Extrapulmonary Effects of Mechanical Ventilation

Cardiovascular Effects

  • Spontaneous Breathing:
    • During inspiration, negative intrapleural pressure draws air into lungs and blood into thoracic vessels and heart.
    • During expiration, increased pressure has the opposite effect.
  • Positive Pressure Ventilation (PPV):
    • During inspiration, airway pressure increases are transmitted throughout the thorax causing increased intrathoracic pressure which can lead to decreased cardiac output.
    • There is a direct correlation between thoracic vascular pressures and mean airway pressure.
    • PPV with PEEP decreases cardiac output further, especially in CMV mode, as PEEP further increases mean airway pressure.
  • Due to PPV, pressure rises in the vena cava, reducing venous return to the heart.
  • Increased Pulmonary Vascular Resistance (PVR):
    • High tidal volumes or high levels of PEEP compress alveolar capillaries, increasing PVR.
    • This leads to increased right ventricular afterload, dilation of the RV, and potentially a left shift of the interventricular septum.
    • The intrusion into the left ventricle causes a decrease in LVEDV and potential decrease in left ventricular stroke volume.
  • Cardiac Tamponade Effect:
    • The heart is compressed between the lungs when tidal volumes are large.
  • Lower cardiac output may be caused or worsened by myocardial dysfunction due to reduced myocardial perfusion and subsequent ischemia caused by a decrease in coronary artery perfusion gradient.
  • People with normal anatomy compensate for these effects through: increasing sympathetic tone (tachycardia, increased SVR and PVR), blood flow shunting away from kidneys and lower extremities to maintain central systemic blood pressure.
    • Vascular reflexes can be blocked by anesthesia or spinal cord transection.
  • Compliance Effects:
    • In patients with stiff lungs (ARDS, pulmonary fibrosis), the cardiovascular system is less affected by increased airway pressure.
    • In patients with stiff chest walls, the effect is increased.

Benefits of PPV on Cardiac Function

  • PPV can benefit patients with LV dysfunction.
  • PEEP improves oxygenation to the myocardium.
  • PEEP and PPV, in general, can reduce preload by increasing intrathoracic pressure and decreasing venous return.

Minimizing Physiologic Effects

  • Reduce intrapulmonary pressure to decrease harmful cardiovascular effects.
  • Mean airway pressure is the most potentially damaging pressure.
  • Mean\ Airway\ Pressure = \frac{1}{2} (PIP – PEEP) \times (Inspiratory\ time/TCT) + PEEP
  • Mean airway pressure increases FRC and improves oxygenation.
  • Mean Airway Pressure is optimized in ARDS to improve oxygenation.
  • Inspiratory Flow:
    • Higher flows decrease inspiratory time and decrease mean airway pressure.
    • High inspiratory flows can cause greater volume to be lost in the tubing.
    • Higher flow means more pressure is required to overcome the increased RAW.
    • Higher flow means the possibility of an increase in uneven distribution of ventilation.
  • I:E Ratio:
    • Higher I:E ratios = higher mean airway pressure.
    • I:E ratio should be kept at 1:2 to 1:4 or lower to reduce mean airway pressure.
  • Inflation Hold:
    • Used historically to improve oxygenation and distribution of ventilation.
    • Increases inspiratory time and mean airway pressure.
    • Now used mostly for measuring plateau pressure for CS calculation.
  • PEEP increases FRC and increases mean airway pressure.
  • In stiff lungs where PEEP is indicated, the pressure is not as easily transmitted.
  • High PIP with increased airway resistance is not transmitted to the alveoli because most of this pressure is transmitted to the conductive airways.
  • The effects of increased flow, decreased I:E and decreased Paw:
    • Benefits: Reduced risk of barotrauma, Reduced risk of cardiovascular effects
    • Hazards: Uneven distribution of gas, Decreased PaO2, Increased PaCO2
  • The effects of increased Paw:
    • Benefits: Increased VA, Recruitment of alveoli, Better gas distribution
    • Hazards: Decreased cardiac output, Decreased O_2 transport, Increased risk of barotrauma
  • SIMV mode can reduce mean airway pressure because the mandatory breaths are only intermittent. However IMV can increase WOB thereby requiring FVS mode for rest and recovery.
  • In using PPV, the clinician must choose a balance that is most beneficial and least harmful to the patient.

Effects of PPV on ICP and CPP

  • CPP = MABP – ICP
    • CPP: Cerebral Perfusion Pressure
    • MABP: Mean Arterial Blood Pressure
    • ICP: Intracranial Pressure
  • PPV can decrease CPP by several factors, like decreased cardiac output, or increased CVP (central venous pressure).

Effects of PPV on the Renal System

  • Changes in renal function can occur by renal responses to hemodynamic changes, humoral responses, or abnormal pH, PaCO2, and PaO2 affecting the kidney.
  • Renal Responses to Hemodynamic Changes:
    • If renal arterial pressure drops below 75 mm Hg, urinary output will diminish.
    • Decreased cardiac output could lead to decreased renal perfusion and cause a decrease in urine output.
    • Redistribution of blood inside the kidney may be a more important factor.
  • Endocrine Effects of Positive Pressure Ventilation on Renal Function:
    • Decreased blood pressure could increase the release of ADH causing the kidneys to retain water and thereby decrease urine output.
    • Volume receptors in the left atrium and baroreceptors in Carotid bodies are both exposed to changes in intrathoracic pressures.
    • NPV has the opposite effect.
  • Arterial Blood Gases and Kidney Function:
    • PaO2 and PaCO2 changes also affect renal function: Decreases in PaO2 as well as increases in PaCO2 cause renal impairment.
    • Decreasing PaO_2 can cause a reduction in renal function.
    • PaCO_2 > 65 mm Hg can also severely impair renal function.
  • In seriously ill PPV can lead to Na and H_2O retention leading to weight gain, which could eventually lead to pulmonary edema.
  • Reduced renal function can also impair fluid and electrolyte management.
  • Altered renal function can prolong effects of drugs excreted by the kidney.

PPV Effects on Hepatic and GI Function

  • Patients on PPV and PEEP can show evidence of liver malfunction caused by decrease in cardiac output, the downward pressure exerted by the diaphragm on the liver, a decrease in portal blood flow, or an increase in splanchnic resistance, which could lead to liver ischemia.
  • PPV increases splanchnic resistance and decreases splanchnic venous outflow, which may also lead to gastric mucosal ischemia which can lead to GI bleeding and ulcers.
  • Ventilated patients, if not being fed, are usually on some type of H2 blocker and or a GI motility accelerator.
  • Another potential problem is gastric distension from swallowing air.
  • For this and other reasons every mechanically ventilated patient should have a gastric tube.
  • Narcotics, which may be required for patients on PPV are known to decrease GI motility.

Nutritional Complications of PPV

  • Nutritional status of patients on PPV must be carefully monitored.
  • Patients need nutrition to recover.
  • Increased temperature and wound-healing cause an increase in metabolism requiring an increase in provided nutrition.
  • Food-intake may be inadequate. Patients may not tolerate “tube-feeding.”
  • Over-feeding can lead to increased O2 consumption and increased CO2 production.
  • REE can be calculated or measured by “indirect calorimetry.”
  • Feedings should be given through the gut if possible, if not, they may be given parenterally.