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