The Ventilator

Question 1

What is inspiratory airflow?

Answer 1

Occurs as a result of a pressure gradient where the extrapulmonary pressure is greater than the intrapulmonary pressure

Question 2

What is expiratory airflow?

Answer 2

Occurs as a result of a pressure gradient in which intrapulmonary pressure is raised to above extrapulmonary pressure

Question 3

What is tidal volume?

Answer 3

• It is the amount of air entering or inhaled or leaving by exhalation with each breath
• When a ventilator mode uses a target tidal volume in adults the expected settings should be between 6-8 ml/kg of the patient’s ideal weight. The goal is not to exceed the plateau pressure of 30 cm of H2) to prevent barotrauma.
• The most common practice is to use 50 ml increments and approximate the tidal volume and adjust to effect. Thus tidal volumes can be 400, 450, 500 ml and so on.

Question 4

What is plateau pressure?

Answer 4

Plateau pressure is the pressure applied to the alveoli and small airways. Lung barotrauma can occur when there is over-inflation causing damage to tissues and circulation. Air embolism can occur.

Question 5

What is a “rate” on the ventilator?

Answer 5

The rate is the number of breaths delivered each minute

• A mechanical breath cycle can be defined by either the tidal volume amount or target OR by a pressure target
• Rates are adjusted according to blood gas analysis to achieve pH and PaCO2 goals

Question 6

What is FIO2?

Answer 6

Fraction of inspired oxygen (FIO2) is the decimal value produced by dividing the partial pressure of oxygen (PO2) by the total pressure of the mixture.

Therefore:

• FIO2 of room air is approximately .21. Using a decimal value rather than a percentage is the correct expression.
• FIO2 of 100% is 1
• In the hospital setting however, the most accepted way to express oxygen concentration that you see will likely be %

Question 7

What are 2 types of inspiratory cycles?

Answer 7

1. Volume cycled: pressure limited (volume target). Preset tidal volume is delivered unless a set pressure limit is reached. At that point, the cycle is terminated
2. Pressure cycled: volume limited (pressure targeted). A preset pressure is delivered unless a volume limit is reached terminating the cycle.

Question 8

What is the minute volume/ventilation?

Answer 8

It is the total volume of new air that enters the respiratory passages per minute

Question 9

What are the benefits of a high PEEP?

Answer 9

It prevents the return of intrapulmonary pressure to equal extra pulmonary pressure at the end of expiration. It accomplishes an incomplete expiration leaving a higher amount of air in the lungs or functional residual volume (FRV)

Question 10

What are the effects of an increase in functional residual volume (FRV)?

Answer 10

• Better gas exchange as more alveoli are left open throughout the ventilatory cycle
• There are additional alveoli that will open at peak inspiration because the delivered tidal volume is “stacked” on the FRV.
• A higher PaO2 may be achieved without having to increase the FIO2
• Typical PEEP settings are 5-10 cm H20 although higher settings may be needed for patients that have low compliance conditions such as acute respiratory distress syndrome
• Risks of PEEP include barotrauma, impedence of central venous return that can result in a lower cardiac output and increased intracranial pressure

Question 11

What is CPAP and when is it used?

Answer 11

• Same as PEEP but with no mechanically delivered inspirations
• Patient must be able to breathe without assistance
• Has the same risks and benefits as PEEP

Question 12

What causes a high-pressure ventilator alarm?

Answer 12

A high-pressure alarm will occur when the proximal airway pressures exceed the limits that are set. Alarm conditions can occur when there is:

• secretion accumulation
• the patient is coughing
• there is spontaneous dysynchrony
• decreasing lung compliance
• pneumothorax
• airway occlusion

Question 13

What causes a low-pressure ventilator alarm?

Answer 13

A low-pressure alarm occurs when the proximal airway pressure does not reflect the current ventilator function. This can happen with disconnected tubing and/or ETT leaks

Question 14

When is a ventilator “trigger”?

Answer 14

Determines how the ventilator initiates a breath. Can either be pressure triggered or volume triggered

Question 15

What is “cycling” when using a ventilator?

Answer 15

Tells the ventilator when to terminate the breath to allow expiration

Question 16

Describe the Controlled mandatory ventilation (CMV ) ventilator mode

Answer 16

During the mode the patient will receive a set tidal volume at a set rate

• Patient cannot add spontaneous breaths
• Sedation or neuromuscular (NM) blockade is required to sedate and prevent the patient from “fighting” the vent

Question 17

Describe the Assist Control (A/C) ventilator mode

Answer 17

This ventilator mode is the primary mode of ventilation used in respiratory failure. Patient will receive a set tidal volume at set rate, BUT:

• Here the patient can add spontaneous breaths** but will continue to **receive the set tidal volume with the initiation of each of the spontaneous breaths
• The ventilator breaths will not be synchronized with the spontaneous breaths
• Sedation may be required to prevent hyperventilation and ventilator dysynchrony

Question 18

Describe the Synchronized intermittent mandatory ventilation (SIMV) ventilator mode

Answer 18

• This mode of ventilation is commonly used in surgical patients
• Patient triggered breaths are unassisted or minimally assisted, meaning the TV delivered will vary with this breath
• The volumes are determined by both the patient effort and lung compliance.

Question 19

What is a good ETT position?

Answer 19

• Should not extend the distal tip of the tube beyond the level of the carina.
• ETT 22 cm mark that is even with the lips is a good approximation for most adults
• On CXR the tip of the ETT should be 2-4 cm above the carina.
• PE should show equal breath sounds bilaterally

Question 20

What is the criteria for a ventilator-associated condition (VAC)?

Answer 20

Any increase of oxygen requirement ( ≥0.20 in FiO2) or PEEP (≥3 cm H2O) after a period of stability (≥2 days) without evidence of infection

Question 21

What is the criteria for an infection-related ventilator-associated complication (IVAC).

Answer 21

1. If temperature is >38° or <36°, or white blood cell count > 12,000/mm³ or <4,000/mm³
2. AND a new antibiotic is added for at least 4 days
3. Any increase of oxygen requirement ( ≥0.20 in FiO2) or PEEP (≥3 cm H2O) after a period of stability (≥2 days)

Question 22

What is the criteria for ventilator associated pneumonia (VAP)?

Answer 22

• Any increase of oxygen requirement ( ≥0.20 in FiO2) or PEEP (≥3 cm H2O) after a period of stability (≥2 days)
• A temperature is >38° or <36°, or white blood cell count > 12,000/mm³ or <4,000/mm³
• A new antibiotic is added for at least 4 days
• Purulent secretions or positive respiratory cultures (irrespective of chest film findings)

Question 23

What are the indications for mechanical ventilation?

Answer 23

1. Cardiac or respiratory arrest
2. Tachypnea or bradypnea with respiratory fatigue or impending arrest
3. Acute respiratory acidosis
4. Refractory hypoxemia (when the PaO2 could not be maintained above 60 mm Hg with inspired O2 fraction (FIO2)>1.0)
5. Inability to protect the airway associated with depressed levels of consciousness of consciousness Indications for Mechanical Ventilation
6. Shock associated with excessive respiratory work
7. Inability to clear secretions with impaired gas exchange or excessive respiratory work
8. Newly diagnosed neuromuscular disease with a vital capacity <10 capacity <10 - 15 mL/kg
9. Short term adjunct in management of acutely increased intracranial pressure (ICP) intracranial pressure (ICP)