PEEP* / CPAP, IMV, and PSV: Fundamentals
PEEP* / CPAP, IMV, and PSV: Fundamentals
Patients with acute respiratory failure characteristically manifest a low PaO2 that responds only minimally to an increase in FIO2. A young otherwise healthy patient with Acute Respiratory Distress Syndrome (ARDS) and breathing room air may have PaO2 of 50-55 mm Hg that increases to only 75 mm Hg after an increase in FIO2 to 0.7 for example. This response is considerably less than expected (PaO2 should be approximately 400 mm Hg while breathing an FIO2 of 0.7). Patients with ARDS present with variety of alveolar ventilation-to-perfusion [pic]abnormalities (Fig. 1). Also, lung compliance is decreased significantly, i.e., the lungs are stiff, requiring increased pressure for inflation.
Loss of lung volume
Loss of lung volume is a major hallmark of ARDS primarily as a result of increased extravascular lung water (EVLW) (Fig. 2). Increased EVLW results from a heightened fluid (and albumin) flux across the leaky pulmonary capillaries. The increased EVLW compromises the patency of small airways, especially in dependent lung regions, predisposing to atelectasis. If fluid continues to move unabated into the pulmonary interstitium, fluid "spills over" into the alveoli, disrupting surfactant and leading to peripheral widespread atelectasis and a further reduction of functional residual capacity (FRC). As a result, areas of low [pic]present, right-to-left intrapulmonary shunting of blood increases, and PaO2 decreases.
• Although PEEP is technically different from CPAP, the terms PEEP and CPAP are/will be used to mean the same thing.
VENTILATOR / PERFUSION CONDITIONS [pic]
Figure 1. Under normal [pic]conditions (center), for every unit of alveolar ventilation [pic]there is a unit of perfusion ([pic]). In the center there are 5 units of [pic]and 5 units of [pic], i.e., [pic]= 1. Areas of low [pic] (left) are absolute (zero [pic]as in atelectasis) and relative (decreased [pic]) intrapulmonary shunts. Areas of high [pic] (right) have increased [pic] relative to [pic]and may have zero [pic]. Ventilated, unperfused alveoli are termed alveolar deadspace volume.
Figure 2. Increased Extravascular lung water (EVLW) in acute respiratory distress syndrome (ARDS) compromises the patency of small airways and thus, predisposes the patient to atelectasis, loss of lung volume, increased absolute and relative intrapulmonary shunting, and
( PaO2.
Positive End Expiration Pressure (PEEP) / Continuous Positive Airway Pressure (CPAP) - Restore lung volume to improve PaO2
PEEP / CPAP restores FRC toward normal by re-expanding collapsed alveoli and converting areas of low [pic](relative and absolute shunting) to more normal gas exchange areas. Thus, PEEP / CPAP corrects a major mechanical abnormality and restores the lung to a more physiologic state (Figs. 3 and 4). In clinical practice the level practice the level of PEEP / CPAP is titrated by assessing intrapulmonary shunt and PaO2. PEEP / CPAP is titrated in increments of 2 to 5 cm H2O to minimize intrapulmonary shunting of blood and improve arterial oxygenation.
Ventilation with PEEP /CPAP allows the patient to inhale a tidal volume spontaneously with subsequent exhalation to a positive pressure plateau. Airway pressure is positive (pressure greater than atmospheric) during both inhalation and exhalation. CPAP pressure "splints" the lung toward a position of normal function. Early return of the lung to a more physiologic state facilitates gas exchange and promotes healing of the lung.
The particular level of PEEP / CPAP to be employed is selected at the end of exhalation. It is incorrect to use inspiratory or mean airway pressure values. Because this form of positive pressure therapy is employed to restore expiratory lung volume (FRC), inspiratory and mean airway pressure values as reflections of a particular level of CPAP are misleading (these values do not reflect the total alveolar distending pressure and end-exhalation).
Intermittent Mandatory Ventilation (IMV) - Normalize alveolar ventilation and PaCO2
Originally proposed by Dr. Robert Kirby as a method to ventilate infants with hyaline membrane disease (HMD) and by Dr. John Downs for use with adults, IMV enables the patient to breath spontaneously with mechanical inflations provided at preselected intervals (Fig. 5). The mechanical ventilatory rate (IMV rate) cannot be influenced by the patient. Between sequential mechanical breaths, an unrestricted flow of gas on spontaneous inspiratory demand is provided.
Decreased lung volume is a salient characteristic of patients with Acute Respiratory Distress Syndrome (ARDS
Fig. 3 Both spontaneous inhalation and exhalation occur above ambient pressure during CPAP.
* Positive pressure is defined as any pressure greater than ambient or atmospheric pressure.
Figure 4. In A, under normal conditions functional residual capacity (FRC) is maintained at end-exhalation. In B, during ARDS the FRC decreases below normal predisposing to alveolar collapse. In C, CPAP/PEEP as a treatment for ARDS © restores the FRC and normalizes alveolar volume.
Figure 5. (A) Intermittent mandatory ventilation (IMV) airway pressure patterns. With IMV, the patient is allowed to breathe spontaneously as desired; however, at set intervals, a mechanical inflation is provided (the ventilator rate is the IMV rate). IMV tidal volume is set and cannot be affected by the patient. (B) With IMV and continuous positive airway pressure (CPAP)/positive end-expiratory pressure (PEEP, the patient breathes spontaneously with CPAP/PEEP between IMV breaths.
influenced by the patient. Between sequential mechanical breaths, an unrestricted flow of gas on spontaneous inspiratory demand is provided.
IMV was introduced initially as a method to wean patients from mechanical ventilation by permitting a smooth transition from mechanical ventilation to spontaneous breathing, as the rate of ventilator cycling was gradually decreased and spontaneous effort increased. As clinical experience has been accumulated, IMV has evolved as a primary ventilatory technique.
The IMV rate should be titrated to deliver only that support which, inc conjunction with spontaneous breathing, maintains normal alveolar ventilation and PaCO2 (Table 1). When used in patients with antecedent pulmonary disease (emphysema, chronic bronchitis), IMV is extremely useful in regulating PaCO2 and pHa.
Table 1. Criteria to adjust IMV rate
• pHa: 7.35 - 7.45
• PaCO2: 35-45 mm Hg (previously eucapnic patient)
50 - 55 mm Hg (patient with chronic obstructive pulmonary disease)
• Spontaneous breathing rate < 30 breaths/min
e.g., Consider a previously eucapnic patient receiving IMV at a rate of 8 mandatory breaths/min and breathing spontaneously at 15 breaths/min with the following arterial blood gas data: pHa = 7.52, PaCO2 = 30 mm Hg, PaO2 = 105 mm Hg, [pic] 24 mEq/L, FIO2 = 0.30. Respiratory alkalemia is the diagnosis, therefore, the IMV rate should be reduced to 4 or 6 per minute in order to normalize PaCO2.
Pressure Support Ventilation (PSV) - Unload the respiratory muscles and decrease work of breathing
Originally described by Dr. Neil MacIntyre, PSV operates in conjunction with demand-flow valve ventilator systems. The airway pressure, flow, and lung volume changes during PSV are more akin to assisted mechanical ventilation than to breathing spontaneously with PEEP/CPAP. Work of breathing is decreased by PSV; however, the technique is based on an entirely different concept than PEEP/CPAP. In the PSV mode, the ventilator is patient-triggered "ON" and continues in the inhalation phase to a preselected positive-pressure limit. As long as the patient's inspiratory effort is maintained, the preselected airway pressure stays constant, with a variable flow rate of gas from the ventilator. Inhalation cycles "OFF" when the patient's inspiratory flow demand decreases to a predetermined percentage of the initial peak mechanical inspiratory flow rate (Fig. 6) The ventilator thus is flow-cycled "OFF," following which passive exhalation occurs. With PSV, the peak inspiratory flows rate, flow waveform, inspiratory time, tidal volume, mean airway pressure, and airway pressure contour depend on the patient's breathing pattern. Tidal volume is determined by patient effort, compliance and resistance of the respiratory system, and the level of PSV
Two approaches to using PSV have been described. Low-level PSV (5-25 cm H2O) to assist spontaneous breathing decrease patient work of breathing by partially unloading the respiratory muscles; and second, high-level PSV (25-50 cm H2O) to decrease work of breathing to zero, totally unloading the respiratory muscles.
Partial unloading of the respiratory muscles results when, after initial decreases in intrapleural pressure at the onset of inhalation, the ventilator is patient-triggered "ON" and airway pressure rises abruptly to the preselected positive-pressure level. (Integration of airway pressure and tidal volume represent work performed by the ventilator to inflate the respiratory
Figure 6. Pressure support ventilation (PSV). The ventilator is patient-triggered "ON" and, as long as inspiratory effort is maintained, airway pressure stays constant with a variable flow rate of gas from the ventilator. The ventilator cycles "OFF" when the patient's inspiratory flow rate demand decreases to a predetermined percentage of the peak inspiratory flow rate (i.e., the ventilator is flow-cycled "OFF"). In the PSV mode, the airway pressure level is preselected while the patient retains control over inspiratory time (TI) and flow rate, tidal and minute volume, and ventilator rate. On the right, the patient increases inspiratory effort and generates a long inspiratory time compared with the PSV breath on the left. Because tidal volume equals inspiratory time times inspiratory flow rate, a greater tidal volume is delivered than the previous breath. Even though peak inflation pressure is constant from breath to breathe, tidal volume is thus variable with patient effort.
system - lungs and chest wall.) A further decrease in intrapleural pressure is indicative of work performed by the patient, who thus interacts with the positive-pressure breath to regulate tidal volume. (Integration of intrapleural pressure to tidal volume represent work performed by the patient to inflate the respiratory system.) Total work, thus, is performed in part by the ventilator in part by the patient, i.e., work is shared between the ventilator and patient, a work-sharing effect. The patient's portion of the workload should be in a tolerable, non-fatiguing range to promote conditioning of the respiratory muscles. Ideally, patient work of breathing should be measured when applying PSV to determine appropriate workloads (normal work of breathing for adults ( 0.3 to 0.7 Joules/L). In clinical practice, the spontaneous breathing pattern is used to infer patient work of breathing. For adults, a spontaneous breathing frequency of approximately 15-25 breaths per minute and a tidal volume of 6 to 8 mL per kg ideal body weight, in the absence of accessory muscle use, is inferred to mean appropriate patient work of breathing during PSV. However, some investigators have questioned the accuracy of predicting patient work of breathing based on the breathing pattern.
Alternatively, for patients with fatigued respiratory muscles (muscles failing as force generators), the level of PSV may be set high enough to totally unload the respiratory muscles, to provide essentially all the work of breathing, and to allow the patient to rest. This approach promotes respiratory muscle recovery. A negligible amount of work is performed by the patient to trigger the ventilator "ON." PSV levels < 40 cm H2O have been used in some patients with large minute ventilation demands and severely impaired pulmonary mechanics. Following an appropriate period (about 24 hours), the level of PSV may be decreased to partially unload the respiratory muscles to a non-fatiguing, tolerable work range.
PEEP, IMV, and PSV - Combined ventilatory therapy
In clinical practice, the aforementioned ventilatory support modes are combined to treat three basic problems of patients with ARDS (Fig. 7):
1) Decreased oxygenation secondary to decreased lung volume (decreased [pic])
2) Compromised carbon dioxide elimination due to inappropriate minute ventilation
3) Loaded respiratory muscles resulting from decreased respiratory system compliance and increased total resistance (respiratory system and imposed breathing apparatus resistances)
Figure 7:
Pressure Support Ventilation (PSV): Used to unload the respiratory muscles and decrease the work of breathing
Positive End Expiratory Pressure (PEEP): Used to restore functional residual capacity (FRC) or lung volume, improve alveolar ventilation to perfusion matching, and thus, PaO2.
Intermittent Mandatory Ventilation (IMV): Used to normalize alveolar minute ventilation and thus, PaCO2.
In the above example, the patient is receiving PEEP 10 cm H2O, PSV 15 cm H2O, and IMV 6 per minute (the ventilator rate is the IMV rate). The PSV breaths, 12 per minute in this example, are spontaneous pressure assisted breaths. Spontaneous inspiratory time is longer for the second PSV breath. The inflation Pressure (PIP) generated for the IMV breath, 40 cm H2O in this example, varies according to the following equation:
Dr. Banner
Anesthesiology
ARDS Simulations
I. Positive End Expiratory Pressure (PEEP) is used for restoring lung volume, thus correcting areas of low [pic] resulting in increased PaO2
|PEEP (cm H2O) |O |10 |15 |20 |
|PaO2 (mm Hg) | | | | |
|PaCO2 (mm Hg) | | | | |
|FIO2 | | | | |
|PAO2 (mm Hg) | | | | |
|Pa - AO2 (mm Hg)* | | | | |
|IMV (per min) | | | | |
|VT (mL)† | | | | |
|PIP / Pplt (cm H2O) | | | | |
|CRS (mL / cm H2O) | | | | |
|[pic] (mm Hg) | | | | |
|PCWP (mm Hg) | | | | |
|C.O. (L / min) | | | | |
Symbols
PAO2: Alveolar PO2 = (FIO2 x 713) - [pic], VT: Tidal volume, PIP: Peak inflation pressure, Pplt: Plateau pressure, CRS: Compliance of Respiratory System = [pic], [pic]: Mean arterial
blood pressure, PCWP: Pulmonary capillary wedge pressure, CO: Cardiac output
†Weight 100 kg (VT = 10 mL/kg)
Dr. M. J. Banner
Anesthesiology
Right-to-left Intrapulmonary Shunt (Qs / Qt) Calculation
CcO2 - CaO2
Qs / Qt = -----------------------
CcO2 - CvO2
Where: Qs is the amount of mixed venous blood directed from the right side of
the heart through atelectatic areas of the lungs (areas of low VA / Q)
and then to the left side of the heart that is not appropriately
oxygenated (shunt flow)
Qt is the cardiac output
CcO2 is the end pulmonary capillary oxygen content
CaO2 is the arterial oxygen content
CvO2 is the mixed venous oxygen content
For example, consider the following conditions, an adult with ARDS is attached to a life-support ventilator, [Hgb] = 9 g /dl and the alveolar partial pressure of oxygen
(PAO2) = 300 mm Hg (FIO2 = 0.50).
Blood gas data: PaO2 = 65 mm Hg SaO2 = 91 %
PvO2 = 30 mm Hg SvO2 = 70 %
Fill in the blanks and calculate Qs / Qt
CcO2 = ( [Hgb] X 1.34) + (0.003 X PAO2) = _____ ml / 100 ml blood
CaO2 = ( [Hgb] X 1.34 X SaO2) + (0.003 X PaO2) = _____ ml / 100 ml blood
CvO2 = ( [Hgb] X 1.34 X SvO2) + (0.003 X PvO2) = _____ ml / 100 ml blood
-
Qs / Qt = -------------------------- = --------- = ______ %
-
(Clinically acceptable Qs/Qt is approximately 15%.)
Thus, if the cardiac output is 4 L / min, then 1.6 L / min (4 L / min X 0.40) is the shunt flow directed from the right to the left side of the heart.
Dr. Banner
Anesthesiology
II. Intermittent Mandatory Ventilation (IMV) is used for maintaining appropriate [pic] and thus, normalizing PaCO2.
| |APNEA |Spontaneously breathing |
|IMV (per min) |0 |12 |8 |8 |4 |
|Spon. f (per min) | | | | | |
|Spontaneous VT (L) | | | | | |
|IMV VT (L) | | | | | |
|Total [pic] (L/min) | | | | | |
|PaCO2 (mm Hg) | | | | | |
|PaO2 (mm Hg) | | | | | |
|PIP (cm H2O) | | | | | |
* Weight 100 kg (IMV VT = 10 mL/kg)
Symbols
f: breathing frequency, [pic]: Minute exhaled ventilation
Dr. Banner
Anesthesiology
III. Pressure Support Ventilation (PSV) is used to unload the respiratory muscles and decrease the work of breathing to appropriate, tolerable levels.
| | |PSV (cm H2O) |0 |10 |20 |
|* Parameters used to |f (per min) | | | |
|INFER | | | | |
|work of | | | | |
|breathing | | | | |
| |VT (mL)+ | | | |
| |Accessory Muscle Use (yes / | | | |
| |no) | | | |
Clinically acceptable range for f is 15 - 25 per minute, VT is ( 6 - 8 mL / kg body weight (Adults).
+ Body weight in this example is 90 kg.
* Normal work of breathing for adults is 0.3 - 0.6 Joule / L. Work of breathing is measured by integrating the change in esophageal pressure (Pes) and tidal
volume (VT) during spontaneous inhalation. (Work of breathing = ( Pes d VT)
-----------------------
[pic]
[pic]
AIRWAY PRESSURE
(cm H2O)
CONTINUOUS POSITIVE*
AIRWAY PRESSURE (CPAP)
AMBIENT PRESSURE
BASELINE PRESSURE INCREASED TO RESTORE LUNG VOLUME
SPONTANEOUS INHALATION
[pic]
[pic]
[pic]
[pic]
[pic]
* Note: The Pa-AO2 is an inference of intrapulmonary right-to-left shunt. The higher Pa-AO2, the greater the intrapulmonary shunt, and vice versa.
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