Galvagno: Patient-ventilator interactions, perfusion-protection strategies

Clinical Pearls (assisted by Dr. Ram Baalachandran)

Physiology:

• Normal intra-thoracic pressure is 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
    • Results in collapse of inferior vena cava and decreased venous return (coupled with PPV leads to a decrease in Preload)
    • Can result in complete CV collapse!

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:

• Remifentanil 4 mcg/kg (super potent opioid and broken down by esterases)
  • +/- NMB
  • +/- propofol
• 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 resistance and 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 yo 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 olds male with h/o 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 dys-synchronous 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
• Goals of 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 hyerinflation, 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