Highlights of Unit 2 mechanical
Physiologic Effects of Mechanical
both hazards and positive effects of
PPV & of negative pressure
Airway pressures
• Spontaneous breathing results in a driving
pressure of 0 - -5 = 5 cmH20
• At no point during quiet spontaneous
breathing is the intrathoracic pressure positive
Airway pressures during PPV
• In the case of positive pressure ventilation
[PPV], the pressure gradient is positive at the
mouth and negative at the alveoli.
• The driving pressure is the difference between
these two pressures.
• the intrapleural pressure will rise because the
airway pressure has transmitted through the
thin walls of the lung.
Cardiovascular effects of PPV
• Positive airway pressure raises pressure in
thoracic cavity
• Prevents the RA from sucking blood back into the
heart so the venous return is decreased
• Less blood in heart- less blood out of the
ventricles so RV and LV both have decreased SV
• Compression of pulmonary capillaries
• Slows down blood return from head
• Decreases Cardiac Output CO
• Might decrease the myocardial perfusion
Who is more likely to suffer the effects
of PPV on cardiovascular system?
• The person with hypotension
• or with low blood volume
Who could benefit from the effects of
PPV on the CV system?
• patient with congestive heart failure is helpful
• need to push back the excess lung fluid
• need to decrease the amount of blood
returning to the stressed heart,
• so that the CO can actually rise with these
Effect of PPV on tissue delivery of
• 02 delivery = 02 content (cardiac output)
– If decreases the CO ---decrease the delivery of 02
to the tissue
– If PPV causes increased VD ventilation, the 02
content can go down
Lung Compliance and CO
• If the patient’s lungs have normal compliance,
50% of pressure in the lungs is transmitted to
the thorax.
• If the patient has stiff, low compliant lung, the
higher alveolar pressure may not transmit to
the thorax.
reverse pulsus paradoxus
• status asthmaticus patients placed on PPV
can have such excessive driving pressures that
this dampens the systemic B/P
• decreased LV after load is so low that systemic
BP changes significantly between inspiratory
phase & expiratory phases.
Excessive VT and inappropriately
high baseline pressures
• Excessive VT and inappropriately high baseline
pressures [PEEP] will result in serious
problems with CO and with myocardial
Effects of PPV on hypoxemia
• alveolar recruitment, increases surface area
for gas exchange so Pa02 rises
• as the alveolar C02 drops due to increased
movement of gas into the lungs and collapsed
alveoli re-inflate the PA02 rises.
PPV and hypoxemia with CHF
• As we increase the alveolar pressure with
PEEP or CPAP, fluid is pushed back and Pa02
rises as more gas diffuses into the capillaries.
• As Pa02 rises, the Fi02 can be dropped to
avoid 02 toxicity
Treatment of hypercapnea with
VE rising decreases the PaC02.
• VT based on disease states:
• 8-10 ml/Kg IBW normal lungs
• 6-8 ml/kg IBW asthma
• 5-8 ml/ kg IBW for ARDS & for COPD
• Keep Pplateau less than 30 cmH20.
PPV can increase V/Q
• Gas distribution to Zone I instead of Zone III
• Pressure on pulmonary capillaries will
decrease Q in some areas and raise perfusion
to under-inflated areas
• Increased Ventilation without perfusion
results in increased VD—so PPV can increase
PPV and acid pH
• If VE is too low, respiratory acidosis will result
in increased Ventilatory demand
• Prolonged acidosis raises the serum potassium
so hyperkalemia can cause cardiac
PPV and alkalosis
• If VE too high:
• Prolonged alkalosis will drop the serum
potassium so that hypokalemia results- this
can also effect the EKG
• Alters the Hb/02 affinity with shift to the left
so that tissue oxygenation can be decreased
• High pH will drop the ventilatory drive and
prolong weaning from mechanical ventilation
Time constants
• Time constant = RAW x C
• we need at least 7 time constants for both I &
PPV and excessive airway pressures
• PIP above 50 is associated with lung tissue
• If increased RAW, check BBS for suctioning or
need for bronchodilator
• Pplateau above 30-35 will cause sheer damage.
Keep below 30, by decreasing the VT [usually
too high for disease]
Mean airway pressures
• Increasing the PEEP will raise the mPAW
• Increasing the Ti will also raise the mPAW
• Increased airway pressures raise the FRC
which can increase lung C and increase Pa02
but excessive FRC will only add to air trapping
Effects of PEEP
• PEEP can drop the Cardiac Output particularly
in persons with good compliance
• PEEP can raise the FRC which may or may not
have good effects
– Low FRC associated with atelectasis –good
– High FRC associated with asthma or COPD- bad
• Auto-PEEP created by prolonged Ti can result
in air trapping and barotrauma
• Excessive VT -- in the face of problems with
gas distribution
Effect of PPV on VT
• If the patient’s parameters are based on
pressure, the delivered VT will vary based on
the patient’s compliance and RAW.
• If ventilation is based on a volume, the airway
pressures will vary based due to changes in
the patient’s compliance and RAW
Effect of PPV on WOB
• If WOB is not decreased, settings are wrong
– WOB decreased because driving pressure is
increased by ventilator
– Increased VE drops the PaC02 so ventilatory drive
– VD/VT decreased because VT is increased
– Pa02 rises
– If sensitivity set appropriately, there is little WOB
Inappropriate settings result in
increased WOB
inspiratory flow rate too low,
sensitivity not responsive to patient effort
failure to correct hypoxemia
failure to correct hypercapnia or acidosis.
if the level of auto-PEEP is interfering with
triggering a breath
Effect on ventilator muscles
• muscle atrophy within 72 hours in adults.
• muscle mass and muscle fibers affected
• resting muscle length increases [due to
increased FRC] results in decreased muscle
• while alterations in the blood flow to the
ventilatory muscles secondary to PPV just
adds to the problem.
Ventilatory-associated pneumonia [VAP]
• VAP more an issue of artificial airways rather
than ventilation
– cuff pressures to prevent aspiration
– oral care Q2 hours,
– oral intubation
– keeping the HOB
Effects on the kidneys
• When CO drops, the body shunts blood away
from kidney so renal perfusion decreases
• Decreased perfusion decreased urine out put
– less than 400 ml/day or 160ml/8 hours shift
– increases in BUN, creatinine , Potassium and
decreases in Na.
– drug clearance may be affected
Serum inappropriate Anti-diuretic
hormone [SIADH]
• baroreceptors located in the walls of the
Aortic Arch and in the carotid arteries respond
to decreases of LV pressures by sending a
message that results in secretion of ADH so
urine output decreases
Effects of negative pressure
ventilation on the body
Major differences
• Negative airway pressures more nature, but
also negative pressure on the outside of the
thorax and on the abdominal cavity
• Immobility inside the device– always supine
• Not effective ventilation in the face of
excessive secretions or other causes of
increased RAW
• None-invasive so unable to protect airway
Negative pressure ventilation
• Pooling of venous blood in the abdomen can
result in decreased CO
• If patient has paralysis, this can result in
positional hypotension
• If person with Guilliam Barre or myotonic
dystrophy may have increased cardiovascular
instability– hard to get to patient for CPR
• Must decrease VE during nursing procedures
Problems with negative pressure
• Chest wall can-- over decades-- become
• chest cuirass cover only the chest and not the
abdomen so that there is less pooling of
venous blood—but maybe less ventilation

Highlights of Unit 2 mechanical ventilation