Artificial Ventilation and Heart-Lung Interaction Flashcards
What is Surfactant?
main function: to lower surface tension between air and alveolar fluid –> to prevent alveolar collapse at the end of expiration
- pulmonary host defense
How to inflate the Alveoli?
- requires an excess pressure inside the alveoli relative to their surroundings –> pressure in thoracic cavity negative with respect to atmospheric pressure
- amount of pressure required for inflation depends on:
- surface tension
- radii of alveoli
LaPlace’s Law
relation of vessel radius and tension
- the larger the vessel radius, the larger the wall tension required to withstand a given internal fluid pressure
- for a given vessel radius and internal pressure, a spherical vessel will have half the wall tension of a cylindrical vessel
Why does the wall tension increase with radius?
- larger radius –> less curvature –> total tension must be greater in order to get the same downward component of tension
Pendelluft
-transient movement of gas out of some alveoli and into others
–> larger alveoles increase their volume while smaller alveoles collapse
What happens in case of ARDS?
The efficiency of ventilation is reduced so much that blood gases are affected
Reasons for ARDS
1) pulmonary: diseases of the lung
- external incidents affecting the lung:
- drowning
- exposure to smoke
2) extrapulmonary: ventilation control problems, Motor Neuron Diseases (ALS), lung edema, Sepsis
Defintion of Respiratory Failure
1) Hypoventilation: paCO2 > 45 mmHg
2) acute respiratory failure: paO2 < 50 mmHg
ventilatory vs. parenchymal failure
pulmonary ventilatory failure: insufficient elimination of CO2
pulmonary parenchymal failure: inadequate oxygenation
Negative Pressure Ventilation
e. g. Tank Respirators
- negative ambient pressure surrounding body
- -> reduced danger of pneumonia
but: high mechanical efforts
Advantages & Disadvantages - Positive Pressure Ventilation
Advantages:
- easy access to patient
- -> surgical procedures possible
Disadvantages:
- unphysiological pressures
- bacteria may enter lung –> pneumonia
Categories of Respiratory Support - Classification Types
1) Classification by work of breathing (WOB)
2) Classification by the dependent variable
- volume control
- pressure control
3) Classification by Access to the Lung
- invasive vs. non-invasive
WOB
Continuous Mechanical Ventilation
- Volume / Pressure controlled Ventilation systems
- PEEP = 0 –> intermittent positive pressure ventilation
- PEEP > 0 –> continuous positive pressure ventilation
WOB Assisted Ventilation (Synchronized!)
Ventilator provides mandatory breath, but patient has to trigger it
WOB
Spontaneous Ventilation with Support
ventilator ensures a minimum minute Ventilation
WOB
Pure Spontaneous Ventilation
patient is capable to breath spontaneously
–> weaning mode
- PEEP is used to prevent lung collapse
intrinsic PEEP
= P(alv) at the end of expiration
but not necessarily = PEEP when deflation is not long enough
Components of Ventilator
- dosing units
- mixing chamber
- pressure sensors
- inspiration & expiration tube
COPD
& effects of COPD
Chronic Obstrusive Pulmonary Disease - increased airway resistance - intrinsic PEEP - increased paCO2 & WOB - often hyperinflated lungs with increased FRC (functional residual capacity) - decreased FEV1/FVC Emphysema
FEV: forced expiratory volume per second
FVC: orced vital capacity
Asthma
muscle contraction makes airways narrower
–> increased expiratory resistance to air flow
Emphysema
increased FRC
FRC: Functional Residual Capacity
- permanent expansion of airspaces
- -> barrel chest
Positive vs. Negative Pressure Ventilation
- positive pressure ventilation may primarily ventilate ventral parts of the lung
- in a lying patient, perfusion is largest in the dorsal regions –> positive pressure ventilation producues V’/Q’ mismatch
How is acute respiratory distress syndrome (ARDS) defined?
paO2/FiO2 < 200mmHg
- clinical signs: lung edema & Atelectasis
- FEV and FVC reduced
Where in the paO2-pressure graph would the ventilation of ARDS lungs be least harmful?
at the beginning of the descending limb of hysteresis
What kind of signal is set in Pressure Controlled Ventilation?
square signal
What induces a hysteresis in the pV - diagram?
Atelectasis
What is the difference between Intermittent and Continuous Positive Pressure Ventilation?
the value of PEEP
The mechanical breathing power is a function of … and …
mechanical breathing power is a function of respiratory rate and tidal volume
- tidal volume is a function of frequency