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Dear Caroline Huber-Brown,

In mechanically ventilated patients, the expiratory time constant is an indication of the time needed to exhale and a valuable tool for better understanding the patient's respiratory mechanics. In this newsletter, we discuss the monitoring of respiratory mechanics in mechanically ventilated patients and explain how you can interpret and use the expiratory time constant in various situations.

We hope you enjoy reading it and would be pleased to hear your comments.

With best regards,

Jens Hallek
Hamilton Medical
Volume 15, Issue 2

  Monitoring respiratory mechanics in mechanically ventilated patients

The term respiratory mechanics describes the mechanical properties of the respiratory system that is inflated during mechanical ventilation, and monitoring them may be useful for assessing the lung condition, diagnosing the lung impairment, and adjusting ventilator settings.

The main properties of respiratory mechanics are compliance and resistance. The time constant, which describes the speed of the change in volume after a step change in pressure, is the product of these two properties measured at inspiration or expiration.

The expiratory time constant is very useful for assessing the overall respiratory mechanics and the changes in them. The measurement is accurate in both passive and spontaneously breathing patients, assuming there is passive expiration. It can also be measured during noninvasive ventilation, provided there are no unintentional leaks.

  Bedside tip: How to use the expiratory time constant

As the product of resistance and compliance, the expiratory time constant gives us useful information about the patient’s respiratory mechanics.

This single variable can help us assess the lung condition and severity of the disease, optimize the ventilator settings, monitor prone positioning and understand certain respiratory events.

Our bedside tip explains how to interpret and use the expiratory time constant for each of these purposes.

  Fact of the day

Pulmonary circulation was first described in the thirteenth century by an Arab physician, Ibn al-Nafis. He rejected the long-held Galen theory that blood moved from the right to the left side of the heart through the cardiac septum, saying that blood passed through the lung, and also believed in connections between the pulmonary artery and vein.

This was several hundreds of years prior to William Harvey's De Motu Cordis or the discovery of pulmonary capillaries by Marcello Malpighi. Al-Nafis is estimated to have written some 110 medical textbooks in his lifetime.

Sources: J Appl Physiol (1985). 2008 Dec; 105(6): 1877–1880; Pediatr Cardiol. 2014 Oct;35(7):1088-90; Wikipedia. Image: Arabic book on ibn al Nafis Takrouri M.S. M & Khalaf M 2003 (author unknown)
  Test your IntelliVence

This patient is passive and ventilated in pressure-control mode. Why is the flow curve shaped this way?

  Product news: Did you know...

...that measurement of the expiratory time constant (RCexp) is included in the monitoring system of all Hamilton Medical ventilators?

RCexp is calculated as the ratio between expiratory tidal volume (VTE) and flow at 75% of the VTE, and displayed in the monitoring window.

RCexp is used by the closed-loop control mode ASV® (Adaptive Support Ventilation) for the automatic, breath-by-breath selection of the optimal breath pattern. 

  Meet us here
Do you have questions or need information? The best way is to ask our staff personally. You can find Hamilton Medical at these upcoming events.
WFPICCS - 9th Congress of the World
Federation of Pediatric Intensive & Critical Care Societies
9 - 13 June, 2018
Booth no.: 4

AIRMED World Congress 2018
12 - 14 June, 2018
Warsaw, Poland
Booth no.: 5

Disclaimer: The content of this newsletter is for informational purposes only and is not intended to be a substitute for professional training or for standard treatment guidelines in your facility. Any recommendations made in this newsletter with respect to clinical practice or the use of specific products, technology or therapies represent the personal opinion of the author only, and may not be considered as official recommendations made by Hamilton Medical. Hamilton Medical provides no warranty with respect to the information contained in this newsletter and reliance on any part of this information is solely at your own risk.