1. Building a Solid Understanding of Mechanical Ventilation
By Chris Kallus, RRT, MEd
Nursing2009, June 2009
2.5 ANCC contact hours
Online:
© 2009 by Lippincott Williams & Wilkins. All world rights reserved.

2. Mechanical ventilation
More patients who are mechanically ventilated are on general units
Follow facility’s procedures and protocols
Assess patient first when problems arise
Obtain physician orders as appropriate
Work with respiratory therapists when making ventilator changes

3. Relationship between ventilator settings and arterial blood gas
Step 1: Evaluate pH and PaCO2
Step 2: Evaluate PaO2 and FIO2
Step 3: Determine the solution

4. Step 1: Evaluate pH and PaCO2
Hypoventilation causes patient’s pH to drop and PaCO2 to rise
Hyperventilation, pH is >7.45; PaCO2 is
<35 mmHg
Changing “minute ventilation” setting will help get values back to normal

5. Step 1: Evaluate pH and PaCO2
“Minute ventilation” is determined by multiplying tidal volume by ventilator rate; expressed as Ve
Tidal volume is based on patient’s ideal body weight (IBW)
Normal tidal volume is between 10 and 12 mL/kg of IBW
Large tidal volumes can cause ventilator-induced lung injury

6. Calculating IBW
Patients of different weights can have same lung size; calculating IBW (in lbs) helps choose right tidal volume
Men: (height in inches-60)
Women: (height in inches-60)

7. Lungs under pressure
Best indicator of alveolar overdistension (too much pressure from mechanically delivered breaths) is peak alveolar pressure, which can be assessed by measuring plateau pressure, or pressure applied to small airways and alveoli during inspiration
Following delivery of tidal volume, you’ll see a number on ventilator called PIP, or amount of pressure it takes to deliver that volume. This number shouldn’t be used for trending or evaluation

8. Lungs under pressure
If you set ventilator to achieve a breath hold following delivery of tidal volume, you should see pressure drop from peak to a holding pressure (the plateau pressure; should be 30 cm H2O or less)
If value is higher, overdistension is likely. Every time you perform ventilator check, assess plateau pressure. If value is trending upward or exceeds 30 cm H2O, talk to respiratory therapist about alternative, lung-protective strategies

9. Lungs under pressure
Alternative strategies include permissive hypercapnia, airway pressure release ventilation, changing the mode to pressure control ventilation
If patient’s peak inspiratory pressure (PIP) is increasing but plateau stays the same, reason for pressure increase is in ventilator tubing or patient’s tracheobronchial tree

10. Lungs under pressure
If patient’s PIP is 35 cm H2O and plateau pressure is 25 cm H2O and an hour later peak pressure is 65 cm H2O but plateau pressure is still 25 cm H2O, patient isn’t in danger of lung damage because reason for high PIP is an increase in airway resistance
Patient may be biting on ET tube or need an inline bronchodilator treatment or suctioning; this is why plateau pressure is more important ventilator pressure to monitor

11. Lungs under pressure
Transairway pressure is difference between PIP and the plateau pressure; is typically <10 cm H2O. Investigate any pressure above this level
For example, a sudden increase means an ET tube may be occluded; a more gradual increase may mean patient is developing bronchoconstriction and may need an inline bronchodilator

12. Step 2: Evaluate PaO2 and FIO2
Oxygenation status is evaluated by calculating P/F ratio
P/F ratio = PaO2 divided by FIO2
>300 is considered normal
Lung injury and values <200 indicate refractory hypoxemia

13. Step 2: Evaluate PaO2 and FIO2
FIO2 is always expressed as a decimal (ex., FIO2 of 1.0 = 100% oxygen)
Only reason to keep FIO2 above 100% is in treating carbon monoxide poisoning
As delivered, FIO2 increases, PaO2 should increase; if not, patient has refractory hypoxemia

14. Step 2: Evaluate PaO2 and FIO2
P/F ratio is good indicator of how much oxygen patient is breathing (FIO2) and how much is moving into circulation (PaO2)
In critically ill patients, 3 common causes of refractory hypoxemia:
- pneumothorax
- atelectasis
- pulmonary edema

15. 3 Common causes of refractory hypoxemia
Pneumothorax - rapid deterioration in patient condition, absent breath sounds, high pressure ventilator alarm
Atelectasis – gradual, usually identified on chest X-ray
Pulmonary edema - may occur in patients with history of heart failure, decreasing SpO2 accompanied by fine crackles in lung bases

16. 3 Common causes of refractory hypoxemia
HCP will rule out pneumothorax first as increasing ventilator volume will worsen pneumothorax
If atelectasis or pulmonary edema, PEEP will be added

17. PEEP Restores or maintains lung volume
Using PEEP also lets you use lower FIO2 to reach a target PaO2
Generally, to reduce the risk of oxygen toxicity, FIO2 should be below 0.5 provided PaO2 is OK and SpO2 is 92% or higher

18. Step 3: Determine the solution
Goal is PaO2 60 to 100 mmHg
Pulse oximetry values are best used for trending rather than spot-checking oxygenation
Follow facility protocol and work with respiratory therapist to make ventilator changes

19. Ventilator modes
Assist/control - tidal volume delivered in response to every patient effort; can be used in most patients except those with COPD due to risk of hyperinflation of lungs
SIMV - delivers tidal volume only at a set time interval; patient can breathe in between, suitable for all patients, including COPD

20. Ventilator modes
Continuous positive airway pressure (CPAP) - can be used for ventilator weaning, and patients with sleep apnea; basically, PEEP with 10 breaths per minute

21. Ventilator strategies to deliver tidal volume
Volume control ventilation - sets tidal volume so patient receives same tidal volume with each mechanical breath; also called volume target, volume cycled, volume limited
ABGs remain constant with this strategy

22. Ventilator strategies to deliver tidal volume
Pressure support ventilation (PSV) - sets PIP; monitor patient for risk of hypo/hyperventilation
Pressure support ventilation (PSV) - add-on strategy for patients with low tidal volume and mild respiratory distress; adds pressure boost on inspiration, decreases work of breathing