Anticipating Physiologic Changes

• Critically ill patients are at high risk of deterioration with intubation and initiation of mechanical ventilation
• Much of this summary is devoted to reviewing the effects of positive pressure ventilation (PPV) can have on pulmonary physiology:
  • However, mechanical ventilation can also have:
    • Extra-pulmonary effects that warrant review:
      • Specifically, PPV leads to:
        • An increase in the intrathoracic pressure:
          • Which has different effects on the right and left ventricles:
            • For the right ventricle, the PPV will lead to decreased preload via decreased venous return
              • This is shown by the blue and white arrowheads indicating increased pressure
            • The distention of the alveoli can also lead to increased afterload on the right ventricle
              • The inset illustrates the compression of small capillaries by distended alveoli:
                • Leading to an increase in pulmonary vascular resistance

• Note, however, that there is a U-shaped curve for changes in the pulmonary vascular resistance:
  • Both atelectasis and overdistention can:
    • Increase the afterload on the right ventricle

• The effects on the left ventricle are slightly different:
  • PPV also decreases the left ventricular preload:
    • Given the impact on the right ventricle
  • However:
    • The increased intrathoracic pressure also decreases the transmural pressure, or:
      • The afterload, on the left ventricle:
        • While we use this principle to care for those with congestive heart failure (CHF):
          • Can lead to an increase in stroke volume and cardiac output

• However, in excess, these impacts on the cardiovascular system can lead:
  • To a decrease in the cardiac output and hypotension:
    • Especially in the intravascularly depleted patient, those with shock physiology, or with air trapping
    • Additionally, PPV leads to a decrease in the left ventricular afterload
• When intubating and placing the patient on the ventilator, the clinician should anticipate these effects:
  • A volume-depleted patient:
    • Such as a patient with a GI bleed, may have hemodynamic collapse with initiation of positive pressure ventilation
• When initiating mechanical ventilation, the practitioner must be conscientious to ensure adequate gas exchange to meet the metabolic demands of the patient:
  • For example, a patient in with metabolic acidosis and respiratory compensation might be very tachypneic:
    • One must be cognizant to increase the respiratory rate on the ventilator to help meet the patient’s metabolic demands:
      • Failure to do so can be detrimental for the patient, and lead to rapid decompensation
• Along the same lines, the practitioner must be careful to set and then adjust the ventilator settings to prevent further decompensation or injury:
  • For example:
    • Excessive volumes on the ventilator can lead to:
      • Volutrauma and impaired gas exchange
    • Excess pressure can lead to:
      • Hemodynamic instability or barotrauma

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