KNOWING THE BASICS of mechanical ventilation is the key to caring for a patient who's endotracheally intubated and on mechanical ventilation. With more ventilated patients on general units, you need to be able to stay in tune with the day-to-day aspects of ventilator care. In this article, I'll outline what you need to know about mechanical ventilation. Follow your facility's procedures and protocols when caring for your patient, assess the patient first when problems arise, obtain a physician's order as appropriate, and work with the respiratory therapist when making ventilator changes.

Let's start with assessing the relationship between ventilator settings and arterial blood gas (ABG) values for a hypothetical patient, Arthur White, 68, who was put on mechanical ventilation because of persistent apnea following his partial colectomy. He has no history of lung disease. His ventilator settings are:

* Synchronized intermittent mandatory ventilation (SIMV) at 10 breaths/minute. (More on this ventilator mode later.)

* Tidal volume (V_T) of 700 mL. Tidal volume represents the volume of gas exchanged during each ventilated breath. Normal tidal volume in a patient who's breathing spontaneously is 5 to 8 mL/kg. In mechanically ventilated patients, the tidal volume is set to prevent lung injury; the value depends on the patient's lung condition. A patient with normal lungs would be ventilated at 10 to 12 mL/kg, one with chronic obstructive pulmonary disease (COPD) at 8 to 10 mL/kg, and one with acute repiratory distress syndrome at 4 to 8 mL/kg.

* The fraction of inspired oxygen (F_IO₂) is 1.0, meaning that he's receiving 100% oxygen.

* Positive end-expiratory pressure (PEEP) is 5 cm H₂O. PEEP is a measure of the pressure remaining in the lungs at end-expiration; in normal, nonventilated patients, 3 to 5 cm H₂O of PEEP is considered physiologically normal.

Mr. White's pH is within the normal range, and he's not compensating for a metabolic or respiratory disorder. Hypoventilation causes the patient's pH to drop below 7.35 and his PaCO₂ to rise above 45 mm Hg-the level of ventilation is insufficient to maintain a normal PaCO₂. In hyperventilation, the patient's pH is greater than 7.45 and PaCO₂ is less than 35 mm Hg-the level of ventilation is excessive. Because his pH and PaCO₂ are both normal, Mr. White's level of ventilation is satisfactory.

If Mr. White's PaCO₂ weren't normal, you'd try to get it back into the normal range by working with the respiratory therapist to change the minute ventilation, which is determined by multiplying the tidal volume by the ventilator rate and is expressed as V_E. The PaCO₂ is the respiratory component affecting pH (the kidneys and the HCO₃^- value are the metabolic component). In a patient with hypoventilation, you'd adjust the minute ventilation by increasing the tidal volume or ventilatory rate. These interventions reduce the PaCO₂ back to normal and bring the pH back within the acceptable range.

However, before adjusting the tidal volume, you need to be sure that the tidal volume used is appropriate for the patient, based on his ideal body weight (IBW), which is used to approximate lung size. (For more information, see Calculating IBW.) Normal lungs typically require a tidal volume between 10 and 12 mL/kg of IBW. Mr. White's IBW is 154.4 pounds (70 kg) and his lungs are normal, so a tidal volume between 700 and 840 mL is appropriate. (An even quicker way to set tidal volume for a patient with normal lungs is to add a zero to the patient's IBW in kilograms.)

So far, so good for Mr. White. His pH and PaCO₂ are normal, and his tidal volume is appropriate for his IBW. Before we move on to the next step, let's look at what you'd do if his tidal volume were inappropriate: For example, suppose Mr. White's tidal volume was set at 1,000 mL and his ventilator rate was set at 7 breaths/minute. The minute ventilation is the same, 7,000 mL (or 7 liters). But the tidal volume exceeds the maximum 840 mL recommended for a patient of Mr. White's IBW. Large tidal volumes can cause ventilator-induced lung injury.

In this situation, you'd reduce the tidal volume to 700 mL and increase the ventilatory rate to 10 breaths/minute. This change retains the same minute ventilation value (7 liters), won't change Mr. White's pH or PaCO₂, and avoids the dangers of excessive delivered tidal volumes. For more on pressure and the risk of ventilator-induced lung injury, see Lungs under pressure.

Step 2: Evaluate PaO

Next, assess your patient's oxygenation status by calculating the P/F ratio, which is PaO₂ divided by F_IO₂. For example, if the patient's PaO₂ is 225 mm Hg and the F_IO₂ is 1.0, the P/F ratio is 225. A number greater than 300 is considered normal. Values between 200 and 300 indicate acute lung injury, and values less than 200 indicate refractory hypoxemia, or hypoxemia that's not responsive to oxygen therapy.

Note that the F_IO₂ value is always expressed as a decimal, not a percentage. An F_IO₂ of 1.0 can also be expressed as 100% oxygen; an F_IO₂ of 0.5 can also be expressed as 50% oxygen, but it's incorrect to say the patient is on an F_IO₂ of 50%.

Let's return to assessing Mr. White's oxygenation status. His PaO₂ is 225 mm Hg-too high. The only reasons to keep a patient's PaO₂ above 100 mm Hg is if you're treating carbon monoxide poisoning, and a patient's F_IO₂ shouldn't exceed 0.5 (more on that later). However, given that Mr. White is receiving 100% oxygen, his PaO₂ should be 500 mm Hg, or five times the percentage of oxygen he's receiving. (Because room air is 21% oxygen, multiplying the F_IO₂ by 5 is a quick way of estimating PaO₂.) For example, a patient breathing 40% oxygen should have a PaO₂ of about 200 mm Hg. As you increase the delivered F_IO₂ you should see a corresponding increase in PaO₂-if not, the patient has refractory hypoxemia.

Mr. White's P/F ratio of 225 already has alerted you that he has acute lung injury. The P/F ratio tells you if there's a good relationship between how much oxygen the patient is breathing (F_IO₂) and how much is moving through the alveoli into the circulation (PaO₂). If a significant amount of atelectasis causes a low PaO₂, your patient won't respond as he should no matter how much oxygen you administer.

In a critically ill patient, the three most common causes of refractory hypoxemia are:

* pneumothorax, characterized by a rapid deterioration in the patient's condition, absent breath sounds, and a high-pressure alarm on the ventilator

* atelectasis, which develops gradually and usually is identified by chest X-ray

* pulmonary edema, which may occur in patients with a history of heart failure and decreasing SpO₂ accompanied by fine crackles auscultated in the lung bases.

Let's look a little closer at PEEP. After delivery of the tidal volume during inspiration, the mechanically ventilated patient is allowed to exhale to a baseline pressure of zero cm H₂O. Low levels of PEEP (3 to 5 cm H₂O) are added to restore the normal volume loss that occurs with intubation, and these levels are well-tolerated by almost all intubated patients. A PEEP level over 5 cm H₂O is considered therapeutic, and is used to treat refractory hypoxemia: PEEP restores or maintains lung volume at end-expiration by either recruiting collapsed alveoli or preventing further loss of lung volume and atelectasis. Remember that in patients with refractory hypoxemia caused by significant atelectasis, increasing the F_IO₂ won't raise the PaO₂ appreciably. In these patients, PEEP is indicated to recruit collapsed alveoli and prevent further loss of lung volume.

Using PEEP also lets you use a lower F_IO₂ to reach a target PaO₂. For example, if a patient requires 60% oxygen (an F_IO₂ of 0.6) to maintain a PaO₂ of 90 mm Hg, adding 5 cm H₂O of PEEP will raise the PaO₂ to 130 mm Hg. Now the F_IO₂ can be decreased to bring the PaO₂ into the clinically accepted range of 60 to 100 mm Hg, as I'll describe shortly. Generally, to reduce the risk of oxygen toxicity, the F_IO₂ should be below 0.5, provided the PaO₂ is acceptable and the patient's SpO₂ is 92% or higher.

Step 3: Determining the solution

Let's go back to our patient now and see what we can do to fine-tune his oxygenation status.

Mr. White is on 5 cm H₂O of PEEP, which is being used to prevent volume loss due to intubation. Our approach now is to wean the F_IO₂ out of the toxic range and keep the PaO₂ between 60 and 100 mm Hg. If you're monitoring SpO₂ values, you'll want to keep the value above 92%-remember that pulse oximeters generally have a 2-percentage-point margin of error if the SaO₂ is 90% or above, so aiming for 92% ensures that his actual SpO₂ stays above 90%, the lowest clinically accepted level. Pulse oximetry values are best used for trending, rather than spot-checking your patient's oxygenation (remember that a patient in acute respiratory acidosis can have an SpO₂ above 90%.)

Instead of guessing how much to decrease the F_IO₂, we'll use the P/F ratio to set up a prediction formula to make sure we don't wean too slowly or overdo it. Follow your facility's protocol or work with the respiratory therapist to make the actual ventilator changes. The formula is a proportion problem: PaO₂/F_IO₂ = desired PaO₂/new F_IO₂. For Mr. White, these numbers are: 225/1.0 = 100/X, where X represents the new F_IO₂ value. Solving for X, you get a new F_IO₂ value of 0.44.

Mr. White's current F_IO₂ is 1.0, so your initial goal is to reduce the level to 0.5. Decrease the oxygen percentage by 10 points every 5 to 10 minutes (from 100% to 90%, then to 80% and so on) while observing the patient's oxygen saturation via pulse oximeter. Because our target PaO₂ is 100 mm Hg, keep weaning the oxygen as long as the SpO₂ is 95% or greater.

Mr. White's P/F ratio was 225. If this value were less than 200, you'd follow your facility's protocol for adjusting the ventilator settings-this is where PEEP levels over 5 cm H₂O come into play. The techniques to find the best or optimal PEEP and to provide significant lung recruitment are beyond the scope of this article.

Staying a la mode

Now let's look at ventilator modes, which determine the respiratory rate during mechanical ventilation. Terminology varies depending on the ventilator manufacturer, but in general, mode of ventilation is selected based on the patient's condition.

If the patient is apneic on a ventilator, it doesn't really matter what mode of ventilation is selected as long as it can deliver a tidal volume at a set time interval. The mode will determine how the ventilator responds when the patient makes an inspiratory effort.

* In assist/control mode, the machine delivers a tidal volume in response to every patient effort. This mode also may be called continuous mandatory ventilation. Assist/control can be used for any patient needing mechanical ventilation, except for patients with COPD. Because patients with COPD can't fully exhale each mechanical breath, this mode raises the risk of hyperinflation. This mode also doesn't let the patient breathe spontaneously, raising the risk of air-trapping.

* In SIMV mode, the machine responds to patient effort by delivering a tidal volume only at a set time interval; in between these breaths, the ventilator will let the patient take spontaneous breaths. This mode is suitable for all patients, including those with COPD.

* In continuous positive airway pressure (CPAP) mode, the machine doesn't respond to patient effort for volume delivery, and lets spontaneous breathing occur. As you know, CPAP is used for patients with obstructive sleep apnea, but in critically ill patients, it's used for ventilatory weaning.

When it's time to wean Mr. White from the ventilator, the ventilator rate will be changed from 10 breaths/minute to 8, and so on, depending on patient tolerance, until the rate is zero and Mr. White is on CPAP.

For a patient on assist/control mode, CPAP is used during a spontaneous breathing trial before extubation. If the patient's vital signs, hemodynamics, and ABGs are stable, he can be placed on CPAP for 5 minutes while his respiratory rate and pattern, SpO₂, and vital signs are assessed. If he's stable after 5 minutes, the trial can continue for up to 2 hours and he may be extubated.

Ventilator strategies

Now let's look at ventilator strategies, which determine how tidal volume is delivered:

* Volume control ventilation sets the tidal volume so that the patient receives the same tidal volume with each mechanical breath. This strategy also is called volume target, volume cycled, and volume limited ventilation. If the patient's lung compliance or airway resistance changes, the pressure will change to maintain a constant tidal volume. The patient's ABG values remain relatively consistent with this strategy.

* Pressure control ventilation sets the peak inspiratory pressure (PIP) for each mandatory breath. If the patient's lung compliance or airway resistance changes, the tidal volume will change. If this strategy is used, monitor the patient closely because of the increased risk for hyperventilation or hypoventilation.

* Pressure support ventilation (PSV) is an add-on strategy on many ventilators. This strategy is used if a patient has a low spontaneous tidal volume and is in mild respiratory distress-the respiratory therapist will titrate pressure support to increase the spontaneous tidal volume. Instead of the patient performing all the work to breathe during inspiration, the ventilator adds a pressure boost to help with inspiration. This added pressure increases the spontaneous tidal volume in the same way that you increase volume when you use bag-valve-mask ventilation. You could start at a pressure of 10 cm H₂O and gradually increase it in increments of 2 (no hard and fast rule here) until you reach the minimum spontaneous tidal volume (5 mL/kg of IBW). You'll know when you reach the appropriate level because the patient's work of breathing should decrease. The PSV strategy works only during a spontaneous breathing mode, so it can't be used if the patient is in assist/control mode.

Table. Mr. White's status

Summarizing the information

Now you can create a bullet box containing all of the data about Mr. White's mechanical ventilation and his clinical status before any interventions. (See Mr. White's status.) Here's what the information tells you: Mr. White is in SIMV mode with volume control strategy, so a set tidal volume is delivered at a rate of 10 breaths/minute, and spontaneous breathing is allowed between the mechanical breaths. His acid-base balance, PaCO₂, and HCO₃^- are normal, but his high F_IO₂ puts him at risk for oxygen toxicity. As discussed earlier, you'll reduce the F_IO₂. Mr. White's P/F ratio indicates some acute lung injury, possibly developing atelectasis.

Mr. White's spontaneous tidal volume is too low--it should be at least 5 mL/kg of IBW (350 mL in his case). Is he groggy? Is he having distress during spontaneous breathing, as indicated by accessory muscles of respiration use and an increase in spontaneous respiratory rate? If so, add pressure support as described above. Mr. White's transairway pressure is 8 cm H₂O (PIP of 32 cm H₂O minus plateau pressure of 24 cm H₂O). If the transairway pressure is above 10 cm H₂O, your patient has a significant amount of airway resistance and may need inline bronchodilator therapy or suctioning. Mr. White's plateau pressure is below the maximum recommended level of 30 cm H₂O.

His total minute ventilation of 9 liters also is fine. (Values greater than 10 liters are considered high.) Auscultation indicates possible atelectasis, which may be why the P/F ratio is low, so in addition to decreasing the F_IO₂, more PEEP may be added. Follow your facility's protocol for handling this.

By understanding the basics of mechanical ventilation and how to summarize your patient's information, you'll be able to care for him appropriately.

Calculating IBW

Patients of different weights can have the same lung size, so calculating IBW helps healthcare providers choose the right tidal volume for the patient. The following formulas give you IBW in pounds.

* For men: 106 + 6(height in inches-60)

* For women: 105 + 5(height in inches-60)

Lungs under pressure

The best indicator of alveolar overdistension (or too much pressure from mechanically delivered breaths) is peak alveolar pressure, which can be assessed by measuring the plateau pressure, or the pressure applied to small airways and alveoli during inspiration.

Here's how to measure plateau pressure:

Following delivery of the tidal volume, you'll see a number on the ventilator called PIP, or the amount of pressure it takes to deliver that volume. As you may know, this number doesn't mean a whole lot and shouldn't be used for trending or evaluation. For example, a PIP of 45 cm H₂O doesn't give you any clinical information, because any change in airway resistance or compliance will change the PIP.

If you set the ventilator to achieve a breath hold following delivery of the tidal volume, then you should see the pressure drop from the peak to a holding pressure. This holding pressure is the plateau pressure, and should be 30 cm H₂O or less. If the value is higher, overdistension is likely.

Every time you perform a patient and ventilator assessment (sometimes called a ventilator check), assess the plateau pressure. If this value is trending upward or exceeds 30 cm H₂O, talk to the respiratory therapist about alternative, lung-protective strategies, a relatively new but important part of mechanical ventilation. Alternative strategies may include permissive hypercapnia, airway pressure release ventilation, or changing the mode to pressure control ventilation.

If the patient's PIP is increasing but the plateau stays the same, then the reason for the pressure increase is in the ventilator tubing or the patient's tracheobronchial tree. Suppose the patient's PIP is 35 cm H₂O and the plateau pressure is 25 cm H₂O. An hour later, the peak pressure is 65 cm H₂O, but the plateau pressure is still 25 cm H₂O. The patient isn't in danger of lung damage, because the reason for the high PIP is an increase in airway resistance. The patient may be biting on the endotracheal (ET) tube, or may need an inline bronchodilator treatment or suctioning. This is why PIP doesn't mean much by itself, and plateau pressure is the more important ventilator pressure to monitor.

Transairway pressure is the difference between the PIP and the plateau pressure, and typically is less than 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 that the patient is developing bronchoconstriction and may need an inline bronchodilator.

RESOURCES

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Malley WJ. Clinical Blood Gases. 2nd ed. W.B. Saunders Co.; 2005.

Oakes D, Shortall S. Ventilator Management: A Bedside Reference Guide. 2nd ed. Orono, ME: Respiratory Books; 2005.

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Wilkins RL, Stoller JK, Kacmarek RM. Egan's Fundamentals of Respiratory Care. 9th ed. Mosby, Inc.; 2009.