Chapter 9: Flow Waveforms and I:E Ratios

Microprocessor-controlled ventilators allow the clinician to choose among various inspiratory flow waveforms. This chapter describes the technical and physiologic aspects of various inspiratory waveforms during mechanical ventilation.

An important principle for understanding pulmonary mechanics during mechanical ventilation is that of the time constant. The time constant determines the rate of change in the volume of a lung unit that is passively inflated or deflated. It is expressed by the relationship:

where Vt is the volume of a lung unit at time t, Vi is the initial volume of the lung unit, e is the base of the natural logarithm, and τ is the time constant. The relationship between Vt and τ is illustrated in Figure 9-1. Note that the volume change is nearly complete in five time constants.

For respiratory physiology, τ is the product of resistance and compliance. Lung units with a higher resistance and/or a higher compliance will have a longer time constant and require more time to fill and to empty. Conversely, lung units with a lower resistance and/or compliance will have a shorter time constant and thus require less time to fill and to empty. A simple method to measure the expiratory time constant is to divide the expired tidal volume by the peak expiratory flow during passive positive pressure ventilation:

where V_T is the expired tidal volume and V̇_e(peak) is the peak expiratory flow. Although this is a useful index of the global expiratory time constant, it treats the lung as a single compartment and thus does not account for time constant heterogeneity in the lungs.

Volume-Controlled Ventilation

The flow, pressure, and volume waveforms produced with a constant flow pattern are shown in Figure 9-2. This is often called square-wave or rectangular-wave ventilation due to the shape of the flow waveform. With the constant flow pattern, the volume (per unit time) is delivered into the lungs equally throughout inspiration. In other words, volume (per unit time) delivery into the lungs at the beginning of inspiration is the same as that at the end of inspiration. Note that airway pressure increases linearly throughout inspiration, following an initial rapid pressure increase due to the resistance through the endotracheal tube. The effect of resistance and compliance on the airway pressure waveform during constant flow volume ventilation is shown in Figure 9-3.

During volume-controlled ventilation, the inspiratory flow can also be set to a descending ramp. With this pattern, flow is greatest at the beginning of inspiration and decreases to a lower flow at the end of inspiration. Typical flow, pressure, and volume waveforms with a descending ramp are shown in Figure 9-4. Note that most of the tidal volume is delivered early during inspiration and the pressure waveform approaches that of a rectangular shape. A descending ramp flow pattern lengthens the inspiratory time unless the peak flow is increased. The descending ramp flow can be provided in several ways (Figure 9-5). With a complete ramp, flow decreases to 0 at end inspiration. With 50% ramp, the flow at end inspiration is half of the initial flow. Flow can also taper to a fixed, manufacturer-specific level at the end of inspiration (eg, 5 L/min).

Pressure-Controlled Ventilation

Typical pressure, flow, and volume waveforms during pressure ventilation are shown in Figure 9-6. Note the shape of the pressure waveform and descending (exponential) flow pattern. Also note that most of the tidal volume is delivered early in the inspiratory phase. During pressure-controlled ventilation, flow is determined by the pressure applied to the airway, inspiratory effort, airways resistance, and the time constant (Figure 9-7):

where ΔP is the transpulmonary pressure (difference between airway pressure and pleural pressure), R is airways resistance, t is the elapsed time after initiation of the inspiratory phase, e is the base of the natural logarithm, and τ is the product of airways resistance and respiratory system compliance (the time constant of the respiratory system). The length of zero flow time at the end of inspiration is determined by the inspiratory time; a longer inspiratory time results in more zero flow time.

With many ventilators, it is possible to adjust the time required to reach the peak inspiratory pressure (rise time). The rise time controls the flow at the beginning of the inspiratory phase (Figure 9-8). With a faster rise time, flow is greater at the beginning of inspiration, which may improve synchrony in patients with a high respiratory drive, but at the cost of a higher V_T.

An inverse ratio can be used in conjunction with pressure-controlled ventilation. This results in pressure-controlled inverse ratio ventilation (PCIRV). This mode has been used in the setting of refractory hypoxemia. Its physiologic effect is to increase mean airway pressure and it is commonly associated with auto-positive end-expiratory pressure (auto-PEEP). The clinical results are determined by the flow pattern, rather than the use of volume-controlled or pressure-controlled ventilation strategies per se. A descending ramp flow pattern with an inspiratory pause can be produced using either pressure-controlled or volume-controlled ventilation. There does not seem to be a benefit from the use of PCIRV on patient outcome.

Despite some clinicians favoring pressure-controlled ventilation in patients with acute respiratory distress syndrome (ARDS), evidence supporting its superiority in this setting is lacking. For the same V_T, the same inspiratory time, and a descending ramp of flow with volume-controlled ventilation, the differences in Pao₂ between pressure-controlled and volume-controlled ventilation are trivial.

Pressure Support Ventilation

The typical waveform for pressure support ventilation (PSV) is shown in Figure 9-9. When the pressure-supported breath is triggered, the ventilator delivers flow sufficient to reach the set pressure (typically, the pressure support is the amount of pressure added to the PEEP). As with pressure-controlled ventilation, all current generation ICU ventilators allow the initial flow (rise time) to be adjusted with PSV, which controls how quickly the pressure reaches the set target.

The pressure-supported breath should terminate when the patient's inspiratory effort ceases. Premature termination may result in double triggering and a prolonged inspiration may result in the patient activating expiratory muscles in an attempt to end the inspiratory phase. The inspiratory phase stops when the flow decreases to a ventilator-determined level (eg, usually 25% of peak flow). To improve synchrony, many current generation ventilators allow adjustment of the flow at which the ventilator cycles to the expiratory phase (Figure 9-10). To avoid unintentional termination or prolongation of inspiration, redundant systems are used to terminate inspiration. These are typically time or pressure-based. Inspiration is terminated if the time or pressure criteria are met before the termination flow criteria. These redundant features are particularly important if a leak is present or if the patient's respiratory mechanics result in a short or long inspiratory phase. One commercially available ventilator uses measures of lung mechanics and assessment of the airway pressure waveform to automatically determine cycle off criteria using closed-loop feedback control.

Proportional-Assist Ventilation, Neurally Adjusted Ventilatory Assist, and Airway-Pressure Release Ventilation

These modes are each pressure-targeted. As such, flow delivery into the lungs is similar to that for pressure control and pressure support ventilation. For proportional-assist ventilation, the inspiratory phase is flow-triggered and flow-cycled. For neurally adjusted ventilator assist, the inspiratory phase is triggered and cycled by the electrical activity of the diaphragm. For airway pressure release ventilation, the time at high pressure is triggered and cycled by the ventilator; the patient's spontaneous breaths are flow-triggered and flow-cycled.

Expiratory flow is normally passive (ie, it does not require expiratory muscle activation) and determined by alveolar driving pressure (Palv), airways resistance, the elapsed expiratory time, and the time constant of the respiratory system:

By convention, expiratory flow is displayed on the flow-time graphic in the direction negative and inspiratory flow is displayed in the positive direction. Also note that end-expiratory flow is present if Palv is greater than the proximal airway pressure. This indicates air-trapping (intrinsic PEEP; auto-PEEP).

The clinical usefulness of inspiratory flow waveform manipulations is controversial. Evidence is lacking that manipulation of the flow waveform affects patient outcome. The choice of waveform is usually one of clinician bias, rather than the desire to achieve a specific therapeutic goal. The following generalizations can be made regarding inspiratory waveform manipulations.

• Mean airway pressure is higher with descending ramp of flow and lower with constant flow.

• Peak inspiratory pressure is lower with descending ramp flow and higher with constant flow.

• For the same respiratory mechanics and tidal volume, peak alveolar pressure (plateau pressure) will be the same regardless of the inspiratory flow waveform.

• Gas distribution is improved with descending ramp flow. This often improves gas exchange, but only to a small degree.

• Mean airway pressure is increased with an end-inspiratory pause. An end-inspiratory pause may improve distribution of ventilation. During the period of no flow, gas from areas of the lungs with low airway resistance (short time constants) may be redistributed to areas of the lungs with high airway resistance (long time constants); this is called pendelluft.

• Asynchrony may be affected by the inspiratory flow waveform. Some patients are more asynchronous on one flow waveform compared to another, but inter-patient differences do not allow one waveform to be recommended over another as always associated with better synchrony.

When the inspiratory flow pattern is changed from constant flow to descending ramp flow during volume-controlled ventilation, the ventilator must adjust either the peak flow or inspiratory time to maintain the delivered tidal volume (Figure 9-11). If peak flow is adjusted, then inspiratory time and I:E ratio are constant. For a descending ramp flow pattern, the peak flow (V̇_pk) is determined by the inspiratory time (T_I) and the flow at the end of inspiration (V̇_f):

For example, if V_T is 0.75 L, T_I is 1.5 seconds, and V̇_f is 5 L/min (0.083 L/s), then V̇_pk will need to be 55 L/min (0.92 L/s).

If inspiratory time is adjusted when the flow pattern is changed, then the peak flow set on the ventilator is maintained at the initial flow. For a descending ramp flow pattern, the increase in inspiratory time will be determined by the peak flow and the flow at the end of inspiration:

For example, if V_T is 0.75 L, V̇_pk is 90 L/min (1.5 L/s), and V̇_f is 5 L/min (0.083 L/s), then T_i will be 0.95 second. For full deceleration (ie, V̇_f = 0), either the inspiratory time or the peak flow must double.

The sigh breath is a deliberate increase in tidal volume for one or more breaths at regular intervals. Sighs during mechanical ventilation were commonly used in the 1970s, but have been rarely used since. There is again interest in the use of sighs during mechanical ventilation as a recruitment maneuver, but the value of this is yet to be determined. One way of producing a sigh during pressure support ventilation is to set an inspiratory pressure of 20 to 30 cm H₂O, for 1 to 3 seconds, 2 to 4 times/min, using a ventilator with an active exhalation valve (Figure 9-12).

The relationship between inspiratory time and expiratory time (I:E ratio) is an important consideration during mechanical ventilation. A longer inspiratory time (and a shorter expiratory time) increases the mean airway pressure, which may increase arterial oxygenation but may also decrease cardiac output. A longer inspiratory time may also result in air trapping (auto-PEEP), particularly if inspiratory time is greater than the expiratory time (inverse I:E ratio). Normally, expiratory time is longer than inspiratory time. An inspiratory time of 0.5 to 1.5 second is usually appropriate during adult mechanical ventilation. A shorter inspiratory time is desirable for patients who are ventilated with a rapid respiratory rate. In patients who are triggering the ventilator, the inspiratory time should be set to match the patient's neural inspiratory time to avoid asynchrony.

Several approaches can be used by ventilators to set the I:E ratio.

• I:E ratio and rate. For example, at an I:E ratio of 1:3 and a rate of 15/min (cycle time of 4 seconds), the inspiratory time is 1 second and the expiratory time is 3 seconds.

• Flow, tidal volume, and rate. For example, suppose there is a constant inspiratory flow of 30 L/min, tidal volume of 0.35 L, and rate of 12/min. The inspiratory time is therefore 0.7 second, the expiratory time is 4.3 seconds, and the I:E ratio is 1:6.

• Inspiratory time and respiratory rate. With an inspiratory time of 1 second and a rate of 10/min, the expiratory time is 5 seconds and the I:E ratio is 1:5. Note that some ventilators adjust inspiratory time and flow independently during volume-controlled ventilation. If the volume is delivered before the set inspiratory time is reached, the additional time is an end-inspiratory pause. For example, if the flow is set at 60 L/min constant flow, the tidal volume at 500 mL, and inspiratory time is set at 1 second, there will be 0.5 second inspiratory hold after the tidal volume is delivered.

• Percent inspiratory time and rate. At a rate of 15/min and 25% inspiratory time, the inspiratory time is 1 second (25% of the total cycle time of 4 seconds), the expiratory time is 3 seconds, and the I:E ratio is 1:3.