Frequency (Rate) of Breathing (f)
Breathing is a cyclical activity consisting of inspirations and expirations repeated over time. The spontaneous rate or frequency of beathing (f) is considered a vital sign and normally measured by counting the number of breaths per minute. Normal rates of breathing vary by age as follows:
Normal Respiratory Rates
|1 year old||30||25-30|
|10 year old||16||15-20|
Inspiratory to Expiratory Time Ratio (I:E ratio)
More detailed assessment of the ventilatory time parameters includes measurement of the inspiratory to expiratory time ratio (I:E ratio). The I:E ratio is simple the ratio of inspiratory time (tI) to expiratory time (tE):
|For example, if a patient has an inspiratory time of 1.5 seconds and an expiratory time of 4.5 seconds, her I:E ratio is 1.5 to 4.5 or 1:3.|
In normal individuals, the I:E ratio ranges from 1:2 to 1:3 (expiration being about two to three times longer than inspiration). In patients with expiratory airflow obstruction (e.g., COPD patients), the expiratory time is typically prolonged. This results in lower I:E ratios, such as 1:4 or 1:5. A prolonged expiratory time and low I:E ratio is a cardinal sign of expiratory airflow obstruction.
Mechanical Ventilation Time Parameters
Total Cycle Time. During time-triggered/time-cycled modes of ventilator support (such as flow-limited CMV and pressure control ventilation), time parameters are set and manipulated by the clinician. The key parameter under clinician control in these modes is the total breath cycle time, or ttot. The total cycle time is computed as either (1) the simple sum of the inspiratory and expiratory times or (2) the frequency of beathing (f) divided into 60 (second/min):
For example, if a patient is receiving PCV with an inspiratory time of 2 seconds and an expiratory time of 3 seconds, his total cycle time is:
ttot = 2 + 3 = 5 seconds
Alternatively, if a patient is receiving flow-limited control mode CMV at a preset rate of 15 per minute, her total cycle time is:
ttot = 60/15 = 4 seconds
In control modes of ventilation, total cycle time determines the frequency of breathing. Specifically, the frequency of breathing equals 60 (sec/min) divided by the total cycle time (sec):
Using the above example, if a patient is receiving PCV with an inspiratory time of 2 seconds and an expiratory time of 3 seconds, his total cycle time is 5 seconds and his frequency of breathing is:
f = 60/5 = 12/min
Percent Inspiratory Time (%tI or 'Duty Cycle'). Another key time parameter used in mechanical ventilation is the percent inspiratory time (%tI) also called the 'duty cycle' (time during which the ventilatory is doing its 'duty' of delivering positive inspiratory pressure). To compute the %tI, one needs to know both the inspiratory time and total cycle time:
Using the above example again, if a patient is receiving PCV with an inspiratory time of 2 seconds and an expiratory time of 3 seconds, his percent inspiratory time:
%tI = 2/5 x 100 = 40%
Relationship of Percent Inspiratory Time to I:E ratio. Obviously, the %tI and I:E ratio are related to one another. In the above example, the %tI is 40%, while the I:E ratio is 2:3 (which reduces to 1:1.5). In both equations, the numerator (on top) is the inspiratory time). The difference lies in the denominators (bottom) of the equations. For the I:E ratio, the denominator is simply the expiratory time. For the %tI, the denominator is the total cycle time (sum of inspiratory and expiratory times). Below is a handy translation table for converting I:E ratios into %tI:
Note in the above table some I:E ratios greater than 1:1. Since these are the reverse of normal (inspiratory time exceeds inspiratory time), they are called reverse I:E ratios. Although controversial, reverse I:E ratios are sometimes used during pressure control ventilation to improve oxygenation and lower PEEP levels.
Relationship of Percent Inspiratory Time to Flow and Minute Ventilation. During flow-limited CMV, the delivered volume is a function of the inspiratory time and flow. To compute the required inspiratory flow for a patient receiving flow-limited CMV, you apply the following formula:
where is the patient's minute volume and %tI the percent inspiratory time.
A patient receiving flow-limited CMV at a rate of 15/min has a tidal volume of 600 mL. The doctor specifies an an I:E ratio of 1:4. What flow would you set to achieve these parameters?
First, compute the minute volume:
= 15 x 600 = 9.0 L/min
Next, compute/lookup the percent inspiratory time:
%tI = 20% (lookup table above)
Last, compute the inspiratory flow needed (be sure to use the decimal equivalnet for %tI)
= 9.0/0.20 = 45 L/min