Intro to mechanical Ventilation pt 1

Positive Pressure Ventilation (PPV)

• PPV is a method of delivering gases to the lungs, an unnatural way of breathing.
• PPV affects various systems, including pulmonary, cardiovascular, hemodynamic, renal, hepatic, abdominal, gastrointestinal, nutritional, and neurologic systems.

Normal Spontaneous Breathing

• Normal breathing involves a negative pressure gradient, with lung pressure lower than atmospheric pressure.
• Gas flow during spontaneous breathing is driven by this pressure difference.

PPV and Volume Delivery

• PPV delivers gas to the lungs with a positive pressure gradient (airway pressure greater than alveolar pressure).
• Volume delivered to the patient is controlled by either set pressure or set volume.

Air Trapping (Auto-PEEP)

• Air trapping, also known as auto-PEEP or intrinsic PEEP, is a common issue related to PPV.
• Using lower tidal volume or frequency, higher peak inspiratory flow and shorter inspiratory time can help reduce air trapping and auto-PEEP.

Cardiovascular Considerations

• Positive pressure in the lungs increases intrathoracic pressure.
• This can reduce cardiac output and oxygen delivery through a chain of physiologic events.

Hemodynamic Considerations

• PPV can negatively affect hemodynamics, impacting pulmonary blood volume, systemic blood volume, central venous pressure (CVP), pulmonary artery pressure (PAP), and ventricular filling pressure and output.
• Higher intrathoracic pressure impedes systemic blood return.

Renal Considerations

• PPV lowers cardiac output and perfusion to organs, including the kidneys.
• Given that the kidneys receive 25% of circulating blood, reductions in blood volume or cardiac output can lead to renal insufficiency and, eventually, renal failure.
• Symptoms of renal insufficiency include reduced urine output, elevated blood urea nitrogen (BUN), and elevated creatinine.

Hepatic Considerations

• PPV lowers cardiac output and perfusion to the liver (receives 15% of circulating blood).
• Hepatic perfusion decreases when PEEP is added to PPV.
• Liver dysfunction indicators include increased coagulation time (prothrombin time >4 sec over control), increased bilirubin (>50 mg/L), and decreased albumin levels (<20g/L).
• Drugs like lidocaine, meperidine, propranolol, and verapamil are affected by liver dysfunction as the liver processes these drugs.

Abdominal Considerations

• Increased intra-abdominal pressure (IAP) due to conditions like bowel edema, ascites, and surgical procedures is negatively affected by PPV.
• High levels of PEEP (positive end-expiratory pressure), if combined with increased IAP, can severely affect cardiovascular, renal, and pulmonary systems.

Gastrointestinal Considerations

• Decreased perfusion to the gastrointestinal (GI) tract can be caused by PPV and associated factors like medications like opioids leading to conditions like erosive esophagitis, stress-related mucosal damage, and diarrhea.

Nutritional Considerations

• Malnutrition can lead to respiratory muscle fatigue which can worsen issues related to PPV.
• Excessive nutritional support (e.g., TPN) can be affected by increased metabolic demand, CO2 production during PPV, and increase in work of breathing.
• Specific considerations for respiratory muscle fatigue include mechanical factors (high airway resistance, low lung compliance) and non-mechanical factors (malnutrition, electrolyte imbalances, drugs).

Neurological Considerations

• Hyperventilation (reducing CO2) can cause cerebral vasoconstriction and decreased cerebral blood flow.
• Using hyperventilation can temporarily lower intracranial pressure (ICP).
• Prolonged hyperventilation can lead to respiratory alkalosis and neurologic dysfunction.
• Other issues like hypercapnia, hypoxia, and electrolyte changes can also compromise neurological function. - Specific neurological indicators of impairment include headache, mental status changes, motor disturbances, and ocular abnormalities.