Monitoring Mechanical Ventilation

Dr Jacqui Brown
Intensive Care Unit
Chris Hani Baragwanath Hospital

Contents
Introduction
Gas exchange
ABG
Oxymetry
Capnography
Respiratory mechanics
Airway Graphics
Other
CXR

Introduction
Monitoring : derived from Latin word monere
meaning ‘to warn’
In the past : manual measurements by nursing staff,
and organs other that the lung e.g. ECG
Goals of continuous monitoring :
follow real time specific physiological values that change
rapidly – alerts for adverse events
Aid in diagnosis
Enables assessment of therapeutic intervention

Introduction
Ideal monitoring systems:
Must have the potential to alter/influence the
management of the patient
Easy to use and interpret
Technically accurate, both specific and sensitive
Safe
Not interfere with patient care
Inexpensive

Introduction
Respiratory function:
Monitoring gas exchange
Oxygenation
Ventilation
Monitoring lung and chest wall mechanics
Complience
Resistance
Pressure

Gas Exchange
Aim of mechanical ventilation:
Maintain gas exchange to keep body in relative
homeostasis
Deliver sufficient oxygen to body
Eliminate CO2

Gas Exchange
The clinical significance of hypoxia/hypercapnia depends on
Chronicity
Compensatory mechanisms
Tolerance of vital organs e.g brain and heart
Degree of oxygenation depends on patient and condition
Not all patients require full O2 saturation or normal CO2
May not achieve “Normal” values

Gas Exchange
Methods to assess Oxygenation
1. Naked eye – poor
2. ABG – PaO2 and Sat
3. Pulse oximetry
Methods to assess ventilation (CO2)
1. ABG – PaCO2
2. Capnography

Arterial Blood Gas
Widely used
Advantages:
1. Direct measurement of PaO2 and PaCO2
2. Also gives values for acid-base status and electrolytes
Disadvantage:
1. Not specific or sensitive
2. Calculates saturation
3. Requires invasive procedure - safety, accuracy
4. Intermittent sampling – miss events

Arterial Blood Gas
Need to know normal values.
Factors influencing values:
1. Age
1. PaCO2 remains relatively constant
2. PaO2 decreases with age
2. Altitude
3. Natural variations
4. Sampling techniques

Arterial Blood Gas
Other information that can be obtained and calculated using
ABG.
Efficacy of oxygen exchange
1. Alveolar gas equation
PAO2 = PIO2 – (PaCO2/R)
N≈ 100mmHg at sea level , and 75mmHg in JHB
AaDO2 = PAO2 – PaO2
1. N≈ 10mmHG in room air
2. Oxygenation index PaO2/(FiO2 X Paw)
3. PaO2/FiO2

Arterial Blood Gas
Ventilation
PaCO2 is directly measured in blood.
PaCO2 is a measure of ventilation – CO2 elimination
Increaesed PaCO2
Airway obstruction and bronchospasm.
Hypoventilation
Increased metabolism

Gas Exchange - Oxymetry
Initially used in WW2 for pilots.
Determines O2 saturation by absorption
spectrophotometry.
Uses the difference in the absorption spectra of oxyHb
and reduced Hb.
Absorption of light by capillary blood is constant.
Pulsatile arterial blood – light absorption is variable, so
saturation can be calculated

Gas Exchange - Oxymetry
Monitor advantages:
1. Inexpensive
2. Accurate
1. - not accurate at sats below 80%
3. Direct measurement
4. Continuous
5. Non-invasive
6. With arterial wave tracing - can see stroke volume
variation

Pulsoximetry

Gas Exchange - Oxymetry
Limitations
1. Underestimates with decreased perfusion
2. Other forms of Hb absorb at different wavelengths
3. Jaundice can cause false low readings
4. CarboxyHb cause false high readings
5. Chaotic pulse waves (AF) can give unpredictable
readings

O2 dissociation curve

Gas Exchange - Capnography
Analysis of expired CO2 against time.
Provides information about respiratory rate and rhythm.
Gives information about ETT placement, including
obstruction, disconnection and kinking
In healthy subjects, when ventilation and perfusion are
equally distributed end-tidal CO2 approximates PaCO2
Can be used to determine dead space, cardiac output and
pulmonary embolism
Placed between ETT and expiratory limb of vent tubing

Gas Exchange - Capnography
Can be used to determine ‘best PEEP’
The arterial to end-tidal CO2 is minimized when
perfused alveoli are recruited maximally.
The PaCO2 – P ET CO2 difference could help identify
‘best PEEP

Gas Exchange - Capnography
4 phases
Phase I-III : exhalation
Phase IV : Inhalation
Phase I : Anatomical dead space. CO2 = O
Phase II : Mixture of dead space and alveolar ventilation
= abrupt rise in CO2
Phase III : Plateau = pure alveolar ventilation
Phase IV : Inhalation = rapid fall in CO2

Respiratory Mechanics
IPPV is non-physiological
Need to monitor the mechanics of mechanical
ventilation
How it affects chest wall and lungs-
Complience, resistence, airway pressures and volumes
Physical examination
Airway graphics

Airway Graphics
Rapidly and continuously identifies the presence or
absence of respiratory pathophysiology and ventilator
performance.
Adverse ventilator effects can be identified, diagnosed
and corrected

Measured
Parameters
Flow
Pressure
Time
Calculated
Parameters
Volume
Compliance
Resistance

Waves
Most common wave forms:
Flow : Time
Airway Pressure : Time
Volume : Time
Time always on x-axis

Loops
Most common loops
Flow : Volume
Pressure : Volume

Waves

Pressure-Time wave
20
Volume Ventilation
Pressure Ventilation
P
Paw
cmH 2 O
Inspiration Expiration
1 2 3 4 5 6
S

Flow / Time Waveform
120
Inspiration
Volume Control Breath
Square Wave (Constant Flow)
Flow Pattern
V. .
L/min
1 2 3 4 5 6
S
Expiration
120

Pressure Control – Flow/Time

20
Volume Control
Breath
Pressure Control
Breath
P
Paw
cmH 2 O
Inspiratory
Time
1 2 3 4 5
S
V .

Volume Time wave
700
I-Time
E-Time
A
B
V T
mL
1 2 3 4 5 6
S
A = inspiratory volume
B = expiratory volume

Pressure Control
Pressure
PC above PEEP
Time
PEEP
Flow
Time
Volume
Time

Volume control
50 cmH 2 O
T
Pressure
70 l/min
Flow
-70
700 ml
Volume

Loops

Pressure /Volume Loop
Patient-Triggered Spontaneous Breath
V T
LITERS
Clockwise
0.6
0.4
0.2
Paw
Inspiration
Expiration
cmH 2 O
-60
40
20
0 20 40 60

Pressure /Volume Loop
Time-Triggered Mandatory Breath
V T
LITERS
0.6
Counterclockwise
0.4
Expiration
0.2
Paw
Inspiration
cmH 2 O
-60
40
20
0 20 40 60

Flow / Volume Loop
Volume Control Mandatory Breath
.
V
L/min
Inspiration
Expiration
Volume Control
Volume
Breath
Control Breath
Square Wave (Constant
Square
Flow)
Wave
Flow Pattern
(Constant Flow)
Flow Pattern
V T
Liters
Tidal Volume
Peak Inspiratory Flow
Peak Expiratory Flow

Flow / Volume Loop
Pressure Control Mandatory Breath
.
V
L/min
Inspiration
Expiration
Volume Control
Pressure
Breath
Control Breath
Square Wave (Constant
Decelerating
Flow)
Flow Pattern
Flow Pattern
V T
Liters
Tidal Volume
Peak Inspiratory Flow
Peak Expiratory Flow

Clinical Significance
Is the trace normal
What looks abnormal
What would explain the abnormality
Does it correlate with clinical findings

Air Stacking and autoPEEP
Exhalation does not return to zero and overlaps with next inspiration.
1.2
A
V T
Liters
-0.4
1 2 3 4 5 6
SEC

Optimal PEEP
PEEP below critical
opening pressure

Optimal PEEP

Overdistention
A = inspiratory pressure
B = upper inflection point
C = no lower inflection point
(adequate alveolar recruitment
V T
LITERS
0.6
0.4
A
0.2
B
C
Paw
cmH 2 O
-60
-40
-20
0 20 40 60

Overdistention

Decreased compliance
A higher pressure for the same volume indicates a drop in compliance.
Pressure Volume Loop
V T
LITERS
0.6
0.4
0.2
Paw
cmH 2 O
-60
40
20
0 20 40 60

Secretions
SAWTOOTH PATTERN
NEEDS SUCTIONING

Conclusion
Airway graphic analysis provides continuous,
noninvasive information which alerts
clinicians to unexpected changes in patients
ventilatory status and aids in the optimal
management of mechanical ventilation
The basic principles of physical
examination should still be the first line
approach of monitoring the critically ill.

Conclusion
Instant data allows for:
•Quicker recognition / response to
changes in patient status
• number of invasive procedures
• number of exposures to radiation
• expense