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21th September, 2011

Inspiratory Pause

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Doctor DarwinDuration of inspiratory pause is set in the range of 15—25% of an inspiration time (5-15% of respiratory cycle time) or 0,3-0,4 seconds. Long inspiratory pause (more than 0,5 seconds) can influence Pmean (and, hence, intrathoracic pressure)
Duration of inspiratory pause is set in the range of 15—25% of an inspiration time (5-15% of respiratory cycle time) or 0,3-0,4 seconds. Long inspiratory pause (more than 0,5 seconds) can influence Pmean (and, hence, intrathoracic pressure)

It is standard that volume controlled ventilation is always carried out with an inspiratory pause. Upon termination of preset volume delivering the flow from the ventilator interrupts, but the expiratory valve remains to be still occluded. Thus lungs remain in a bloated state, and in their various segments a gradual equalization of pressures occurs. The coming inspiratory pause can be considered as a phase of additional distribution of the inhaled respiratory volume to the smaller respiratory airways. This can be visualised at the inspiratory pause trend - Pplat < Ppeak. Pressure in the distal (lower) respiratory airways is counterpoised with pressure in proximal (upper) respiratory airways - bronchi and it corresponds to a plateau pressure (Pplat). Lung ventilation modes with an inspiratory pause are considered to promote the best distribution of air in the lungs, therefore reducing probability of atelectasis appearance during long-term lung ventilation.

Duration of inspiratory pause is set in the range of 15—25% of an inspiration time (5-15% of respiratory cycle time) or 0,3-0,4 seconds.

Change of inspiratory pause duration essentially influences both time of an inspiration and ratio inspiration:expiration (I:E) irrespective of a flow rate at the beginning of inspiration. Enlarging an inspiratory pause, we enlarge I:Е ratio and shorten an expiration. At patients with restrictive pathology a larger inspiratory pause is usually applied — it enlarges an inspiration time and improves gas distribution in lungs. At patients with an obstructive pathology of lungs inspiratory pause should be more shorten — thus, the expiration elongates and development of autoPEEP is prevented.

Pressure at the end of a plateau (Pplat) is closely with alveolar pressure and reflects a lung compliance. That's why it is being used to count a static compliance:

The difference between Rpeak and Pplat reflects a resistive state of respiratory airways, hereupon it is used for calculation of airway resistance:

Monitoring of these indexes is of great importance for dynamic lungs observation at intensive care units. The inspiratory pause is considered to be a great tool, allowing:

  • to improve Tidal volume distribution
  • to reduce increased PaCO2, and to enlarge PaO2, (due to involving a large part of lungs into gaseous exchange)
  • to refine V/Q ratio (ventilation/perfusion ratio)
  • to reduce dead space volume.

During an inspiratory pause a Pendelluft effect can appear. Pendelluft effect consists in a redistribution of an air flow from rapidly filled (blowed) segments of lungs (a small time constant) to those with major time constant (sluggishly filled segments) during equalization of pressure between different lungs regions with various degree of pathological changes. Pendelluft effect may have a negative meaning, especially at the long-term inspiratory pause: in sluggishly straightened departments of lungs there is a redistribution of air which has already participated in gaseous exchange and consequently is oxygen poor.

There is also a collateral ventilation in distal departments of lungs through canals in alveoli walls (interalveolar pores of Kohn) and through communications between bronchioles (canals of Lambert).

It is necessary to remember that long inspiratory pause (more than 0,5 seconds) can influence a middle airways pressure - Pmean (and, hence, intrathoracic pressure), that may have possible negative haemodynamic effect. So, at the lung ventilation with an inspiratory pause anesthesiologists should monitor Pmean constantly.