WaveformsWaveforms

RC 270

Pressure Volume Curves

Graphic display of changes in compliance and resistance

Used for TREND analysis!– One set of waveforms must be compared to

another set Pressure is on the x-axis; volume on the y-

axis Separate curves for static and dynamic

Static Curve

If static curve shifts, than static compliance and elastic resistance have changed– Plateau pressure also will change

A change in the static curve causes the dynamic curve to shift also– Just as a plateau pressure change also affects peak pressure

Rightward shift: decreased static compliance and increased elastic resistance

Leftward shift: increased static compliance and decreased elastic resistance

Dynamic Curve

Is affected by both elastic and airway resistance

If the dynamic curve shifts, but the static curve doesn’t, than only airway resistance has changed– Rightward means increased airway resistance– Leftward means decreased airway resistance

When to plot P-V Curves

Initiation of CMV Q 8-12 PRN

– Change in patient status– After any therapeutic intervention

P-V Loop: Overdistension(Hyperinflation)

Flow-Time Curve: Air Trapping(AutoPeep)

Continuous Positive Continuous Positive Pressure BreathingPressure Breathing

Breathing at a baseline airway pressure that is greater than ambient

Types of CPPB

Positive End Expiratory Pressure (PEEP)– When patient is on

CMV

Continuous Positive Airway Pressure– When patient is

breathing without CMV

Indication for Indication for PEEP/CPAP: PEEP/CPAP:

Refractory HypoxemiaRefractory Hypoxemia

PaO2 < 60mmhg on an FIO2 of 60% or >

Refractory Hypoxemia Usually Indicates Alveolar Instability

Decreased surfactant Increased interstitial pressure Nitrogen washout Oxygen toxicity

Alveolar Instability

Atelectasis Decreased FRC Increased shunting Decreased static compliance (increased

elastic resistance) Increased W.O.B.

PEEP/CPAP

Airway pressure does not drop to ambient Helps stabilize alveoli and small airways

Without PEEP/CPAP

With PEEP/CPAP

PEEP/CPAP acts like a “splint” to stabilize alveoli/airways

PEEP/CPAP EFFECTS

Increases FRC back towards normal Decreases shunt Increases static compliance (decreases

elastic resistance) Decreases W.O.B. Acceptable PaO2 at lowest possible FIO2

PEEP/CPAP Precautions

Potential Side Effects

Increased mean intrathoracic pressure (ITP)– Hemodynamic compromise

• Decreased venous return• Decreased cardiac output• Decreased blood pressure• Increased intracranial pressure (ICP)

– Pulmonary baro/volutrauma– Fluid retention

If PEEP/CPAP is indicated and applied appropriately, these effects should be minimized

PEEP/CPAP Precautions

Unilateral lung disease Neuro patients COPD

PEEP/CPAP Controversy

Classical indication: increased elastic resistance

Controversy: Increased airway resistance too?

Can we please take a break!

PEEP/CPAP Systems: Flow Resistor

PEEP/CPAP Systems: Threshold Resistor

Selecting a PEEP/CPAP Level

Random– Start at 5 cmH2O and watch PaO2

Optimal/Best -Set PEEP/CPAP 2 cmH2O greater than lower inflection point (LIP or Pflex) on P-V curve Best/Optimal PEEP alternative

– Peep only—use PEEP level that gives best static compliance

LIP or Pflex

LIP

Best PEEP Best PEEP always always occurs at the lowest /\ Poccurs at the lowest /\ P

Remember /\P is gauge plateau pressure

minus the peep

What if more than one PEEP level gives the same highest

Cstat?

Look at blood pressure Look at PvO2 Look at C.O. Look at %shunt Look at mixed venous saturation Look at P/F ratio

– PaO2 divided by FIO2 (expressed in decimal form)

Recruitment Maneuver

Also known as Open Lung Tool

A Sustained Maximal Inspiration (SMI) performed after optimal/best PEEP/CPAP has been determined and set

Example: 40 cmh2o of CPAP applied for 40 seconds (the 40/40 technique)

PEEP/CPAP works best PEEP/CPAP works best for diseases that cause for diseases that cause

increased elastic increased elastic resistanceresistance

May be of help in diseases with

increased airway resistance

Lets see what we can dig up