2/ A patient with lobar pneumonia is on FiO2 1.0 and PEEP 10 cm H2O.
After PEEP is increased to 20 cm H2O:
🔹SpO2 drops from 90% to 79%
🔹BP drops
🔹ScvO2 drops from 60% to 40%
1. How does PEEP improve hypoxemia?
2. Why did PEEP worsen hypoxemia and⬇️BP & ScvO2 in this case?
After PEEP is increased to 20 cm H2O:
🔹SpO2 drops from 90% to 79%
🔹BP drops
🔹ScvO2 drops from 60% to 40%
1. How does PEEP improve hypoxemia?
2. Why did PEEP worsen hypoxemia and⬇️BP & ScvO2 in this case?
3/ Let's start with what PEEP is: positive end-expiratory pressure.
This pressure increases airway & alveolar pressure throughout the respiratory cycle.
A low level of PEEP (~ 5 cm H2O) is used to offset the ⬇️in FRC w/supine positioning.
This pressure increases airway & alveolar pressure throughout the respiratory cycle.
A low level of PEEP (~ 5 cm H2O) is used to offset the ⬇️in FRC w/supine positioning.
4/ How does PEEP improve hypoxemia?
💡By improving the transmural (Ptm) pressure!
Ptm= Pin - Pout
Ptm= Palveolar – Patmospheric (0)
Ptm = Palveolar
⬆️Ptm= increased volume of the respiratory system, improving alveolar ventilation and gas exchange
💡By improving the transmural (Ptm) pressure!
Ptm= Pin - Pout
Ptm= Palveolar – Patmospheric (0)
Ptm = Palveolar
⬆️Ptm= increased volume of the respiratory system, improving alveolar ventilation and gas exchange
5/ Let’s look at the V-P curves of the resp system (RS)
Panel B: PEEP= 0 and RS returns to FRC
Panel C: ⬆️PEEP leads to ⬆️end-exp volume, maintaining V/Q ratios and improving oxygenation
And now closer to steeper portion of curve: less deltaP needed to achieve deltaV
Panel B: PEEP= 0 and RS returns to FRC
Panel C: ⬆️PEEP leads to ⬆️end-exp volume, maintaining V/Q ratios and improving oxygenation
And now closer to steeper portion of curve: less deltaP needed to achieve deltaV
6/ Then why did PEEP worsen hypoxemia?
The 2 types of vessels in pulm microcirculation (intra-alveolar & extra-alveolar) respond differently to changes in lung volume.
These vessels are in series, so pulmonary vascular resistance (PVR) = sum of resistance of each vessel group.
The 2 types of vessels in pulm microcirculation (intra-alveolar & extra-alveolar) respond differently to changes in lung volume.
These vessels are in series, so pulmonary vascular resistance (PVR) = sum of resistance of each vessel group.
7/ In lobar pneumonia, high PEEP causes larger volume changes in healthy areas of lung, overdistending intra-alveolar vessels and increasing PVR.
Blood flow is diverted to areas of lower PVR (in diseased lung).
💡⬆️blood flow through low V/Q & shunt units worsens gas exchange
Blood flow is diverted to areas of lower PVR (in diseased lung).
💡⬆️blood flow through low V/Q & shunt units worsens gas exchange
8/Why did PEEP⬇️BP?
⬆️intrathoracic pressure
⬆️right atrial pressure
⬇️pressure gradient driving venous return to♥️
⬇️venous return/ventricular preload
=
Hypotension (especially with volume depletion)
⬇️cardiac output can decrease oxygen delivery!
DO2 = CO x CaO2
⬆️intrathoracic pressure
⬆️right atrial pressure
⬇️pressure gradient driving venous return to♥️
⬇️venous return/ventricular preload
=
Hypotension (especially with volume depletion)
⬇️cardiac output can decrease oxygen delivery!
DO2 = CO x CaO2
9/ And what about the⬇️in ScvO2?
Tissue response to⬇️DO2 is to⬆️oxygen extraction, decreasing the oxygen content and saturation of both central venous (ScvO2) and mixed venous (SvO2) blood.
⬇️ScvO2 will worsen hypoxemia with shunt physiology – note⬇️in PaO2 from panel A to B.
Tissue response to⬇️DO2 is to⬆️oxygen extraction, decreasing the oxygen content and saturation of both central venous (ScvO2) and mixed venous (SvO2) blood.
⬇️ScvO2 will worsen hypoxemia with shunt physiology – note⬇️in PaO2 from panel A to B.
10/ Summary:
💡PEEP⬆️Ptm and RS volume and allows ventilation at steeper portion of P-V curve
💡W/focal process, PEEP overdistends good lung and⬆️PVR, diverting blood to diseased lung
💡High PEEP can⬇️venous return and Q
💡⬇️DO2 can⬇️SvO2, worsening hypoxemia
/end
💡PEEP⬆️Ptm and RS volume and allows ventilation at steeper portion of P-V curve
💡W/focal process, PEEP overdistends good lung and⬆️PVR, diverting blood to diseased lung
💡High PEEP can⬇️venous return and Q
💡⬇️DO2 can⬇️SvO2, worsening hypoxemia
/end
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