When lung compliance and spo2 decrease with increase in peep

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Here is a complete explanation:

When PEEP Increases but Both Compliance and SpO2 Fall: Overdistension

This is a classic and clinically important pattern. When you increase PEEP and see both compliance and SpO2 decrease simultaneously, the diagnosis is alveolar overdistension.

Why This Happens

Compliance = Tidal Volume / Driving Pressure (or Change in Volume / Change in Pressure)

As PEEP rises, there are two opposing effects on the lung:

Effect	Low-to-Optimal PEEP	Past Optimal PEEP
Alveolar recruitment	Collapsed alveoli open	No more recruitable alveoli left
Overdistension	Minimal	Already-open alveoli are over-stretched

When PEEP exceeds the optimal level:

Already-open alveoli are overinflated - they become stiff, reducing compliance
Overdistended alveoli compress adjacent pulmonary capillaries - this increases dead space and V/Q mismatch, paradoxically worsening oxygenation and dropping SpO2
Cardiac output may fall - elevated intrathoracic pressure reduces venous return, further reducing oxygen delivery

The Compliance-PEEP Relationship (Pressure-Volume Curve)

The optimal PEEP sits at the "sweet spot" on the pressure-volume curve:

Below lower inflection point: Alveoli collapse at end-expiration (atelectasis, shunt, low SpO2)
Between inflection points: Recruitment zone - compliance improves, SpO2 rises
Above upper inflection point: Overdistension - compliance falls, SpO2 also falls ← this is your scenario

Clinical Approach: Decremental PEEP Titration

Per Miller's Anesthesia and Current Surgical Therapy, the standard method to find optimal PEEP is the decremental PEEP trial:

Perform a recruitment maneuver (30-40 cmH2O for 30-40 seconds)
Set PEEP high (20-25 cmH2O)
Decrease PEEP stepwise by 2-5 cmH2O every 3-5 minutes
Monitor compliance (Tidal Volume / Driving Pressure) at each step
Set PEEP at the level that maximizes compliance
Stop decreasing when compliance starts to fall (atelectasis) - that level is too low; optimal is just above it

Miller's Anesthesia, 10e - Recruitment Maneuvers with PEEP Titration
Current Surgical Therapy 14e - PEEP Titration

What to Do When You See This Pattern

When increasing PEEP causes both compliance and SpO2 to fall:

Reduce PEEP back toward the prior level - you are in the overdistension zone
Consider a driving pressure-guided approach: ΔP = Plateau Pressure - PEEP; target ΔP < 15 cmH2O
Use transpulmonary pressure monitoring (esophageal manometry) for more individualized titration if available
Rule out auto-PEEP as a confounding factor (total PEEP = set PEEP + intrinsic PEEP)
Assess hemodynamics - overdistension reduces venous return and cardiac output, which also drops SpO2

Key Memory Hook

SpO2	Compliance	Interpretation	Action
↑	↑	Recruitment - still in the zone	Continue titrating up
↑	↓	Overdistension of some units, but net oxygenation still OK	Caution - approaching limit
↓	↓	Overdistension - past optimal PEEP	Decrease PEEP
↓	↓ (with low PEEP)	Atelectasis / derecruitment	Increase PEEP or recruit

The simultaneous fall in both compliance and SpO2 with rising PEEP is the clearest signal that you have crossed into overdistension - the lung has no more recruitable units and is only being stretched. This increases alveolar dead space, compresses capillaries, and reduces cardiac output, all of which worsen oxygenation. The correct response is to decrease PEEP to the last level where compliance (and SpO2) were maximized.

Here the p/f ratio is 95

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P/F Ratio of 95 + Falling Compliance + Falling SpO2 with PEEP: The Full Clinical Picture

Step 1: Classify the ARDS Severity

A P/F ratio of 95 places this patient in Severe ARDS by the Berlin Definition:

Category	P/F Ratio	PEEP Required
Mild	200-300	≥ 5 cmH2O
Moderate	100-200	≥ 5 cmH2O
Severe	< 100	≥ 5 cmH2O

P/F of 95 = severe, and this is already measured on PEEP, making the hypoxemia even more alarming.

Step 2: What Does This Combination Mean?

In severe ARDS with P/F = 95, when you increase PEEP and see:

Compliance ↓ + SpO2 ↓ = Overdistension (as covered before)

But here there is an important additional implication: the lung has minimal remaining recruitable tissue. In severe ARDS, there are two phenotypes:

Phenotype	Recruitable Lung	Response to High PEEP
Type H (Heavy) - diffuse injury, consolidated, heavy	Low	Overdistension likely, high PEEP harmful
Type L (Light) - less consolidated, more aerated	Higher	May tolerate and benefit from higher PEEP

A P/F of 95 with compliance falling at higher PEEP strongly suggests the Type H phenotype - this lung is already maximally injured, mostly consolidated/collapsed, with very little recruitable alveoli left. Every additional cmH2O of PEEP is just stretching the few remaining open alveoli.

Step 3: Why SpO2 Falls Despite Higher PEEP in This Scenario

Three mechanisms converge in severe ARDS:

Capillary compression by overdistended alveoli - increases dead space, reduces effective gas exchange
Impaired right ventricular output - high intrathoracic pressure reduces RV preload and increases RV afterload; RV failure further drops cardiac output and oxygen delivery
Diversion of blood to poorly perfused units - overdistension of well-perfused alveoli diverts blood to collapsed (shunt) regions, per Murray & Nadel's Respiratory Medicine - worsening V/Q mismatch and right-to-left shunt

Step 4: What to Do - Immediate Management

Since increasing PEEP is making things worse (not better), the strategy must pivot:

1. Reduce PEEP to last tolerated level

Return to the PEEP where compliance and SpO2 were best
Use driving pressure (ΔP = Plateau - PEEP) to guide: target ΔP < 15 cmH2O

2. Do NOT keep increasing PEEP hoping for improvement

The ART trial (2017) showed that aggressive recruitment + compliance-guided PEEP in severe ARDS actually increased 28-day mortality - Murray & Nadel's, Current Surgical Therapy 14e

3. Escalate to rescue therapies for P/F < 100

Since oxygenation is critically low and PEEP titration has failed, now activate salvage strategies:

Therapy	Rationale
Prone positioning (16+ hrs/day)	Recruits dependent lung, improves V/Q, reduces mortality in P/F < 150 (PROSEVA trial)
Neuromuscular blockade	Eliminates dyssynchrony, reduces oxygen consumption, improves plateau pressure
Inhaled pulmonary vasodilators (NO, prostacyclin)	Diverts blood to ventilated alveoli, improves V/Q temporarily
ECMO (VV)	Indicated when P/F < 80 with optimal ventilation, or P/F < 100 with rising pressures

4. Check for confounders before any PEEP change

Auto-PEEP? (Total PEEP = Set + Intrinsic)
Pneumothorax? (Tension PT can mimic this exact pattern)
New pleural effusion or mucus plug causing acute desaturation?

Bottom Line

Finding	Meaning
P/F = 95	Severe ARDS - critically ill lung
Compliance ↓ with ↑ PEEP	Overdistension - past optimal PEEP
SpO2 ↓ with ↑ PEEP	Shunt worsening from capillary compression + reduced cardiac output
Combined	Non-recruitable lung - PEEP is not your friend here; escalate to prone/ECMO

This patient likely needs prone positioning immediately (given P/F < 150) and should be assessed for VV-ECMO candidacy (P/F < 100 with refractory hypoxemia despite optimal ventilator settings).

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