Critical Care Pearls

Perfusion Protection Strategies in Mechanically Ventilated Patients

Physiology

  • Normal intra-thoracic pressureis zero to negative
    • Positive pressure ventilation (PPV) reverses this physiology, making intra-thoracic pressure positive
  • Afterload Reduction
    • Negative intra-thoracic pressure contributes to afterload
      • PPV decreases afterload by increasing intra-thoracic pressure
  • Hypovolemia
    • Induction agents cause venodilation and decrease mean systemic pressure

Post-intubation hypotension

  • Positive pressure ventilation + drugs → hypotension
    • Incidence: 9.6-60%
  • Predictors:
    • Use of Neuro-muscular blockade – OR 2.7 (1.12-6.53)
    • Pre-existing low mean arterial pressure – OR 1.25 (1.01-1.55)
    • Shock index (HR/SBP) >0.8 – 67% sensitivity, 80% specificity
    • ESRD
    • Age
    • mSI (HR/MAP) >1.3– more sensitive and specific than SI

Prevention

  • Premedication: Fentanyl:Ketamine:Rocuronium (3:2:1 vs 1:1:1)
    • Ketamine associated with less myocardial depression, less AV dissociation and arrhythmias
  • Pre-intubation volume resuscitation
  • Awake intubation
  • Have a push-dose pure vasoconstrictor ready (phenylephrine, epinephrine)
  • Proper PPV settings
    • PEEP: 5
    • Tidal Volume: 6-8 ml / kg PBW

CASE: 1

82 year-old with h/o alcohol abuse, spinal stenosis, cerebral atrophy, transferred for esophageal perforation to the ICU. P-peak=15 and decreasing.

  • Differential Diagnosis
    • Air leak, BP fistula
  • Ventilator pressures:
    • Peak Pressure (P-peak)
      • Indicates airways resistanceand elastance
    • Plateau pressure (P-plat) (occlude ETT at end-inspiration)
      • Marker of compliance
    • Increased P-peak (normal P-plat):
      • Aspiration
      • Bronchospasm
      • Secretions
    • Increased P-plat→ Decreased compliance
      • Abdominal distention
      • Atelectasis
      • PTX
      • ARDS
      • Pulmonary edema

CASE: 2

39 year-old female with postpartum hemorrhage, with irregular and rapid RR (42) and her flow/time curve shows lack of return to baseline

  • Auto-PEEP:
    • Preventing Auto-PEEP is one of the important goals of ventilation
      • Increased RR→ decreased expiratory time → increased risk of Auto-PEEP
    • Detection:
      • Flow wave: failure of flow to return to baseline at end expiration
      • Pressure-time curve: steady increase baseline pressure
      • Volume wave: lack of return to baseline
      • Flow-volume loop: end-expiration flow does not return to baseline
      • Pressure-volume loop: incomplete loop
    • Treatment:
      • Disconnect the ventilator+ Observe blood pressure
      • Decrease I-time
      • Increase expiratory time
      • Decrease TV
      • Sedation/NMB

CASE: 3

18 year-old male in CSICU awaiting heart transplantation, echo reveals severely depressed LV function, EF 20%

  • Patient is intubated, milrinone and epinephrine infusions started
    • Follow up echo shows moderately depressed LV function
  • Following extubation patient becomes hypotensive, tachycardic. What is most likely cause?
    • Hypotension most likely caused by increase in LV afterload after extubation

CASE: 4

45 year-old male with history of liver transplantation for amyloidosis+ cardiomyopathy (EF = 25%), patient has been on and off milrinone “for weeks”. Intubated for respiratory distress and subsequently became profoundly hypotensive shortly after intubation. Swan-Ganz catheter is inserted: PA pressure 60/42 and CO 2.9.

  • Right Heart Failure – highly preload dependent
    • PPV decreases preload, resulting in hypotension
  • Prevention of hypotension after intubation in right heart failure:
    • Maintain preload
    • Normal to slow heart rate
    • Maintain afterload
    • Maintain contractility
    • PVR – maintain to decrease if possible

CASE: 5

76 year-old woman with 60 pack-years smoking history is admitted to SICU following exploratory laparotomy for small bowel obstruction. Patient is dyssynchronous with ventilator. PaO2 is 66 on Fio2 70%, high P-peak is noted.

  • Problems in COPD patients + MV:
    • Dynamic hyperinflation (auto-PEEP ↑, compliance ↓)
    • Displacement of P-V curve to upper flat portion
    • High alveolar pressure (WEST zones 1,2)
    • Narrow ETT
    • Decreased respiratory system compliance
    • Insensitive triggering
  • Goalsof ventilation in COPD:
    • Decrease RR to avoid dynamic hyperinflation
    • Use controlled modes of ventilation for limited time only
      • Potential for respiratory muscle atrophy
    • Adjust vent based on dynamic hyperinflation, NOT PaCO2
      • Allow permissive hypercapnia
    • AC/PC with prolonged I:E preferred
    • Add low levels of PEEP
      • Improves triggering and synchrony
    • Controlled hypoventilation
      • Ph>7.20 is ok
      • Moderate academia ok
    • Avoid over-ventilation
  • DON’T try to normalize ABGs
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