Session 1: ABGs, Ventilators, and ARDS



Session 1 Arterial blood gasses, Acute respiratory distress syndrome, and Mechanical ventilation.


Arterial Blood gasses and the acid base balance.


  • 20:1 ratio is the ratio of Base(Sodium bicarbonate) to acid (carbonic acid) in the body
  • 7.35-7.45 is the normal pH (which is the H ion concentration)
  • 35-45 is the normal CO2 levels in the blood =  PaCO2
  • 22-26 is the normal Bicarb levels in an ABG


  • The kidneys control Bicarb and H excretion
  • The Lungs  control the excretion of CO2
  • Acidosis – too much acid or not enough base
    • Respiratory Acidosis – Narcotics to high, COPD, TBI, Breathing slowly in any form
    • Metabolic Acidosis – DKA, and Lactic acidosis, Diarrhea, Renal failure (excrete bicarb)
  • Alkalosis – not enough acid or too much base
    • Respiratory Alkalosis – Breathing to fast, Asthma attack, pain, anxiety
    • Metabolic Alkalosis – Vomiting, NG suction, Antacids, too much bicarb
  • Compensation
    • Fully compensated – pH is normal, PaCO2 and HCO3 are outside of normal range
    • Partially compensated – pH is outside of 7.35-7.45 and PaCO2 and HCO3 are outside of normal range
    • Uncompensated – pH is outside of 7.35 to 7.45 and one level is outside of the normal range and the other value is normal and “not compensating”


ABG interpretation examples:

  • pH – 7.3 Acidosis*
  • PaCO2 – 50 Acidosis*
  • HCO3 – 28 Alkalosis
  • Partially compensated Respiratory acidosis


  • pH – 7.47 Alk*
  • PaCO2 – 53 Acid
  • HCO3 – 32 Alk*
  • Partially compensated Metabolic Alkalosis


  • pH – 7.45 compensated Alk
  • PaCO2 – 60 Acid
  • HCO3 – 34 Alk
  • Fully Compensated Metabolic alkalosis


  • pH – 7.2
  • paCO2 – 60
  • HCO3 – 24
  • Uncompensated resp alk


  • 7.3
  • 60
  • 30
  • Partially compensated resp acidosis


Acute Respiratory Distress Syndrome (ARDS)

  • Sudden progressive Respiratory failure
    • Non cardiogenic pulmonary edema
    • Refractory hypoxemia (cannot be fixed with O2 admin)
    • Lung infiltrates, and decreased compliance
  • Pases:
    • Phase 1: injury or exudative phase
      • Interstitial pulmonary edema due to increased alveolar-capillary membrane permeability, Leaky capillaries, atelectasis, aveloi edema(pulmonary), shunting (blood goes from the right side heart to the left without exchanging gas at the lungs) VQ mismatch (Ventilation:diffusion)
    • Phase 2: Reparative
    • Phase 3: Fibrotic


  • Signs and symptoms:
    • initial tachypnea with low CO2 (respiratory alkalosis) then high CO2 (acidosis) when the respiratory rate decreases.
    • Change in LOC
    • Tachycardia, chest pain, changes in BP
    • Crackles


  • PaO2/ FiO2 ratio
    • Normal value is > 350 mmhg
    • Example of shunting 72 mmHg on a Fio2 of 60%
      • 72/.60 = 120


  • Pulmonary artery wedge pressure (PCWP) = Left ventricular end diastolic pressure (LVEDP)  = Preload
    • Use a swan ganz catheter
      • Inflate the balloon at the end of the catheter with 1-1.5 ml of AIR, and this will occlude a portion of the pulmonary artery.
      • You do this to get the pressure that it takes to occlude this artery, the wedge pressure
      • Once the pressure is found DEFLATE the balloon so that blood can flow through the artery again.
      • A wedge pressure (PCWP) that is normal is 5-12 mmhg
      • A PCWP that is indicative of ARDS or a pulmonary cause of pulmonary edema is <18 mmhg
      • A PCWP > 18 mmhg is indicative of cardiogenic caused pulmonary edema.
  • Fluid balance is hard in these patients
    • Colloids can leak out of the pulmonary vessels, unlike in a normal healthy patient. This can exacerbate the problem by drawing fluid out of the vessels towards the leaked colloid.


Mechanical ventilation: always have an Ambu bag near by if the patient self extubated.

  • Tidal volume – TD normal is about 5 ml/kg and on mechanical vent is 5-7 ml/kg
  • Resp Rate –
  • FiO2 – concentration of O2
  • Pressure limits – pressure at peak can be 12-20 cm H20


  • Modes (we will only talk about 2)
    • AC – Assist control – patient initiates breath but the parameters of I/E and minimum rate are controlled by the machine.
      • Less comfortable  
      • Ex: AC 10, TV 700, FiO2 40%
    • SIMV – synchronized intermittent mandatory ventilation
      • A minimum Rate is set and if the patient spontaneously breathes and gets the TV that the patient can physically breath in.
      • More comfortable, less risk of hyperventilation
      • Synchronizes with the patient’s breathing
      • SIMV 10, TV 700, FiO2
    • PEEP – Positive end expiratory pressure
      • Allows for us to defend from atelectasis
      • Drops CO and BP due to increase in Intrathoracic pressure.
      • Can be at risk for barotrauma leading to a pneumothorax if there is too much PEEP pressure.
    • PS – Pressure Support
      • Adding pressure to aid in a patient’s normal breathing
    • CPAP – Continuous positive airway pressure


Care for patient on a vent

  • Suction, oral care, eye drops, skin breakdown
  • Educate
  • Auscultate for symmetrical breath sounds, the ET tube may migrate to one side of the bronchus.


  • VAP ventilator associated Pneumonia
    • We are bypassing the patient’s natural defences that stop infection from getting to the lower airway
    • Leading cause of hospital associated infection ending in death.
    • Bundle
      • HOB at 30-45 degrees
      • Sedation interruption
      • PUD peptic ulcer disease prophylaxis
      • DVT prophylaxis
      • Oral care with Chlorhexidine

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