Author Dr Shazia Khan1, Dr Mahismita Patro2, Dr Dipti Gothi3*
1- Senior resident, 2- Assistant Professor, 3- Professor; Department of Pulmonary and critical care medicine, ESI PGIMSR, New Delhi, India *Corresponding author, mail id- diptigothi@gmail.com
Case
A 55 year old man with chronic obstructive pulmonary disease (COPD)
was admitted for an infective exacerbation with hypercapnic respiratory
failure. He was initially managed with non-invasive ventilatory support.
He had to be intubated after one hour due to lack of improvement. The
patient was put on invasive mechanical ventilation in the Assist
Control-Volume Control. On the 5 th day of mechanical
ventilation, the patient improved gradually and was planned for
weaning. But there were some sudden changes in the ventilator graphs
due to which the weaning criteria were not satisfied (Video 1). The
ventilator settings at that time were Assist Control-Volume Control
mode; FIO2 24%; TV 320 ml, Set Rate 18, PEEP 6cm H20, decelerating flow
waveform at 60 litres/minute. The volume-time (VT) scalar, flow-time
(FT) scalar, pressure-volume (PV) loop and flow-volume (FV) loop are
shown in video 1.
Video 1: Ventilator waveform changes observed in our patient
Question
What do the ventilator waveforms indicate?
A. Auto PEEP B. Double triggering C. Leakage in the ventilator circuit D. Secretions in the ventilator circuit
Answer
C. Leakage in the ventilator circuit.
Discussion
Air leak in mechanically ventilated is a major problem in clinical
practice contributing to the ineffectiveness of assisted ventilation.
The leakage can be from any part of the circuit. The identification of a
leak from the ventilator circuit requires constant monitoring of the
ventilatory parameters and graphs. The expiratory tidal volume (Vte)
becomes less than the inspiratory tidal volume (Vti) as a consequence of
air leak. In our patient the Vti was 320ml and the Vte had suddenly
dropped to 190 ml raising suspicion for an air leak. The volume-time
(VT) scalar showed that the expiratory limb was not returning to
baseline (video 1).
AutoPEEP also shows changes similar to air leak in the VT scalar and
hence can be confused with air leak. AutoPEEP, also known as intrinsic
PEEP occurs due to air trapping and over-distension. This is usually due
to an insufficient expiratory time. The identification and
differentiation of auto-PEEP and air leak is important as both interfere
with weaning from mechanical ventilation and their management is
different. The changes in various ventilator graphs in the presence of
leak and auto PEEP have been compared in figure 1.
Figure 1: Ventilator waveform patterns in assist control/volume control mode in decelerating ramp in presence of air leak, autoPEEP, airway secretions and double triggering.
In the VT scalar the expiratory limb does not return to baseline (red
coloured double arrow) in both air leak and auto PEEP. The
differentiation of air leak and auto PEEP on FT scalar requires minute
observation. In presence of air leak on the FT scalar, the peak
expiratory flow rate is reduced, thus diminishing the overall size of
the expiratory graph. In presence of auto PEEP, the expiratory flow does
not return to baseline on the FT scalar (red arrow). The FV loop is
ideal for differentiating the air leak from auto PEEP. Air leak is
characterised by the lack of return of the expiratory limb of FV loop to
baseline along the volume axis (i.e., the x-axis). But in auto PEEP,
the expiratory limb does not return to baseline along the flow axis
(i.e., the y-axis) and another breath is started. In the PV loop, the
presence of air leak can be detected when the expiratory limb does not
return to baseline. The auto PEEP in PV loop can be identified from the
shift of the loop to right on the x axis (answer choice A is incorrect).
The changes in the ventilator graphs associated with double
triggering and secretions in the circuit have also been shown in figure
1. Double triggering is characterised by two inspirations occurring in
succession in a single respiratory cycle (answer choice B is incorrect)
whereas the presence of airway secretions can cause a saw-tooth pattern
in the expiratory limb of the flow-time curve (answer choice D is
incorrect).
In our patient, when the ventilator waveforms suggested presence of
air leak, the whole circuit was checked. The leak was due to a crack in
the water trap of the inspiratory limb of the circuit (figure 2).
Figure 2: Crack in the water trap of the inspiratory limb of the ventilator circuit.
After the inspiratory circuit was replaced with a new one, there was
no further leak. The patient was successfully weaned off ventilatory
support and was extubated the next day.
