Extrapulmonary Effects of Positive Pressure Ventilation

Vocabulary practice flashcards covering the extrapulmonary effects of positive pressure ventilation (PPV), including cardiovascular, intracranial, renal, hepatic, gastrointestinal, and nutritional considerations.

Mean Airway Pressure ($\bar{P}_{aw}$)

The average pressure applied to the airways across both inspiration and expiration, influenced by peak inspiratory pressure (PIP), inspiratory time (I-Time), respiratory rate (RR), tidal volume ($V_T$), and positive end-expiratory pressure (PEEP).

Effect of PEEP on Mean Airway Pressure

The application of positive end-expiratory pressure (e.g., $10\,cm\,H_2O$) prevents airway pressure from returning to zero at end-expiration, thereby elevating both the baseline pressure and the mean airway pressure ($\bar{P}_{aw}$).

Decreased $\bar{P}_{aw}$ Strategy (Increased Flow / Decreased I:E)

A ventilator approach that provides the benefits of reduced risk for barotrauma and reduced cardiovascular complications, but carries the hazards of uneven gas distribution, decreased $P_aO_2$, and increased $P_aCO_2$.

Increased $\bar{P}_{aw}$ Strategy

A ventilator management approach that yields benefits of increased alveolar ventilation ($\dot{V}_A$), alveolar recruitment, and improved gas distribution, but poses hazards of decreased cardiac output, decreased $O_2$ transport, and increased risk for barotrauma.

Thoracic Pump Mechanism (Spontaneous Breathing)

The generation of negative intrapleural pressures during spontaneous inspiration as the diaphragm moves downward, which decreases vena cava pressure, increases the pressure gradient back to the heart, and promotes venous return.

PPV Effect on Venous Return

The transmission of elevated positive intrapleural and intrathoracic pressures to thoracic vessels during positive pressure ventilation, resulting in increased pressure within the vena cava and a reduction in venous return to the heart.

Alveolar Overdistention (Pulmonary Capillary Compression)

The excessive enlargement of alveoli due to high ventilatory pressures or volumes that compresses adjacent pulmonary capillaries, leading to thinning of the vessel walls and an increase in pulmonary vascular resistance.

Right Ventricular Shift (Ventricular Interdependence)

A phenomenon in which right ventricular volume overload causes dilation of the right ventricle, flattening of the interventricular septum, and compression of the left ventricle into a D-shape under increased pericardial constraint.

Cardiovascular Compensatory Mechanisms in PPV

Autonomic responses triggered to maintain blood pressure and cardiac performance in response to PPV-induced reductions in venous return, specifically tachycardia (increased heart rate) and systemic vasoconstriction.

Cerebral Perfusion Pressure (CPP)

The pressure gradient driving blood flow to the brain, calculated as $\text{CPP} = \text{MAP} - \text{ICP}$, with a normal targeted range of $60\text{ to }70\,mmHg$.

PPV Effect on Intracranial Pressure (ICP)

The impediment of cerebral venous drainage through the superior vena cava and jugular venous system caused by increased positive intrathoracic pressure, which can lead to an increase in intracranial pressure (ICP).

Renal Effects of Positive Pressure Ventilation

Alterations in kidney function characterized by hypotension, increased antidiuretic hormone (ADH) secretion, reduced atrial natriuretic peptide (ANP) secretion, and resultant sodium and water retention, compounded by hypoxemia and hypercarbia.

Hepatic and Gastrointestinal Effects of PPV

Organ complications caused by hypotension, direct mechanical compression, and increased splanchnic resistance, resulting in decreased liver function, increased bilirubin levels, and the development of gastric ulcers.

Histamine 2 ($\text{H}_2$) Blockers

Pharmacologic agents administered during mechanical ventilation to increase gastric pH, thereby providing prophylaxis against the development of stress-related gastric ulcers.

Nutritional Considerations in Mechanical Ventilation

Adverse systemic effects that occur when nutrition is inadequate during positive pressure ventilation, including respiratory muscle atrophy, decreased immune response, reduced surfactant production, impaired epithelial regeneration, and lower serum albumin levels.