part 2:
mechanical ventilation
shiva birdi m.d.
respiratory insufficiency
ventilation

oxygenation

work of breathing
apnea
hypoventilation
PaCO2 > 50 and pH <7.30
hypoxemia
PaO2 < 60 with FiO2 80%
high RR
accessory muscles
high airway pressures
pre-operative

intra-operative

post-operative
the surgical patient
underlying severity of disease
anesthesia related
sedation / paralysis
intubation
drugs
pain
weakness
direct injury
pleural effusion
atelectasis
surgery related
position
type of surgery
operative field
non-invasive
invasive
Respiratory condition expected to improve in 48-72 hours
Alert, cooperative
Hemodynamically stable
Able to control airway secretions
Able to coordinate with ventilator
No contraindications
advantages
avoids complications of intubation
May prevent re-intubation preserves airway reflexes
improved patient comfort
less need for sedation
? shorter hospital/ICU stay
? improved survival
cpap
bipap
equivalent of PEEP

improves oxygenation

maintains airway
IPAP and EPAP

improves ventilation

maintains airway
tracheal intubation
jet ventilation
extra-corporeal oxygenation
data from CHF/COPD
very few post-operative patients
trigger
cycle
patient
ventilator
flow
pressure
time
flow - cycled
time - cycled
volume - cycled
end of inspiration
when a certain flow is reached
start of inspiration
when set time is reached
when set volume is reached
oxygen
nasal cannula

venturi mask

non-rebreather
pressure
volume
"pressure-limited" breath

time or flow-cycled

target peak airway and alveolar pressures

Vt depends on compliance
targets of ventilation
target a set volume

volume-cycled

ventilation maintained

airway pressures depend on compliance
initial flow is rapid
to meet target
"decelerating"
modes of ventilation
controlled (cmv)
decelerating flow
flow stops
when time is reached
inspiratory time
square (constant) flow
flow stops when
volume reached
assist/control (a/c)
synchronized intermittent mandatory
(simv)
pressure support (psv)
complete control
pressure or volume
patient cannot trigger the ventilator
anesthesia machines, apneic patient
controlled mode allowing for patient triggered breath
patient triggered breath is fully supported
volume assist or pressure assist
Vt, backup rate, trigger,
flow rate, inspiratory time, flow waveform,
PEEP, FiO2
target pressure above PEEP,  backup rate, rise time, trigger,
flow rate, inspiratory time,
PEEP, FiO2
combines spontaneous with assist/control modes
mandatory breath rate set
patient breaths are supported with a pressure-limited breath
volume or pressure
advantage
patient does no work

patient does no work
hyperventilation
disadvantage
advantage
patient-ventilator synchrony
improvement in V/Q
diaphragmatic strength

work of breathing
disadvantage
purely assist mode
no set rate, patient-trigerred
pressure-limited, flow-cycled breath
inspiration is terminated when flow is 25% of peak
decelerating flow
pressure limited
advantage
patient has full control
ideal for weaning
work of breathing
needs to meed patient demand
disadvantage
expiratory sensitivity

rise time
adjust peak flow if patient wants to expire early
time for gas flow to go from zero to peak
varies the slope of the pressure curve
ventilator assossiated injury
barotrauma
volutrauma
biotrauma
atelectrauma
VILI
hyperinflation of normal lung regions


trans-pulmonary pressure


stress fractures in alveolar-capillary interface
excessive pressure in the alveoli
excessive volume in the alveoli
alveolar pressure - pleural pressure
pro-inflammatory mediators
tnf-alpha, IL-6
systemic and pulmonary effects

oxygen toxicity (free radicals)
repetetive opening and closing
shear force
epithelial injury
VILI
can be indistiguishable from ARDS
decreased compliance, decreased surfactant
pulmonary edema
SIRS
pumonary hemorrhage
airway injury (ptx, pneumomediastinum)
lung-protective ventilation
low volume, low pressure
optimal peep
lung recruitment
ARDS
Vt 4-8 cc/kg
Plateau pressure < 30 cm H2O

>10cc/kg increases mortality
use 8-10 cc/kg PBW
no benefit of low volumes

>10cc/kg may cause harm
non-ARDS
sustained CPAP of 40-50 cm H2O for 30-40 sec

PCV with peak pressure 40-50 cm H2O and PEEP 20-30 cm H2O for 2-3 min
rethink "normal" PaO2
permissive hypercapnia
prevents repetitive opening and closing of alveoli
can help inflate recruitable alveoli
improve the lung compliance

however:
difficult to quantify even with waveform analysis
dys-synchrony
patient and ventilator need to be in sync

pressure modes preffered (alter flow rate and rise times)

match peak flow rate to the patient's demand

tigger sensitivity (flow preffered over pressure)

inspiratory time (match neuroinspiratory time)

sedation
increases work of breathing, discomfort, repiratory rate
peak flow
most adults > 80L/min
needs to match neuroinspiratory time

prolonged i-time can benefit slower "time-constant" units
time needed to inflate or deflate 63%
of alveolar volume
inspiratory time
plateau pressure
reflects transpulmonary pressure

pressure applied to the alveoli

end-inspiratory equilibrium pressure

(pressure control)
(volume control)
pressure limit
end-inspiratory hold
compliance
resistance
auto-peep
alveolar pressure above set levels at end-expiration

major cause of dys-synchrony

causes include high inspiratory times, high RR,
expiratory muscle activity, dynamic hyperinflation

goals of therapy
back to the basics
airways compress during expiration

adding peep improves synchrony
and reduces effort
high frequency

aprv

prvc
no proven benefits
experimental
role in pediatric
rethink "normal" PaO2

pH matters NOT PCO2

permissive hypercapnia 
weaning from support
Spontaneous Breathing Trial

Partial reversal of factors contributing to ventilator dependence

Assessment of oxygenation
PaO2/FiO2 150 mm Hg
Positive end-expiratory pressure 8 cm H2O
FiO2 0.5
pH 7.25

Hemodynamic stability

Spontaneous inspiratory efforts present
Failure of a Spontaneous Breathing Trial

Respiratory Rate >35 breaths/min
SaO2 <90%
Pulse >140 beats/min or sustained increase of 20%
Systolic blood pressure >180 mm Hg or diastolic blood pressure >90 mm Hg
Increased anxiety
Increased diaphoresis