Principles of Mechanical Ventilation
RET 2284
Module 6.0 Ventilator Management
- Improving Ventilation/Oxygenation
Improving Ventilation / Oxygenation
The first 30 – 60 minutes following initiation of ventilation
are generally spent evaluating vital signs, breath sounds,
ventilator parameters, lung compliance and resistance,
the artificial airway, and documenting patient response to
therapy
After that initial phase, the RT is often concerned with
improving ventilation and oxygenation and managing the
patient-ventilator system
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

A change in
will often be needed when a
patient is first placed on mechanical ventilation to
correct for respiratory alkalosis or acidosis; this is
facilitated by making a change in VT or rate (f)
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Methods of Changing Ventilation Based on PaCO2 and pH
If it is appropriate to keep rate (f) constant and change
VT, the equations is as follows:
Desired VT = Known PaCO2 x Known VT
Desired PaCO2
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Methods of Changing Ventilation Based on PaCO2 and pH
If it is appropriate to keep VT the same and change rate
(f), then the equations is as follows:
Desired f = Known PaCO2 x Known f
Desired PaCO2
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Respiratory Acidosis
 Volume and Pressure Ventilation Changes


When PaCO2 is elevated (>45 mm Hg) and pH is
.
decreased (<7.35), respiratory acidosis is present and VA
is not adequate
Causes
 PE, Pneumonia
 Airway disease (e.g., severe asthma attack)
 Pleural abnormalities (e.g., effusions)
 Chest wall abnormalities
 Neuromuscular disease
 CNS problems
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Respiratory Acidosis
 Volume and Pressure Ventilation Changes
Guideline:



VT to 8 – 12 mL/kg ideal body weight (based on patient’s
pulmonary problem)
Maintain plateau pressure <30 cm H2O
If VT is already high and/or Pplateau are already high, then f
should be increased

Read example 1, 2 and 3: Respiratory Acidosis,
Increasing VT, page 259 – 260 (Pilbeam)
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Respiratory Alkalosis
 Volume and Pressure Ventilation Changes


When PaCO2 is decreased (<35 mm Hg) and pH
increases (>7.35), then respiratory alkalosis is present
and alveolar ventilation is excessive
Causes
 Hypoxia with compensatory hyperventilation
 Parenchymal lung disease
 Medications
 Mechanical ventilation
 CNS disorders
 Anxiety
 Metabolic disorders
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Respiratory Alkalosis
 Volume and Pressure Ventilation Changes
Guideline:


Volume ventilation: f, and if necessary, VT
Pressure ventilation: f, and if necessary, pressure

Read example 1 and 2: Respiratory Alkalosis,
Decreasing the rate, page 261 (Pilbeam)
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Metabolic Acidosis and Alkalosis
 Treatment of metabolic acidosis and alkalosis should
focus on identifying those metabolic factors that can
cause these acid-base disturbances
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Metabolic Acidosis and Alkalosis
 Metabolic Acidosis

Causes
 Ketoacidosis (alcoholism, starvation, diabetes)
 Uremic acidosis (renal failure to excrete acid)
 Loss of bicarbonate (diarrhea)
 Renal loss of base following administration of
carbonic anhydrase inhibitors (e.g., Diamox)
 Overproduction of acid (lactic acidosis)
 Toxin ingest that produce acidosis (salicylate,
ethylene glycol [antifreeze], methanol
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Metabolic Acidosis and Alkalosis
 Metabolic Acidosis


Treatment should first deal with the cause of the acidosis
Secondly, assess the need to reverse the acidemia with
some form of alkaline agent
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Metabolic Acidosis and Alkalosis
 Metabolic Acidosis


These patients are often struggling to lower their PaCO2
to compensate for the metabolic acidemia. As a
consequence, these patients are at risk for developing
respiratory muscle fatigue
If the patient is losing the struggle to maintain high
with spontaneous breathing, assisted ventilation may be
necessary to avoid respiratory failure. It is then
appropriate to keep the pH (7.35 – 7.45)
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Metabolic Acidosis and Alkalosis
 Metabolic Alkalosis

Causes
 Loss of gastric fluid and stomach acids (vomiting,
nasogastric suctioning)
 Acid loss in the urine (diuretic administration)
 Acid shift into the cells (potassium deficiency)
 Lactate, acetate, citrate administration
 Excessive bicarbonate loads (bicarbonate
administration)
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Metabolic Acidosis and Alkalosis
 Metabolic Alkalosis
 Treatment involves correcting the underlying cause and
reversing those factors leading to the alkalosis. In
severe cases, carbonic anhydrate inhibitors, acid
infusion, and low bicarbonate dialysis my be required

Only in rare circumstances does partial respiratory
compensation of metabolic alkalosis occur – PaCO2 will
usually not rise higher than 55 mm Hg (Remember that as
the CO2 rises, the PaO2 falls)
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Mixed Acid – Base Disturbances
 Combined Respiratory Alkalosis and Metabolic Acidosis

Read case studies: Pilbeam, pg. 262 – 263
 Combined Respiratory Acidosis and Metabolic Alkalosis

Read case study: Pilbeam, pg. 263
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Increased Physiological Dead Space
 If pure respiratory acidosis persists even after alveolar
ventilation has been increased, the patient may have a
problem with increased dead space

Causes
 Pulmonary emboli
 Low cardiac output  low pulmonary perfusion
 High alveolar pressure (PEEP)   pulmonary blood
flow
 Air trapping   pulmonary perfusion
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Increased Metabolism and Increased CO2
Production
 Read case study: Pilbeam, pg. 264
.
 Metabolic rate and VCO2 are increased in the following
patients:






Fever
Sepsis
Burns
Multiple trauma and multiple surgical procedures
Hyperthyroidism
Seizures
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Increased Metabolism and Increased CO2
Production
 In these patients
is increased and WOB is elevated
Treatment Options



Increase machine rate to WOB: may cause auto-peep
Add pressure support for spontaneous breaths to WOB
through ET and circuit
Switch to PC-CMV, use sedation to WOB
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Intentional Iatrogenic Hyperventilation
 Definition

Deliberate hyperventilation in patients with acute head
injury and increased intracranial pressure (ICP)

Hyperventilation reduces PaCO2 which causes
vasoconstriction of cerebral blood vessels and
decreases blood flow to the brain and is believed to
lower increased intracranial pressure ICP
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Intentional Iatrogenic Hyperventilation
 Current therapy guideline for head injuries with
increased ICP do not recommend prophylactic
hyperventilation (PaCO2 <25 mm Hg) during the first 24
hours - may cause cerebral ischemia and cerebral
hypoxemia
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Intentional Iatrogenic Hyperventilation
 Hyperventilation may be needed for brief periods when
acute neurological deterioration is present and ICP
elevated
 Mild hyperventilation (PaCO2 30 – 35 mm Hg) may be
used for longer periods in a situation in which increased
ICP is refractory to standard treatment
The practice of iatrogenic hyperventilation still remains
controversial
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Permissive Hypercapnia (PHY)
 Definition



Deliberate limitation of ventilatory support to avoid lung
overdistention and injury of lung
 ARDS
 Status asthmaticus
PaCO2 values are allowed to rise above normal
 ≥50 – 150 mm Hg
pH values are allowed to fall below normal
 ≥7.10 – 7.30
 Most researchers agree pH ≥7.25 is acceptable
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Permissive Hypercapnia (PHY)
 PaCO2 accompanied PaO2

O2 administration must be provided and monitored closely
 PaCO2 stimulates the drive to breath

Appropriate to provide sedation to patients in whom PHY
is being employed
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Permissive Hypercapnia (PHY)

Procedures for Managing PHY
1.
2.
Allow PaCO2 to rise and pH to fall without changing
mandatory rate or volume
a.
Sedate the patient
b.
Avoid high ventilating pressures
c.
Maintain oxygenation
Reduce CO2 production
a.
Paralyze
b.
Cool
c.
Restrict glucose
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Permissive Hypercapnia (PHY)

Procedures for Managing PHY
3.
Keep pH >7.25
a.
Sodium bicarbonate
b.
Tris-hydroxiaminomethane (an amino buffer)
c.
Carbicarb (mixture of sodium carbonate and
bicarbonate
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Permissive Hypercapnia (PHY)

Contraindications and Effects of PHY



Head trauma
Intracranial disease
Intracranial lesions
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Permissive Hypercapnia (PHY)

Relatively contraindicated in the following
 Cardiac ischemia
 Left ventricular compromise
 Pulmonary hypertension
 Right heart failure
Improving Ventilation / Oxygenation
Correcting PaCO2 Abnormalities

Permissive Hypercapnia (PHY)
The use of PHY is restricted to situations in which the
target airway pressure is at its maximum and the highest
possible rates are being used
The risks of hypercapnia are considered by some to be
preferable to the high Pplat required to achieve normal CO2
levels
Read Case Study: Pilbeam, pg. 265 – 266
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Adjusting FiO2

Every attempt should be made to maintain the FiO2
<0.40 to 0.50 to prevent the complications of O2
toxicity while keeping the PaO2 between 60 and 90
mm Hg


This goal is not always possible and sometimes a
.
higher
FiO2 is required
The SpO2 can be used to titrate FiO2, with the goal
of maintaining the SpO2 >90%

The SaO2 on an ABG is used to establish the
relationship with the current SpO2
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Adjusting FiO2

ABGs are obtained after mechanical ventilation is
initiated and compared with FiO2 being delivered and
the SpO2 to establish their relationships

A linear .relationship exists between PaO2 and FiO2 as
long as VE, CO, Shunt, VD/VT remain fairly constant
(cardiopulmonary status)
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Adjusting FiO2

Because of the linear correlation between PaO2 and
FiO2 the following equation can be used to select the
desired FiO2 to achieve a desired PaO2:
Desired FiO2 = PaO2 (desired) x FiO2 (known)
PaO2 (known)
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Adjusting FiO2
Exercise
After being supported on a ventilator for 30 minutes, a
patient’s PaO2 is 40 mm Hg on an FiO2 of 0.50. Acidbase status is normal and all other ventilator
parameters are within the acceptable range. What
FiO2 is required to achieve a desired PaO2 of 60 mm
Hg?
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Adjusting FiO2
Desired FiO2 = PaO2 (desired) x FiO2 (known)
PaO2 (known)
Desired FiO2 = (60 mm Hg) (0.50 FiO2)
40 mm Hg
Desired FiO2 = 0.75
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP
_

Selection of FiO2 or Adjustment of Paw

Maintaining an FiO2 >60 may lead to:


Lower limits of target PaO2 is 60 mm Hg
 Lower limits of target SpO2 is 90%

_
O2 toxicity
Absorption atelectasis
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP
_

Selection of FiO2 or Adjustment of Paw

When PaO2 remains very low on high FiO2, significant
shunting, V/Q abnormalities , and/or diffusion defects
are present - other methods to improve oxygenation,
besides increasing FiO2, must be considered
 Paw
_

PEEP

HFOV

APRV
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP
_

Selection
of FiO2 or Adjustment of Paw
_

Paw can be used to increase the PaO2
_

Factors that affect Paw during PPV






PIP
PEEP
Auto-PEEP
I:E ratio
Respiratory rate
Inspiratory flow patterns
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP
_

Selection
of FiO2 or Adjustment of Paw
_

Paw is a major determinant of oxygenation in patients
with ARDS



Mean alveolar pressure  oxygenation
Alveolar recruitment  oxygenation
_
Typical method to increase Paw

PEEP
_

Other methods to increase Paw


HFOV
APRV
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP
_

Selection
of FiO2 or Adjustment of Paw
_

Paw must be monitored closely to prevent:





Air trapping
Overdistention
Barotrauma (e.g. pneumothorax)
Venous return
CO
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Goals of PEEP




Enhance tissue oxygenation
Maintain a PaO2 above 60 mm Hg, and SpO2 ≥90% at
an acceptable pH
Restore FRC
These goals my be accompanied by the opportunity to
reduce the FiO2 to safer levels (<0.50) as PEEP
becomes effective

Must maintain cardiovascular function and avoid lung
injury
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Minimum or Low PEEP


PEEP at 3 – 5 cm H2O to help preserve a patient’s
normal FRC
Therapeutic PEEP


PEEP >5cm H2O
Used in the treatment of refractory hypoxemia caused by
increased intrapulmonary shunting and V/Q mismatching
accompanied by a decreased FRC and pulmonary
compliance
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Optimal PEEP

The level of PEEP at which the maximum beneficial
effects of PEEP occur

O2 transport

FRC

Compliance

Shunt
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Optimal PEEP


The level of PEEP is considered optimum because it is
not associated with profound cardiopulmonary side
effects

Venous return

CO

BP

Shunting

VD/VT

Barotrauma

Volutrauma
Accompanied by safe levels of FiO2
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Indications for PEEP Therapy






Bilateral infiltrates on chest radiograph
Recurrent atelectasis
Reduced CL
PaO2 <60 mm Hg on high FiO2 of >0.5
PaO2/FiO2 ratio <200 for ARDS and <300 for ALI
Refractory hypoxemia: PaO2 increases <10 with FiO2
increase of 0.2
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Specific clinical disorders that may benefit from PEEP




ALI
ARDS
Cardiogenic PE
Bilateral, diffuse pneumonia
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Application of PEEP


Increased in increments of 3 – 5 cm H2O in adults, 2 –
3 cm H2O in infants
Target acceptable PaO2/FiO2 ratio at a safe FiO2

>300 (e.g., PaO2 = 100, with FiO2 = 0.33
(optimal, but not always realistic)
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Application of PEEP



Patient Appearance

Color, level of consciousness, anxiety – a sudden
deterioration may indicate cardiovascular collapse
or pneumothorax
Blood Pressure

BP of 20 mm Hg systolic drop is significant
Breath Sounds

Barotrauma, e.g., pneumothorax
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Application of PEEP

Ventilator Parameters

VT, Flow, PIP, plateau pressure, VE
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Application of PEEP

Static Compliance (CS)

As PEEP progressively restores FRC, compliance
should increase
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Application of PEEP

Static Compliance (CS)

Too Much PEEP  Overdistention  CS
Optimized Lung Volume “Safe Window”
Overdistension




Edema fluid accumulation
Surfactant degradation
High oxygen exposure
Mechanical disruption
Zone of
Overdistention
Injury
Derecruitment, Atelectasis





Repeated closure / reexpansion
Stimulation inflammatory
response
Inhibition surfactant
Local hypoxemia
Compensatory
overexpansion
“Safe”
Window
Volume
Zone of
Derecruitment
and Atelectasis
Injury
Pressure
Application of PEEP
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Application of PEEP

Arterial PO2, FiO2, and PaO2/FiO2

The usual approach to the management of FiO2
and PEEP is to start with high FiO2 and
incrementally decrease it as PEEP improves
oxygenation
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Application of PEEP

Arterial to End-Tidal Carbon Dioxide Tension Gradient

Normal P(a-et)CO2 gradient is 4.5 ± 2.5 (Pilbeam)

Is lowest when gas exchange units are maximally
recruited without being overdistended

If P(a-et)CO2 gradient increases minimal
acceptable values, it signifies that too much PEEP
has been added and is producing a drop in cardiac
output and in increase in VD/VT
Application of PEEP
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Application of PEEP

Arterial-to-Venous Oxygen Difference (C(a-v)O2)
reflects O2 utilization by the tissues

Normal value is 5 vol%

Increases in C(a-v)O2 with increases in PEEP may
indicate hypovolemia, cardiac malfunction,
decreased venous return to the heart, and
decreased cardiac output from PEEP
Application of PEEP
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Application of PEEP

Mixed Venous O2 Tension or Saturation

Normal PvO2 = 35–40 mm Hg
(minimal acceptable is 28 mm Hg)

Normal SvO2 = 75%
(minimal acceptable is 50%)

PEEP usually improves PvO2 and SvO2

When PvO2 and/or SvO2 decrease, with a increase
C(a-v)O2 increase, this indicates a decrease in
cardiac output – TOO MUCH PEEP
Application of PEEP
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Application of PEEP

Cardiac Output

Cardiac output provide key information about the
body’s response to PEEP

PEEP improves V/Q  Oxygenation  CO

Too much PEEP  Overdistention  Venous
return  CO
Application of PEEP
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Application of PEEP

Pulmonary Vascular Pressure Monitoring

When using PEEP >15 cm H2O, it is important to
closely evaluate the patient’s hemodyamic status,
which may require the placement of a pulmonary
artery catheter


If pulmonary artery occluding pressure (PAOP),
also known as “wedge pressure,” rises markedly as
PEEP is increased, the lungs may be overinflated
On the other hand, when PEEP rises, PAOP may be
markedly decreased because of pulmonary blood
flow is reduced as a result of decreased venous
return to the right side of the heart
Application of PEEP
Improving Ventilation / Oxygenation
Data From a Patient with ARDS on MV 24 Hours after Admission
VT: 700
f: 6
VE: 6.6
PEEP
BP
0
130/65
5
120/55
10 135/65
15 130/70
20 110/50
HR
130
135
125
120
130
PCWP
16
13
18
19
25
FiO2: 0.8
CO
4.8
4.2
5.8
5.9
4.1
Can you find the optimal PEEP level?
CS
28
31
33
36
27
PIP
50
58
60
55
63
PaO2
40
45
50
115
150
PVO2
27
37
35
37
29
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Weaning From PEEP





Patient should demonstrate an acceptable PaO2 on an
FiO2 of <0.40
Must be hemodynamically stable and nonseptic
Lung conditions should have improved

CS, PaO2/FiO2 ratio
Reduce PEEP in 5 cm H2O increments
Evaluate SpO2 within 3 minutes to determine effect – if
it falls <20% from previous PEEP level, the patient is
ready to tolerate lower PEEP level. If SpO2 drops
>20% place PEEP at previous level
Improving Ventilation / Oxygenation
Oxygenation Using FiO2 and PEEP

Positive End Expiratory Pressure (PEEP)

Weaning From PEEP


Wait between reductions in PEEP and reevaluate the
initial criteria. If the patient is stable, reduce PEEP by
another 5 cm H2O. This may take 1 hour or may
require as long as 6 hours or more
When the patient is at 5 cm H2O, an additional
evaluation is necessary. If reducing the PEEP to zero
result is a worsening of the patient, then it may be
appropriate to leave the patient at 5 cm H2O until
extubation