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Mechanical Ventilation: The Basics Plus some pointers on interpreting blood gas values

Case: What would you do?. 50 yr-old 60 kg woman presents with 4 days of progressive respiratory distress. CXR shows diffuse infiltrates suggestive of pulmonary edema. The patient is breathing 95% FIO2 (non-rebreather face mask)ABG: pH 7.2, PCO2 50, PO2 60, HCO3 18, SaO2 90%Pulse-ox SaO2 is 8

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Mechanical Ventilation: The Basics Plus some pointers on interpreting blood gas values

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    1. Mechanical Ventilation: The Basics Plus some pointers on interpreting blood gas values Ted Carter, MD Pulmonary Division Seattle Childrens Hospital

    2. Case: What would you do? 50 yr-old 60 kg woman presents with 4 days of progressive respiratory distress. CXR shows diffuse infiltrates suggestive of pulmonary edema. The patient is breathing 95% FIO2 (non-rebreather face mask) ABG: pH 7.2, PCO2 50, PO2 60, HCO3 18, SaO2 90% Pulse-ox SaO2 is 88% HCO3 on serum electrolytes is 20 Meq/L

    3. Case: What would you do? How well is the patient oxygenating and ventilating, and what is the acid/base status? You need to place the patient on mechanical ventilation. (The patient needs to be intubated. Need to know rapid sequence induction and ET tube size) What do you need to know to set up the ventilator? What ventilator settings will you choose? How will you monitor the success of mechanical ventilation?

    4. Blood gas interpretation Blood gas values pH, PCO2, PO2 measured HCO3, base deficit, O2 sat calculated HCO3 on serum electrolytes - measured SaO2 by pulse oximeter measured, but indirectly

    5. Different blood sources ABG arterial blood CBG (capillary blood finger prick) Capillary pH arterial pH PCO2cap PCO2art PO2cap < PO2art, so PO2 cap is disregarded VBG (venous blood, peripheral or central vein) Need a free flowing venous sample Normal venous pH is 7.34 Venous PCO2 is 6 mm Hg > arterial PCO2 Venous PO2 does not correlate with arterial PO2

    6. Blood gas interpretation Know the normal values and the differences between values obtained from arterial, capillary, and venous blood Assess acid base status Assess ventilation status (PCO2) Assess oxygenation status (PO2 and SaO2)

    7. Assessing acid-base status pH = pKa + log [HCO3/.03PCO2] An increase in serum HCO3 and a decrease in PCO2 result in alkalosis A decrease in serum HCO3 and an increase in PCO2 result in acidosis Compensation occurs when either the PCO2 or HCO3 changes to bring the pH towards 7.4 In compensated states, the pH tends to stay on the side of the primary disorder

    8. Assessing ventilation PCO2 = k (CO2 production/Alveolar ventilation) If PCO2 is high, then ventilation is sub-par Low PCO2 = hyperventilation High PCO2 = hypoventilation

    9. Assessing oxygenation ABGs measure arterial PO2; VBGs and CBGs do not. Arterial PO2 is associated with SaO2 per the PO2-Hgb sat curve Know what FIO2 your patient is breathing

    10. Mechanical Ventilation: Purposes: To move air into the lungs and allow air to leave the lungs in order to: Eliminate CO2 (ventilation) Bring O2 into contact with the blood (oxygenation)

    11. The ventilator inflates the lungs (balloons) Airways Resistance 8 1/r4 Flow = [Pm Palv]/R Alveoli (compliance = ?V/?P) The driving pressure determines how fast (insp. time) and how much (tidal volume) the balloon is inflated The lung stiffness and airways resistance determine the tidal volume for a given PIP

    12. The ventilator vs. natural breathing Natural breathing Inspiration results from the generation of negative pressure: air is sucked into the lungs Exhalation is passive Respiratory rate is determined by the patient The patient overcomes increased resistance and decreased pulmonary compliance by breathing harder Mechanical ventilation Inspiration results from the generation of positive pressure: air is pushed into the lungs Exhalation is passive You set the ventilator rate, but the patient can also breathe over that rate The ventilator does the work, but you have to tell it what to do

    13. Basic points of mechanical ventilation: The ventilator delivers a certain amount of air per each breath - tidal volume (VT) Air flows into the lung due to a pressure gradient, and the flow rate depends on the pressure gradient and the airway resistance: Flow = (Pmouth Palv)/R The pressure required to deliver a set volume depends primarily on the distensability (compliance) of the lung. Compliance = ?V/?P

    14. The major ventilator settings Tidal volume (VT) = delivered volume Peak inspiratory pressure (PIP) Positive end expiratory pressure (PEEP) Pressure set above PEEP (PIP PEEP) Inspiratory pressure, pressure control (PCV), driving pressure Pressure support Ventilator breath rate Inspiratory time; Flow rate

    15. The 2 primary modes of ventilation Volume control ventilation You set the tidal volume for each ventilator breath, and the PIP varies Pressure control ventilation You set the PIP for each breath, and the VT varies

    16. Pros and Cons of Ventilator Modes Pressure Ventilation PIP is set, so there is less danger of over-pressurizing the lung VT and Vmin will change if lung mechanics change Compensates for leaks around the ET tube PIPs are about the same for all ages Volume Ventilation Volume is set, and PIP increases to deliver the set volume VT and Vmin are constant even if mechanics change Does not compensate for leaks around the ET tube VTs are different for all ages (usually 6-10 cc/kg)

    17. Additional Ventilator Settings Pressure support A set amount of pressure is added to each spontaneous breath (a pressure boost that assists the patients normal breathing) Assist control With each spontaneous breathing effort the ventilator delivers a full ventilator breath Synchronized intermittent mandatory ventilation (SIMV): the ventilator delivers a set number of breaths and synchronizes those with the patients spontaneous breaths

    18. Terminology: PIP, PCV, PEEP,

    19. Pressure Ventilation AC or SIMV modes Settings: Rate PIP (Insp. pressure pressure + PEEP) Inspiratory Time PS (SIMV only) PEEP FI02

    20. Volume Ventilation AC or SIMV modes Settings Rate Volume (VT) I-time PS (SIMV only) PEEP FI02

    21. Initial Settings Volume ventilation: Set the VT at 6-10 cc/kg Keep pressures < 35-40 cmH20 to avoid barotrauma Pressure ventilation: Set the PIP: PIP = Inspiratory (Driving) Pressure +PEEP Normal PIP: 18-25 cm H2O; High PIPs: > 35 cm H2O You need to adjust PIP to achieve the desired VT PEEP: normal 4-6; high >8 cm H2O Pressure support: low 4-6, mod 6-10; high >10 cm H2O Inspiratory time: infants 0.5; children 0.7; adults 1 sec Ventilator rate: Infants: 20-40; Children:15-30; Adults:10-20 bpm FIO2: 21-100%

    22. Choosing the Ventilator Settings Know what disease processes have caused your patient to require mechanical ventilation Decide on the mode of ventilation (Volume or Pressure) Determine if your initial settings are effective Oxygenation, ventilation (PCO2), air movement Make adjustments

    23. Ventilator strategy: airways obstruction Need time to let the lungs empty (long expiratory time) and to avoid auto-PEEP Need a high driving pressure to overcome resistance, but watch out for barotrauma! Limit PIP to = 40 cm H2O, if possible Low ventilator rates (6 12/min) to allow plenty of time for exhalation Permissive hypercapnea (high PCO2s dont kill) Low-moderate PEEP (4-8 cm H2O)

    24. Ventilator strategy: Diseases affecting the alveoli Pulmonary edema, ARDS, pulmonary hemorrhage (alveoli filled with fluid) Low VTs to keep PIPs from being excessive Rapid ventilator breath rates, e.g. small and frequent breaths High PEEPs to push fluid out of the alveoli and to keep alveoli from collapsing

    25. Case: What would I do? 50 yr-old 60 kg woman presents with 4 days progressive respiratory distress. CXR shows diffuse infiltrates suggestive of pulmonary edema. The patient is breathing 95% FIO2 (non-rebreather face mask) ABG: pH 7.2, PCO2 50, PO2 60, HCO3 18, SaO2 90% Pulse-ox SaO2 is 88% HCO3 on serum electrolytes is 20 Meq/L

    26. Interpret the blood gas values ABG: pH 7.2, PCO2 50, PO2 60, HCO3 18, SaO2 90%; Pulse-ox SaO2 is 88%; HCO3 on serum electrolytes is 20 Meq/L Mixed respiratory and metabolic acidosis Severe hypoxemia (remember the pt is breathing 95% FIO2!)

    27. Understand the disease Pulmonary edema alveolar process Will want to set up the ventilator with low volumes, relatively high rates and PEEPs, and will likely need high FIO2.

    28. Now set up the ventilator My initial settings: Volume ventilation in the assist control mode Set the VT at 6 cc/kg (6 x 60 kg = 360 cc) Since PIP will vary based on the patients lung stiffness I need to set a max pressure (pressure pop off) will set this at 45 cm H2O Set ventilator flow rate to give an insp. time of 1 sec Set PEEP at 12 cm H2O (high) and FIO2 at 70% Will monitor PCO2 for ventilation, SaO2 for oxygenation, and pH and serum HCO3 for acid-base status

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