Ch. 16 Extrapulmonary Effects of Mechanical Ventilation

Extrapulmonary Effects of Mechanical Ventilation

Cardiovascular Function

• Spontaneous Inspiration:
  • Negative intrapleural pressures increase venous return.
  • Pressure gradient to the heart increases due to drop in vena cava pressure.
• Right Ventricular Preload:
  • Increased during inspiration.
• Left Ventricular Preload:
  • Decreased during passive expiration.
  • Venous return decreases as intrapleural pressure becomes less negative.
  • Right ventricular stroke volume decreases.
• Pulmonary Capillaries:
  • Overdistension narrows capillaries, increasing pulmonary vascular resistance and right ventricular afterload.
• High Positive Pressure:
  • Levels >15 cm H2O and volume depletion reduce cardiac output and coronary artery perfusion, potentially causing myocardial ischemia.
• Compensatory Mechanisms:
  • Include increased sympathetic tone, systemic vascular resistance, peripheral venous pressure, and peripheral shunting of blood.
• Measuring Blood Pressure:
  • Essential soon after initiating PPV.
• Venous Return:
  • PPV reduces venous return, improving stroke volume by optimizing length-tension relationships.

Mean Airway Pressure

• Formula: Mean\ Airway\ Pressure = \frac{1}{2}[PIP × (\frac{TI}{TCT})] where TCT = \frac{60}{Respiratory\ Rate}
• Inspiratory Hold: Increases mean airway pressure.
• Formula with PEEP: Mean\ Airway\ Pressure = \frac{1}{2}[(PIP - PEEP) × (\frac{TI}{TCT})] + PEEP

Ventilation Parameters

• High Inspiratory Flow Rates: Can cause uneven ventilation.
• I:E Ratio: Ratios of 1:1, 2:1, or higher can lead to air trapping and hemodynamic complications.
• Factors Influencing Cardiovascular Effects: Inspiratory flow and pattern, I:E ratio, inflation hold, PEEP, and ventilator mode.
• Rapid Flow Rates: Deliver desired VT quickly, shortening inspiratory time and lowering mean airway pressure.
• PEEP: Decreases cardiac output if pressure transmits to intrathoracic space; less impact with "stiff" lungs.

Neurological Function

• CPP (Cerebral Perfusion Pressure): CPP = MABP - ICP
• PPV Impact: Increases central venous pressure, potentially raising ICP.
• Increased ICP Sign: Jugular vein distention.
• Hyperventilation: Temporarily constricts cerebral vessels by lowering PaCO2.

Renal Function

• PPV Effects: Alters kidney function due to hemodynamic changes, humoral responses (ADH, ANF, renin-angiotensin-aldosterone), and pH/PaCO2/PaO2 abnormalities.
• Reduced Urinary Output: Occurs when glomerular capillary pressure drops below 75 mm Hg.
• Hormonal Influence:
  • ADH: Released due to blood pressure changes from PPV, causing oliguria.
  • Atrial Natriuretic Factor: Reduced by PPV and PEEP, leading to water and sodium retention.
  • Renin-Angiotensin-Aldosterone: Activated during PPV, causing sodium and water retention.
• Blood Gas Impact: Decreased PaO2 and increased PaCO2 impair renal function.
• Drug Effects: Altered renal function from PPV can prolong drug effects.

Gastrointestinal Function

• Liver Effects: PPV and PEEP can elevate serum bilirubin, potentially from reduced cardiac output, increased diaphragmatic force, decreased portal venous flow, or increased splanchnic resistance.
• Gastric Distention: Can result from air swallowing; managed with a gastric tube.
• Malnutrition Risk: Seriously ill patients are at risk due to inadequate food intake and increased metabolic rate.

Additional Considerations

• Complications of Malnutrition: Reduced response to hypoxia/hypercapnia, muscle atrophy, respiratory infections, decreased surfactant, slowed tissue healing, and lower serum albumin.
• Overfeeding Risks: Increases oxygen consumption, carbon dioxide production, and work of breathing.
• Nutritional Assessment: Includes body composition, weight, anthropometric measurements, protein deficiencies, and immune function.
• Mitigation: Early recognition and intervention are crucial for reducing complications associated with PPV.