Resistance
What is Resistance?
• Resistance is a measurement of the frictional
forces that must be overcome during
breathing.
• It can also be described as the force (pressure)
necessary to maintain a specific flow in a
particular system.
– Bronchioles
– Vessels
What is Resistance?
• As gas molecules pass through the ETT and the
bronchi, the particles collide with the ETT and the
bronchial wall, causing them to exert energy, or
pressure, causing bronchial walls to expand. This
causes some of the particles to remain in the
airway, thus giving incoming gas molecules
resistance as they flow over the incumbent
molecules, causing resistance to increase (due to
particle friction), or creating turbulent flow,
opposed to free flowing laminar flow.
How do we calculate resistance?
• Ohm’s Law for airway resistance
– RAW=PIP- Platuea/flow (Mechanical ventilation)
• Normal value of a intubated patient: 6 cmH2O/L/s
• Raw increases as ETT size decreases, increase secretions,
bronchospasm, tube biting, circuit kinks and condensation.
• Ohm’s Law for vascular resistance (VR)
– VR=
P/cardiac output
• SVR= MAP-CVP/CO.
– Normal ranges: 900-1500 dyne x s x cm -5th.
• PVR= MPAP-PAWP/CO
– Normal ranges: 100–250 dyne x s x cm -5th.
How do we calculate resistance?
• Poiseuille’s Law
– The resistance to airflow through the conductive
airways (airway resistance) depends on gas viscosity,
density, and length and diameter of the tube.
– Flow= Pr4th/8nl
– Flow is related to the fourth power of the radius. Flow
is inversely related to the viscosity of the fluid and the
length of the tube through which the gas passes and
directly related to the pressure gradient.
What can increase resistance?
• Bronchospasm
– An abnormal contraction of the smooth muscle in
the bronchi, resulting in acute narrowing and
obstruction of the respiratory airway.
– Asthma, Chronic Bronchitis, HAAD
• Inflammation of the airways causing lumen diameter to
narrow and obstructed.
– Solution- administer bronchodilator, cool mist
treatment, corticosteroids (severe).
What can increase resistance?
• Secretions
– An increase in mucus secretions can cause airway
obstruction. Also, secretions can become viscous and
immobile, also causing airway obstruction.
– Asthma, Chronic Bronchitis, HAAD, inadequate
humidification (bypassed airway), decreased MEP.
• Asthma, CB, HAAD can increase mucus production that can narrow
lumen and cause obstruction. Inadequate humidification can
cause secretions to become dried and thicker. A decrease in MEP
can result in a weak cough resulting in less secretion mobility
causing mucus to remain in the airways.
– Solutions- perform suctioning as needed, PD&P if
tolerated, coughing techniques, mucolytics and proper
humidification.
What can increase resistance?
• Ventilator Associated Resistance
– Biting the ETT
• Patients can bite down on the ETT causing the airway to become
occlude.
• Solution- insert bite guard, artificial airway.
– Kink in the ventilator circuit
• Circuit can become lodged between bed rail handles occluding flow.
• Solution- keep circuits in circuit holder (hanger) and away from
moving parts.
– Condensation in the ventilator circuit
• Condensation may build up into low points of circuits causing water to
pool up, obstructing airflow (heated humidification only). When using
an HME, the condensation may fill the HME filter and occlude airflow.
• Solution- drain condensation into reservoir or circuit drain bag.
Change HME once a shift.
What can increase resistance?
• Ventilator Associated Resistance
– ETT smaller than patient requirement
• Patient can be given a smaller ETT due to inflammation
of the glottis or vocal chords or by physician error.
• Solution- if possible, switch out ETT with bigger size. If
unable to change ETT (pt is still inflamed), increase flow
to compensate for the smaller ETT.
• Calculate with Poiseuille’s Law
How can we recognize resistance?
• Patient
– Increased WOB
• Patient may be using inspiratory accessory muscles to
compensate for resistance. Patient may also become
diaphoretic as WOB increases.
– Tube biting
• Patient may have jaw clenched occluding tube.
– Auscultation
• Patient may have wheezes (bronchospasm) and/or rhonchi
(secretions) narrowing/occluding airways. Auscultating the
throat can show stridor or performing MOV/MLT (ventilated
patients) can show inflammation/edema due to lack of leak
with cuff deflated.
How can we recognize resistance?
• Ventilator
– Peak Inspiratory Pressures
• PIP is directly related to Raw, therefore if there is an increase
in Raw, the PIP will also increase.
– E.G. Patient is biting the ETT. Therefore the ETT becomes
occluded, causing the ventilator to increase pressure to deliver
the set amount of volume (ACVC).
– Low Tidal Volume
• Not as directly related to Raw as PIP, but can show
decreased values when volume cannot be fully delivered
through occluded area.
– E.G. Patient has bronchospasm that is causing lumen to become
narrow. The PIP has reached the limit and stops delivery of
breath, thus causing set volume to not be delivered (ACVC).
PIP- 30 cmH2O
Vt – 374 ml
How can we recognize resistance?
• Ventilator Waveform Graphics
– Pressure/Volume Loop
• Loop can show secretions (wavy line) and resistance (delay
in line rise) during inspiration.
resistance
secretions
How can we recognize resistance?
• Ventilator Waveform Graphics
– Flow/Time waveform
– Wavy line in expiratory side of graphic