Quiz 11: Vent Support I Quiz Notes

Vent Support I – Quiz 11 Notes

Question 1: Cardiac Output Reduction

• Question: Which mode causes the greatest reduction in cardiac output during ventilation?
• Answer: VC-CMV with PEEP.
• Explanation:
  • VC-CMV (Volume-Controlled Continuous Mandatory Ventilation) with PEEP increases intrathoracic pressure significantly.
  • Increased intrathoracic pressure leads to decreased venous return.
  • Decreased venous return results in a greater reduction in cardiac output compared to other modes.
  • PEEP exacerbates this effect by further impeding venous return and increasing right ventricular afterload.

Question 2: High Tidal Volumes & PEEP

• Question: High tidal volumes and/or high levels of PEEP can cause which of the following?
• Answer: Increased right ventricular afterload.
• Explanation:
  • High tidal volumes and PEEP can overdistend alveoli.
  • Overdistension compresses pulmonary capillaries.
  • Compression raises pulmonary vascular resistance.
  • Increased pulmonary vascular resistance increases the afterload on the right ventricle.

Question 3: PEEP and Cardiac Output in Normovolemic Patients

• Question: Normovolemic patients experience decreases in cardiac output above what level of PEEP?
• Answer: 15 cm H2O
• Explanation: Cardiac output in normovolemic patients tends to decline significantly when PEEP exceeds 15 cm H₂O, due to a substantial drop in venous return and increased intrathoracic pressure.

Question 4: Hemodynamic Changes and Alveolar Pressures

• Question: Which patient condition is least likely to experience hemodynamic changes with high alveolar pressures during mechanical ventilation?
• Answer: Third-degree chest wall burns

Question 5: Harmful Cardiovascular Effects of PPV

• Question: The harmful cardiovascular effects of PPV are influenced most by which of the following pressures?
• Answer: Mean airway pressure.
• Explanation:
  • Mean airway pressure (Paw) reflects the average pressure applied to the lungs throughout the respiratory cycle.
  • It is the most influential pressure affecting venous return, intrathoracic pressure, and ultimately cardiac output.
  • While PEEP and peak inspiratory pressures contribute, the sustained elevation of pressure (mean airway pressure) has the greatest cumulative hemodynamic effect.

Question 6: Physiological Dead Space in Apneic Patients

• Question: Which of the following can increase the physiological dead space in apneic patients receiving volume control ventilation?
• Answer: Adding an inflation hold

Question 7: Spontaneous Breathing and Intrapleural Pressure

• Question: During spontaneous breathing, the fall in intrapleural pressure that draws air into the lungs during inspiration also draws blood into the major thoracic vessels. This phenomenon increases which of the following?
• Answer: Right ventricular preload

Question 8: Cerebral Perfusion Pressure Calculation

• Question: Calculate the cerebral perfusion pressure when the mean arterial blood pressure is 120 mm Hg and the ICP is 14 mm Hg.
• Answer: 106 mm Hg
• Explanation:
  • The formula for cerebral perfusion pressure is: CPP = MAP - ICP
  • CPP = 120 - 14 = 106 mm Hg

Question 9: Urinary Output and Renal Arterial Pressure

• Question: Urinary output is severely reduced when the renal arterial pressure decreases below what level?
• Answer: 75 mm Hg
• Explanation: Renal perfusion becomes compromised below 75 mm Hg, leading to reduced glomerular filtration and marked decrease in urine output. This threshold reflects the kidney’s autoregulatory limit for maintaining function.

Question 10: Positive Pressure Ventilation and the Kidneys

• Question: Positive pressure ventilation has which of the following effects on the kidneys?
• Answer: Redistributes blood flow in the kidneys
• Explanation:
  • Positive pressure ventilation (PPV) can reduce renal perfusion and cause a redistribution of blood flow within the kidneys, often shifting blood away from the cortex to the medulla.
  • This contributes to decreased glomerular filtration rate and reduced urine output.
  • It does not increase renal blood flow or urinary output.

Question 11: Mechanical Ventilation and Hormones

• Question: Mechanical ventilation has an effect on which hormone that causes a decrease in urinary output?
• Answer: Arginine vasopressin
• Explanation:
  • PPV stimulates the release of arginine vasopressin (AVP), also known as antidiuretic hormone (ADH).
  • ADH promotes water reabsorption in the kidneys and decreases urine output.

Question 12: Positive Pressure Ventilation and Splanchnic Resistance

• Question: Positive pressure ventilation increases splanchnic resistance, decreases splanchnic venous outflow, and may contribute to gastric mucosal ischemia, which can contribute to which of the following?
• Answer: Gastric ulcers
• Explanation: The hemodynamic changes caused by PPV (especially high PEEP) can impair perfusion to the gastrointestinal tract, particularly the gastric mucosa, leading to ischemia and gastric ulcer formation.

Question 13: Interventricular Septum Movement

• Question: Which of the following condition must exist to cause the interventricular septum to move to the left during positive pressure ventilation?
• Answer: Mean airway pressure >15 cm H2O
• Explanation: When mean airway pressure exceeds ~15 cm H₂O, intrathoracic pressure rises enough to significantly increase right ventricular afterload. This can cause right ventricular dilation and force the interventricular septum to shift leftward, compromising left ventricular filling and output.

Question 14: Uneven Ventilation

• Question: Which of the following is the most likely cause of uneven ventilation?
• Answer: High inspiratory flow rates
• Explanation: High inspiratory flow rates can lead to short inspiratory times, resulting in maldistribution of gas—especially in lungs with heterogeneous time constants (as in obstructive or restrictive disease). This causes uneven alveolar filling.

Question 15: Effects of PEEP on Heart Function

• Question: Which of the following are potential effects of PEEP on heart function?
  1. Decreased airway pressure
  2. Decreased functional residual capacity
  3. Increased airway pressure
  4. Increased functional residual capacity
• Answer: 3 and 4 only
• Explanation: PEEP increases airway pressure and raises functional residual capacity (FRC) by preventing alveolar collapse. However, it does not decrease airway pressure or FRC. These effects can improve oxygenation but may negatively impact preload and cardiac output at high levels.