Transcript Highlights of RSPT 2414 Mechanical Ventilation: Unit 1

Highlights of RSPT 2414 Mechanical
Ventilation: Unit 1
By
Elizabeth Kelley Buzbee AAS, RRTNPS, RCP
Normal ventilation
• With normal compliance of 100 ml/cmH20
pressure
• and normal RAW of .5 to 2.5 cm H20/L/second
the WOB is easy because the driving pressure
is low.
Persons need help breathing when:
• The driving pressure might be excessive
• or the patient may lack the ventilatory
muscles
• The patient may lack the ventilatory drive
Define respiratory failure
• Inability to oxygenate the tissues (Pa02 less
than 60 mmHg) and /or to remove C02 (PaC02
more than 50 mmHg)
• in persons with chronic hypercapnia, the
person is ok until his C02 rises to the point
that he has partially compensated respiratory
acidosis—he has de-compensated
Classify Respiratory failure
• Acute hypoxemic respiratory failure- if refractory hypoxemia
02 will not help
– Decreased Pi02 such as with smoke inhalation or high altitude
– Diffusion problems such as increased alveolar-capillary or decreased
surface area due to atelectasis
• Acute hypercapnia respiratory failure- uncompensated
respiratory acidosis
– Associated with decreased ventilatory drive or decreased alveolar
ventilation
– Patient may be hypoxic only because the PA02 is decreased due to
increased PaC02—may or may not need increased Fi02 once
ventilation starts
• Chronic hypercapnia respiratory failure- partially
compensated respiratory acidosis
– Exacerabation of existing problem such as COPD, neuromuscular
disorder or morbid obesity
Differentiate between the V/Q mismatch and a
shunt or shunt-like effect:
• V/Q mismatch :
– Acute respiratory failure that will respond to supplementary 02
TX
– When there is low V/Q, we have low ventilation with good
perfusion
– When there is high V/Q, we have good ventilation with poor
perfusion
• Shunt: Acute respiratory failure with refractory hypoxemia
– Supplementary 02 will not help
– Physiological shunt of 10% in WLN—more is pathological shunt
02 indices to determine if patient is in
refractory hypoxemia
– Use the Pa02:Fi02 as Pa02:Fi02 to determine if we
can correct hypoxemia
– Use the a/A ratio to determine if patient is above
.14 to .17
– Use rule of 50: Fi02 more than 50% with Pa02 less
than 50 mmHg
02 indices to determine if patient’s
hypercapnia is the only reason he is
hypoxemic
• If the P[A-a]D02 is not elevated (10 mmHg for
young and 25 mmHg for elderly), the hypoxemia
may only be due to the rise in alveolar C02
replacing the alveolar 02.
Once his alveolar ventilation is increased by
our putting him on mechanical ventilation, his
Pa02 is corrected
Conditions that result in increased WOB
due to a need for driving pressures higher
than they can handle
• increased RAW
• decreased lung compliance
• Persons at risk for muscle fatigue
– Persons with severe muscle fatigue need to rest on
mechanical ventilation for 24 – 48 hours
Lung function studies that demonstrate situations that result in
ineffective ventilator muscle action
• VC of less than 20 ml/kg IBW requires some ventilator support.
• VC of less than 25 ml/kg IBW is associated with decreased ability to
cough effectively.
• [PI max] inspiratory max pressure measures weakness of inspiratory
chest wall muscles and diaphragm. a need for mechanical
ventilation is seen with a[PI max] of less -20 cmH20
• [PE max] expiratory max pressure measures weakness of the
abdominal muscles. a need for mechanical ventilation is seen with a
[PE max] less than + 40 cmH20
• be aware that facial weakness can result in false values for these
two figures
– if the patient cannot seal properly—needless to say, that alone tells us
we have problems with patient being able to protect his airway
Situations that result in increased VD ventilation will make a
person need mechanical ventilation
• anatomical VD conducting airways. Comprises about 30% of the
VT of the body.
• is equal to 1 ml / pound of IBW
• Is always present, but can be reduced by tracheostomy
which bypasses upper airways
• VD/VT ratio will change, as the patient’s VT varies but the VD
will stay the same
• alveolar VD when an alveoli gets ventilation but no perfusion, it is
considered alveolar VD
• as CO drops or there are problems with pulmonary blood
flow the alveolar VD will rise above baseline
• physiological VD
• is the sum of the anatomical VD + the alveolar VD
Problems with VD/VT
• physiological VD is the sum of the anatomical
VD + the alveolar VD
– the normal VD /VT is about .3 or 30%. It is not
uncommon for mechanically ventilated persons to
have VD /VT of .6 and higher.
– if physiological VD is excessive, we can increase the
VT to get the alveolar ventilation back to an
effective level
– Failure to get the VD /VT below .6 will prevent
successful weaning of a patient from mechanical
ventilation.
Clinical signs and symptoms of respiratory failure in
the adult patient.
– inadequate alveolar ventilation:
• hypercapnia above 55 torr & pH below 7.20
– inadequate lung expansion:
• VT less than 5 ml/kg IBW, VC less than 10 ml/kg IBW requires full
ventilator support, and RR over 35 bpm
– poor muscle strength:
• MIP less than -20 cmH20, VC less than 10 ml/kg and MVV less than
2x VE
– increased WOB:
• VE more than 10 LPM & VD/VT more than .6
– hypoxemic respiratory failure:
• P(A-a)D02 on 100% more than 350 mmHg
• Pa02/Fi02 less than 200.
ABG associated with respiratory failure.
• Acute respiratory acidosis with
moderate/severe hypoxemia
• Partially compensated respiratory acidosis
with moderate / severe hypoxemia. Chronic
patient is no longer compensating effectively.
• Panic values on ABG:
• PaC02 above 55 torr & pH below 7.20
• Serial ABG in which the PaC02 rises each time
Bedside measurements of increased
WOB
• Calculation of the RAW
– If RAW increased, WOB increased
– High RAW – high driving pressure needed
• Calculation of the lung Compliance [CL]
– If CL decreased, WOB increased
– Low CL high driving pressure needed
Mechanical ventilation:
• a machine that can perform bulk transfer of
gas into the lung for a patient who cannot
perform this task effectively enough to
exchange gases.
• The ventilator works during inspiration, while
exhalation is usually passive.
Phases of ventilation
• Inspiratory phase: inspiration in which gas
enters the lung.
– The TI is a function of the flow rate, the VT and the
patient’s RAW
• Expiratory phase: the portion of the breath
that is concerned with the passive flow of gas
out of the lung.
– The TE will be a function of the TI, and to a great
part to the patient’s RAW
I:E ratio:
• comparisons of the TI to the TE.
• Normal I:E ratio during spontaneous breathing is
1:1.5, but to minimize some of the hazards of
mechanical ventilation, with positive pressure
ventilation, this ratio needs to be 1:2 or more.
• A patient with significant air-trapping may require
much longer 1:E ratio such as 1:3 or 1:4.
Cycle time:
• cycle time = TI + TE
• cycle time = 60 seconds/BPM
Airway pressures
• PIP- highest pressure during the inspiratory phase– at the
end of inspiration.
– This is P1 of the RAW formula
• P plateau: during a breath hold, this is the second pressure
during inspiration.
– On a graphic, it looks like a flat plateau.
– This pressure is the P2 of the RAW formula and the Δ P of the
static compliance formula
• Baseline pressure: After the positive pressure breath is
given, the airway pressure returns to the baseline, which
may be zero or a positive number if there is PEEP or CPAP.
Airway pressure
• PAW: the “mean airway pressure” is the
average airway pressure.
– It is a function of the inspiratory time (Ti),
– the PIP,
– the baseline pressure
– and the I:E ratio.
PAW = [PIP ( I )] + [PEEP (E)]
[I + E]
Different types of ventilation
•
•
•
•
Positive pressure ventilation
Negative pressure ventilation
Invasive mechanical ventilation
Non-invasive mechanical ventilation:
Ventilator modes:
• Full-support mechanical ventilation: Most fatigued patients need
to be rested for 24-48 hours, but a serious complication of fullsupport is that after a few days, the patient’s respiratory muscles
start to atrophy quickly.
•
• Partial-support mechanical ventilation: SIMV or IMV are examples
of partial-support mechanical ventilation. Frequently patients are
started on full support and are moved to partial support after the
mandatory rest period.
• Spontaneous modes: When a patient is past the point of needing
full or even partial support, we can challenge the patient with the
machine acting only as a monitoring device with/without alarms
and mechanical intervention in case of apnea or hypoventilation.
– Patients on spontaneous modes of ventilation must have an intact
ventilatory drive, and must be able to maintain their PaC02 with little
or no help from the machine.
flow
• Wave forms/graphics: electronic devices convert
airway pressures, volumes or flows into a graphic
• Peak flows /flow rates: All modern positive
pressure ventilators have peak flow rates. If you
select the flow/time wave form you can see the
flow pattern:
– Constant flow
– Descending ramp
– Sine wave
VT tidal volume
• To adjust the VE for the PaC02, we can alter
the RR or the VT.
• Set VT: the VT the RCP selects that may or
may not be the same as the delivered VT
• Return VT: the delivered VT that is measured
at the exhalation point
• Corrected VT: the VT that is corrected for
volume that is lost in the tubing as it swells
during positive pressure