Adjusting Ventilator Settings


Ventilator settings are adjusted to (1) normalize blood gases (ventilation, oxygenation); (2) improve patient-ventilator synchrony/decrease respiratory distress; and/or (3) wean a patient from ventilator support. Here we focus primarily on normalizing blood gases, with an overview of ventilator adjustments to improve synchrony or alleviate respiratory distress.

Normalizing Blood Gases

A blood gas may indicate either abnormal ventilation, abnormal oxygenation, or both.

Adjusting Ventilation

Abnormal ventilation is indicated by an abnormal pH due to an abnormal PCO2. With a normal bicarbonate, restoring the PCO2 to normal will result in a normal pH. If the patient is in respiratory alkalosis (hyperventilation; high pH, low PCO2), you can restore a normal PCO2 by decreasing the minute volume. If the patient is in respiratory acidosis (hypoventilation; low pH, high PCO2), you can restore a normal PCO2 by increasing the minute volume. To estimate how much you should increase or decrease the minute volume, use the following formula:

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For example, if a mechanically ventilated patient with a minute volume of 6 L/min has a pH of 7.25 and a PCO2 of 60 mm Hg, you can restore a normal PCO2 by increasing the minute volume. The new minute volume would be 6.0 L x 60/40 = 9.0 L/min, with 40 mm Hg being the desired normal PCO2.

Exactly how you change the minute volume depends on the mode of ventilation being used. The table below indicates the best ways to increase or decrease minute volume for the common modes of ventilatory support.

How to Change the Minute Volume Depending on Ventilator Mode

MODE TO INCREASE VE TO DECREASE VE
VOLUME-TARGETED    
CMV-Control increase VT decrease rate
CMV-Assist/Control increase VT decrease rate 
add deadspace
SIMV increase rate 
add pressure support
decrease rate
PRESSURE-TARGETED    
PCV increase {short description of image}
increase rate
decrease 
decrease rate
PSV increase P decrease P
BiPAP® increase 
(IPAP-EPAP)
decrease 
(IPAP-EPAP)
APRV increase 
increase release frequency
decrease 
decrease release frequency

Adjusting Oxygenation

Whereas PCO2 varies mainly with one parameter (minute volume), blood oxygen levels can be affected by two - the FIO2 and PEEP levels. In general, the higher the FIO2 and PEEP level, the higher the PO2/SaO2.

If the PO2 is excessive (usually > 100 mm Hg), you should lower the parameter (FIO2 or PEEP) that is potentially most dangerous to the patient. For example, if a patient on 10 cm H2O PEEP and 75% oxygen has a PO2 of 150 mm Hg, the high FIO2 is of most concern (O2 toxicity) and should be lowered. On the other hand, if a patient on 18 cm H2O PEEP and 45% oxygen has a PO2 of 150 mm Hg, the high PEEP pressure is of most concern (barotrauma) and should be lowered.

If the PaO2 or SaO2 is low (< 60 mm Hg or < 90%), hypoxemia is present and either the FIO2 or the PEEP level should be increased. Which you choose to raise depends on the cause of the hypoxemia. If the problem is a simple V/Q imbalance (indicated by an PO2 > 60 mm Hg on a FIO2 < 0.6), increasing the FIO2 will probably due the job. If the problem, however is shunting (indicated by an PO2 < 60 mm Hg on a FIO2 > 0.6), simply raising the FIO2 won't help. Instead, PEEP must be added or increased. See the following Rule of Thumb:

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Use the "60/60 Rule" to determine the cause and treatment of hypoxemia. If the PO2 is > 60 mm Hg on a FIO2 < 0.6, the problem is mainly a V/Q imbalance that will respond to a simple increase in FIO2. If the PO2 is < 60 mm Hg on a FIO2 > 0.6, the problem is shunting and PEEP/CPAP must be added or increased. (A variation of this rule uses 50/50 as the standard)

ARDS Network Guidelines. Guidelines for setting the 'best' PEEP level when patients have hypoxemia due to shunting (as in ARDS) have been established by the Acute Respiratory Distress Syndrome Network. They recommend using one of the following specific combinations of FIO2 and PEEP to achieve a PaO2 of 55-80 mm Hg or an SpO2 88-95%

PEEP

FIO2

 

PEEP

FIO2

5

0.3

 

14

0.8

5

0.4

 

14

0.9

8

0.4

 

16

0.9

8

0.5

 

18

0.9

10

0.5

 

18

1.0

10

0.6

 

20

1.0

10

0.7

 

22

1.0

12

0.7

 

24

1.0

14

0.7

 

 

 

Using this table, if level of oxygenation is below the minimum target (PaO2 < 55 mm Hg or SpO2 < 88%), they recommend moving 'up' one step. Conversely, if level of oxygenation is above the targeted range (PaO2 > 80 mm Hg or SpO2 > 95%), they recommend moving 'down' 1 step.

Example: Adjusting Oxygenation Using ARDS Network

Problem: A patient on 10 cm H2O of PEEP with an FIO2 of 0.60 has a PaO2 of 50 and a SPO2 of 83%. Ehay do yuou recommend?

Solution: Since this level of oxygenation is below the ARDS Network recommended minimum target, you would consult the above table and move the patient up to the next higher level of support, i.e. from 10 cm H2O of PEEP with an FIO2 of 0.60 to 10 cm H2O of PEEP with an FIO2 of 0.70, and then repeat your assessment. If this change does not bring the patient above the minimum target levels, the next step up would be to raise the PEEP from 10 cm H2O to 12 cm H2O.

A more complex process used to determine 'best' PEEP requires access to cardiac output and mixed venous blood gas data (via a pulmonary artery or Swan-Ganz catheter). In this method (often called a 'PEEP study'), increasing levels of PEEP are applied while simultaneous measures of oxygenation, pulmonary mechanics and hemodynamics are made (see example below). Most criteria define the 'best' PEEP as the lowest pressure that yields satisfactory oxygen delivery/tissue oxygenation at a safe FIO2 with minimal cardiovascular compromise. In the example below, cardiac output and O2 delivery reach their maximums (4.5 L/min and 869 mL/min, respectively) at a PEEP level of 15 cm H2O. However, this level of PEEP results in what most would consider potentially dangerous peak and plateau pressures (51 and 48 cm H2O repectively). Comparable cardiac output (4.2 L/min), O2 delivery (811 mL/min) and venous PO2s (37 mm Hg) appear possible at lower PEEP, peak and plateau pressures, in this case 5 cm H2O PEEP, which probably represents the best starting point for this patient.

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Above table from: Pilbeam, SP. Mechanical Ventilation: Physiological and
Clinical Applications
. 3 ed. St. Louis: Mosby; 1998.

More recently, the best PEEP level has been associated with the lowest pressure needed to exceed the lower inflection point (LIP) on the lung's pressure-volume curve, i.e., the point at which the slope initially steepens and compliance rises (see figure below). Since this point, also called Pflex, conceptually represents maximum alveolar recruitment, maintaining a slightly higher PEEP pressure should keep alveoli open, while avoiding overdistention associated with higher pressures.


From Hicks, GH and Scanlan CL. Initiating and adjusting ventilatory support.
In Scanlan, CL, Wilkins, RL & Stoller, JK (Eds.). Egan’s Fundamentals of
Respiratory Care
(7th Ed.). St. Louis, MO: Mosby, 1999.

Unfortunately, to determine the best PEEP level using this method is rather complex, requiring generation of a static pressure-volume curve for the patient. Done manually, this normally involves sedation or paralysis of the patient and either incremental or slow inflation using a calibrated super syringe, while measuring static airway pressures. An alternative way to generate a static pressure-volume curve is the rapid end-inspiratory occlusion maneuver. This technique involves manually clamping the circuit between the Y piece and pressure/volume sensor at end-inspiration over a 10-step range of tidal volumes (usually rbetween 0.1 and 1.2 L). Because static pressure-volume curves are sometimes unobtainable using either of these methods, they are not commonly used in clinical practice. However, they should be understood by NBRC exam takers.

A more reliable and less risky alternative to determine best PEEP via pulmonary mechanics is to assess static compliance over a range of PEEP levels (with the VT or ΔP held constant). Based ot the results of this test, one selects the PEEP level at which the highest compliance is observed. For example, in the following case, one would select a PEEP level of 15 cm H2O, corresponding to the the highest observed static compliance (38 mL/cm H2O)..

PEEP
(cm H2O)
Static Compliance
(mL/cm H2O)
6 23
9 24
12 27
15 38
18 33
14 31
16 31

A similar approach takes advantaqge of current ventilator graphics software. When used during pressure-limited ventilation, it is referred to as the equal pressure method. As shown in the figure below, the equal pressure method involves measuring the volume exhaled after inflating the lung at the same distending pressure of 20 cmH2O during the course of the pressure support ventilation at different levels of PEEP (equivalent to BiPAP). At the end of each inflation, the patient is disconnected from the ventilator, and the exhaled volume (VE) is recorded. The VE obtained with this maneuver at each level of PEEP is then compared with the corresponding exhaled volume at ZEEP, with the difference being the volume recruited at that level of PEEP. In the example below, the volume recruited above ZEEP (a, b, c) increases progressively as the PEEP level is increased, with maximum recruitment (c) at 15 cm H2O.

Improving Patient-Ventilator Synchrony/Decreasing Respiratory Distress

In addition to adjusting ventilators to assure adequate ventilation and oxygenation, one may need to manipulate ventilator settings to improve patient-ventilator synchrony and/or decrease respiratory distress. In some cases, such as when starting noninvasive ventilation, alleviating respiratory distress may be the primary goal. The most common adjustments helpful in improving patient-ventilator synchrony and/or alleviating respiratory distress are summarized in the following table. In terms of decreasing respiratory distress, one should first eliminated patient-related problems as the cause before proceeding with ventilator adjustments.

Problem/Need Action
Inadequate FIO2
  • Titrate FIO2 to SpO2 of 90-92%
  • Check/confirm for adequate O2 delivery - Hb, cardiac output, etc
Trigger problems
Flow problems
  • If using flow-limited ventilation:
    • Increase inspiratory flow to eliminate post-trigger patient effort
    • Use ventilator that provides flow compensation
    • Switch to pressure-limited ventilation
  • If using pressure-limited ventilation: adjust rise time to provide good pressure plateau without spiking
  • If using PSV, adjust off-cycling to assure effort-free and complete exhalation
Rate problems
  • If using CMV, set rate to assure adequate expiratory time and proper I:E ratio
  • If using CMV, consider SIMV
  • If using SIMV, increase mandatory rate until spont rate is < 15-20/min
  • If using SIMV, increase pressure support level until spont rate is < 15-20/min
Tidal volume/
Pressure limit
  • Inspect pressure-volume curve for overdistention; lower P or V if 'beaking' apparent
Inadequate minute volume
  • Assure a minimum mandatory ventilation of at least 4-6 L/min for adults (if normal metabolism; higher as needed)
Mode
  • Give preference to pressure-limited modes and/or those that provide flow compensation during inspiration
auto-PEEP

Adapted from: Hicks, GH and Scanlan CL. Initiating and adjusting ventilatory support. In Scanlan, CL, Wilkins, RL & Stoller, JK (Eds.). Egan’s Fundamentals of Respiratory Care (7th Ed.). St. Louis, MO: Mosby, 1999.

FIO2/PEEP recommendations for ARDS patient from The Acute Respiratory Distress Syndrome Network. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med 342(18):1301-1308, 2000.

Equal pressure method description and graph from Ward NS, Lin DY, Nelson DL, et al. Successful determination of lower inflection point and maximal compliance in a population of patients with acute respiratory distress syndrome. Crit Care Med 30(5):963-8, 2002