Some ventilators allow the setting of either a fixed T I and rate, or a fixed I:E. If the RT sets a rate of 10 breaths/min, the TCT is 6 seconds (60 sec/10 breaths= 6 seconds/breath).

 

Suppose T I % is set at 33% or 1/3 of the total breath, then the T I will equal 33% of 6 seconds or 2 seconds (0.33 x 6 sec = 2 sec). T E will be TCT - T I = 6 sec - 2 sec = 4 sec. I:E ratio = T I / T E = 2 sec/4 sec = 2:4 or 1 :2.

 

Usually the I:E ratio is expressed so that the T I is equal to 1. For example, if the l:E ratio is 2:3, then it is expressed as 1:1.5. Dividing the numerator and denominator by T I reduces the expression to 1:X.

 

I:E ratios of 2: 1 or 3: 1 are called inverse I: E ratios. When l:E ratios are inverse (I> E), then T E takes on the value of 1. For example, if the T I is 3 sec and the T E is 2 sec, then the I:E ratio is 3:2 or 1.5:1. With inverse ratio, where T I equals or exceeds T E , complications are likely to result, and mean airway pressure (Paw) can increase significantly. Some rare situations occur when an inverse I:E ratio is used to improve oxygenation. These situations are restricted to very specific conditions, such as problems with oxygenation in patients with ARDS. In other settings, inverse ratio ventilation (IRV) can lead to increase in physiological dead space, decrease in venous return to the thorax, decreased cardiac output, and increase in air trapping (auto-PEEP). For this reason, I:E ratios are usually set at 1:1.5 to 1:4, so that expiration is longer than inspiration and the adverse effects of positive pressure are reduced.