6 - Effects of Mechanical Ventilation

decreased venous return

• decreased C.O.

• hypotension in hypovolemia or CNS depression

• tachycardia → better BP

• treatment: IV fluids, decrease MAP

decreased venous return

• increased ICP

• head injury, brain tumor, post-surgery

• monitor if PEEP needed (normal: < 10 torr)

• treatment: hyperventilation, meds

decreased venous return

• BP changes

• normal ventilation: arterial pressure decreases with inspiration (pulsus paradoxus)

• PPV: arterial pressure increases with positive pressure (except in hypovolemia)

pneumothorax

• manifestations

• chest pain, dyspnea, cyanosis

• increase in airway pressure (PIP and P_plat)

• absent/diminished breath sounds

• hyper-resonance

• tracheal deviation, asymmetric chest excursion, subcutaneous emphysema

• hypertension → hypotension

• tachycardia → bradycardia

• cardiac arrest

pneumothorax

• treatment

• MV: 100% F_iO₂, low V_T, low pressure

• needle decompression

  • 14 gauge needle, 2nd/3rd intercostal space, mid-clavicular line

• chest tube

renal effects of MV

• decreased urine

• sodium and water retention (CPPV > IPPV)

renal effects of MV

• causes

• low BP

• redistribution of renal blood flow

• abnormal pH/P_aCO₂/P_aO₂

renal effects of MV

• manifestations

• < 400 mL/24 hours urine

• increased BUN (normal: 10-20 mg/dL)

• increased creatinine (normal: 0.7-1.5 mg/dL)

• maybe abnormal sodium/potassium levels

GI and liver effects of MV

downward diaphragm movement decreases portal venous flow and increases splanchnic resistance

• liver ischemia → increased bilirubin

• increased resistance → gastric mucosal ischemia

• stress → gastric acid hyper-secretion, ulcers, and bleeding

GI and liver effects of MV

• diagnosis

• increased prothrombin (PT): > 4 sec

• hyperbilirubinemia: ≥ 50 mg/L

• hypoalbuminemia: ≤ 20 g/L

troubleshooting possible machine and airway malfunction

1. bag patient if unsure

2. use PEEP if patient has set PEEP at > 10 cmH₂O

3. if patient improves, suspect machine/circuit issue

4. if patient does not improve, check for pneumothorax, ET tube kink, or mucus plugging

prevention of atelectasis and infection

• proper V_T

• aseptic airway care

• bronchial hygiene

• reposition patient often

management of immobilized patient

• reposition often to prevent atelectasis and promote secretion mobilization

• good lung down in unilateral disease (West Zone Theory)

risks for air trapping/auto-PEEP

• obstructive disease

• ARDS

• burns

• small ETT

minimizing air trapping/auto-PEEP

• lower R_aw (bronchodilators, suction, use larger ETT)

• increase e-time

  • increase flow to 80-100 L/min (↓i-time)

  • replace standard circuit with low compressible volume circuit

  • COPD: add PEEP at/below auto-PEEP to reduce WOB, withdraw as auto-PEEP decreases

  • ↓V_E if possible (P_aCO₂ rise as long as pH > 7.30)

oxygen hazards

• O₂-induced hypoventilation

• absorption atelectasis

• O₂ toxicity

preventing/minimizing biotrauma (volu-/baro-/atelectrauma)

• use small V_T in COPD

• decrease V_T with restriction

• PEEP cautiously in:

  • unilateral disease

  • necrotizing/cavity-filled lung disease (ie, TB)

  • COPD, asthma

  • late ARDS

• monitor lung-thorax compliance

• avoid sighs

• correct auto-PEEP

prevention of VAP

• elevate HOB (30-45°)

• sedation vacations

• assess readiness to wean daily

• prevent pressure ulcers

• subglottic suction

• mouth care daily

• avoid breaking circuit unless necessary