismthromboembolism

• Asymmetric breath Asymmetric breath soundssounds– PneumothoraxPneumothorax– Mainstem intubationMainstem intubation– Mucus plugging with Mucus plugging with

atelectasisatelectasis

• Decreased breath Decreased breath sounds bilaterallysounds bilaterally– Tube occlusionTube occlusion– Ventilator malfunctionVentilator malfunction– Loss of airwayLoss of airway

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UPressure PatternsPressure Patterns

• Elevated peak and Elevated peak and plateau pressuresplateau pressures– PneumoniaPneumonia– Pulmonary edemaPulmonary edema– PneumothoraxPneumothorax– AtelectasisAtelectasis– Right mainstem Right mainstem

intubationintubation

• Elevated peak Elevated peak pressure, normal pressure, normal plateau pressureplateau pressure– Airflow obstructionAirflow obstruction– Mucus pluggingMucus plugging– Partial tube occlusionPartial tube occlusion

• Reduced peak and Reduced peak and plateau pressureplateau pressure– Cuff leakCuff leak– Ventilator malfunctionVentilator malfunction– Large bronchopleural Large bronchopleural

fistulafistula

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UExtubation CriteriaExtubation Criteria

• NeurologicNeurologic• CardiovascularCardiovascular• PulmonaryPulmonary

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UNeurologicNeurologic

• Patient must be able to protect his airway, e.g, Patient must be able to protect his airway, e.g, have cough, gag, and swallow reflexes.have cough, gag, and swallow reflexes.

• Level of sedation should be low enough that Level of sedation should be low enough that the patient doesn’t become apneic once the the patient doesn’t become apneic once the ETT is removed.ETT is removed.

• No apnea on the ventilator.No apnea on the ventilator.• Must be strong enough to generate a Must be strong enough to generate a

spontaneous TV of 5-7ml/kg on 5-10 cm Hspontaneous TV of 5-7ml/kg on 5-10 cm H22O PS O PS or have a negative inspiratory force (NIF) of or have a negative inspiratory force (NIF) of 25cm H25cm H22O or higher.O or higher.

• Being able to follow commands is preferred.Being able to follow commands is preferred.

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UCardiovascularCardiovascular

• Patient must be able to increase cardiac output Patient must be able to increase cardiac output to meet demands of work of breathing.to meet demands of work of breathing.

• Patient should have evidence of adequate Patient should have evidence of adequate cardiac output without being on significant cardiac output without being on significant inotropic support.inotropic support.

• Patient must be hemodynamically stable.Patient must be hemodynamically stable.

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UPulmonaryPulmonary

• Patient should have a patent airway.Patient should have a patent airway.• If no air leak, consider decadron and racemic If no air leak, consider decadron and racemic

epinephrine.epinephrine.• Pulmonary compliance and resistance should be Pulmonary compliance and resistance should be

near normal.near normal.• Patient should have normal blood gas and work-Patient should have normal blood gas and work-

of-breathing on the following settings:of-breathing on the following settings:– FiOFiO22 <40% <40%

– PEEP 3-5cm HPEEP 3-5cm H22OO– Rate: 6bpm for infants, 2bpm for toddlers, CPAP/PS for Rate: 6bpm for infants, 2bpm for toddlers, CPAP/PS for

1hr for older children and adolescents1hr for older children and adolescents– PS 5-8cm HPS 5-8cm H22OO– Spontaneous TV of 5-7ml/kgSpontaneous TV of 5-7ml/kg

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UBlood Gas InterpretationBlood Gas Interpretation

NORMAL VALUESNORMAL VALUES

ArterialArterial Venous Venous CapillaryCapillary

pH pH 7.4 (7.38-7.42) 7.4 (7.38-7.42) 7.36 (7.31-7.41)7.36 (7.31-7.41) 7.35-7.407.35-7.40pOpO22 80-100 mm Hg80-100 mm Hg 35-40 mm Hg35-40 mm Hg 45-60 mm Hg45-60 mm Hg

pCOpCO22 35-45 mm Hg35-45 mm Hg 41-52 mm Hg41-52 mm Hg 40-45 mm Hg40-45 mm Hg

SatSat >95% on RA >95% on RA 60-80% on RA60-80% on RA >70%>70%HCOHCO33 22-26 mEq/L22-26 mEq/L 22-26mEq/L22-26mEq/L 22-22-

26mEq/L26mEq/LBEBE -2 to +2 -2 to +2 -2 to +2-2 to +2 -2 to +2-2 to +2

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URules Of InterpretationRules Of Interpretation

• ∆∆ in pCOin pCO22 of 10mm Hg should of 10mm Hg should ∆∆ pH by 0.08. pH by 0.08.

• pH ∆ of 0.15 is equal to ∆ in HCOpH ∆ of 0.15 is equal to ∆ in HCO33 of 10mEq/L. of 10mEq/L.

• Normal pCONormal pCO22 in the face of respiratory distress is a in the face of respiratory distress is a sign of impending respiratory failure.sign of impending respiratory failure.

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UAcid-Base DiagramAcid-Base Diagram

From Goldberg, M., From Goldberg, M., Green, S.B., Moss, Green, S.B., Moss, M.L., et al.: JAMA M.L., et al.: JAMA 223:269-275, 1973223:269-275, 1973

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URespiratory DisturbancesRespiratory Disturbances

• Acute respiratory acidosis occurs when COAcute respiratory acidosis occurs when CO22 is is retained acutely.retained acutely.

• Chronic respiratory acidosis occurs when the Chronic respiratory acidosis occurs when the retained COretained CO22 gets buffered by renal retention of gets buffered by renal retention of HCOHCO33. The pH is higher than in acute . The pH is higher than in acute respiratory acidosis, but it is still <7.4.respiratory acidosis, but it is still <7.4.

• Acute respiratory alkalosis occurs when COAcute respiratory alkalosis occurs when CO22 is is blown off acutely.blown off acutely.

• Chronic respiratory alkalosis occurs when the Chronic respiratory alkalosis occurs when the reduction of COreduction of CO22 is compensated for by the is compensated for by the renal excretion of HCOrenal excretion of HCO33. The pH is lower than in . The pH is lower than in acute respiratory alkalosis, but it is still >7.4.acute respiratory alkalosis, but it is still >7.4.

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UMetabolic DisturbancesMetabolic Disturbances

• Acute metabolic acidosis gets compensated by COAcute metabolic acidosis gets compensated by CO22 reduction within 12-24 hours. The pH is still reduction within 12-24 hours. The pH is still usually <7.4.usually <7.4.

• Metabolic alkalosis is rare. Usual causes are Metabolic alkalosis is rare. Usual causes are pyloric stenosis, chronic diuretic use, and pyloric stenosis, chronic diuretic use, and bicarbonate infusions.bicarbonate infusions.

• Otherwise healthy people do not usually retain COOtherwise healthy people do not usually retain CO22 to compensate for metabolic alkalosis.to compensate for metabolic alkalosis.

• Patients who are severely dehydrated or have lung Patients who are severely dehydrated or have lung disease will retain COdisease will retain CO22 to compensate for metabolic to compensate for metabolic alkalosis.alkalosis.

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UHypoxemiaHypoxemia

There are five reasons for hypoxemia:There are five reasons for hypoxemia:

• FiOFiO22 too low (high altitude) too low (high altitude)

• Global alveolar hypoventilationGlobal alveolar hypoventilation• Right-to-left shuntsRight-to-left shunts• V/Q mismatchV/Q mismatch• Incomplete diffusionIncomplete diffusion