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.