Building a Solid Understanding of Mechanical Ventilation By

Building a Solid Understanding of Mechanical Ventilation By Chris Kallus, RRT, MEd Nursing 2009, June 2009 2. 5 ANCC contact hours Online: www. nursingcenter. com © 2009 by Lippincott Williams & Wilkins. All world rights reserved.

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

Relationship between ventilator settings and arterial blood gas u Step 1: Evaluate p. H and Pa. CO 2 u Step 2: Evaluate Pa. O 2 and FIO 2 u Step 3: Determine the solution

Step 1: Evaluate p. H and Pa. CO 2 u u u Hypoventilation causes patient’s p. H to drop and Pa. CO 2 to rise Hyperventilation, p. H is >7. 45; Pa. CO 2 is <35 mm. Hg Changing “minute ventilation” setting will help get values back to normal

Step 1: Evaluate p. H and Pa. CO 2 u “Minute ventilation” is determined by multiplying tidal volume by ventilator rate; expressed as Ve u Tidal volume is based on patient’s ideal body weight (IBW) u Normal tidal volume is between 10 and 12 m. L/kg of IBW u Large tidal volumes can cause ventilatorinduced lung injury

Calculating IBW u Patients of different weights can have same lung size; calculating IBW (in lbs) helps choose right tidal volume u Men: 106 + 6(height in inches-60) u Women: 105 + 5(height in inches-60)

Lungs under pressure u u 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

Lungs under pressure u u 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 H 2 O 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 H 2 O, talk to respiratory therapist about alternative, lung-protective strategies

Lungs under pressure u u 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

Lungs under pressure u u If patient’s PIP is 35 cm H 2 O and plateau pressure is 25 cm H 2 O and an hour later peak pressure is 65 cm H 2 O but plateau pressure is still 25 cm H 2 O, 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

Lungs under pressure u u Transairway pressure is difference between PIP and the plateau pressure; is typically <10 cm H 2 O. 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

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

Step 2: Evaluate Pa. O 2 and FIO 2 u u u FIO 2 is always expressed as a decimal (ex. , FIO 2 of 1. 0 = 100% oxygen) Only reason to keep FIO 2 above 100% is in treating carbon monoxide poisoning As delivered, FIO 2 increases, Pa. O 2 should increase; if not, patient has refractory hypoxemia

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

3 Common causes of refractory hypoxemia u u u 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 Sp. O 2 accompanied by fine crackles in lung bases

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

PEEP u u u Restores or maintains lung volume Using PEEP also lets you use lower FIO 2 to reach a target Pa. O 2 Generally, to reduce the risk of oxygen toxicity, FIO 2 should be below 0. 5 provided Pa. O 2 is OK and Sp. O 2 is 92% or higher

Step 3: Determine the solution u u u Goal is Pa. O 2 60 to 100 mm. Hg 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

Ventilator modes u u 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

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

Ventilator strategies to deliver tidal volume u u 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

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