An elevated dead-space-to-tidal-volume ratio (V
D
/V
T
) has been proposed as
a predictor of successful extubation in children
. We hypothesized that a higher V
D
/V
T
value would be associated with extubation failure and higher postextubation respiratory support.
What is the normal dead space to tidal volume ratio?
The ratio of physiologic dead space to tidal volume is usually
about 1/3
.
How does dead space affect tidal volume?
At a fundamental level, increasing the
dead space functionally
indistinguishable from hypoventilation: Dead space is a fraction of the total tidal volume. Of the tidal volume, only the non-dead fraction participates in gas exchange. Ergo, increasing dead space has the same effect as reducing the tidal volume.
What does it mean if your tidal volume is high?
Tidal volume is a measure of the
amount of air a person inhales during a normal breath
. Traditional preset tidal volumes higher than 10 ml/kg have been proved to be associated with increased risk of pulmonary barotrauma and should be avoided. High tidal volumes also decrease venous return and reduce cardiac output.
What is the relationship between the tidal volume and the dead space volume in the context of high frequency ventilation?
With conventional ventilation where tidal volumes (V
T
) exceed dead space(V
DEAD
), gas exchange is largely related to bulk flow of gas to the alveoli. With high-frequency ventilation,
the tidal volumes used are smaller than anatomical and equipment dead space and therefore alternative mechanisms of gas exchange occur
.
Which disease is the alveoli ventilated but not perfused?
In
pulmonary shunt
, alveoli are perfused but not ventilated. Blood flowing past poorly ventilated alveoli doesn't pick up additional oxygen. This poorly oxygenated blood returns to the heart and mixes with oxygenated blood coming from other areas of the lungs that are ventilated.
Is PE dead space or shunt?
A decrease in perfusion relative to ventilation (as occurs in pulmonary embolism, for example) is an example of increased
dead space
. Dead space is a space where gas exchange does not take place, such as the trachea; it is ventilation without perfusion.
What conditions cause dead space ventilation?
V/Q Mismatch/Decreased Perfusion: Perfusion to the alveoli is decreased in clinical scenarios such as
pulmonary embolism and hypotension
, increasing the V/Q ratio and creating dead space ventilation.
What increases lung compliance?
Two important factors of lung compliance are
elastic fibers and surface tension
. More elastic fibers in the tissue lead to ease in expandability and, therefore, compliance. Surface tension within the alveoli is decreased by the production of surfactant to prevent collapse.
What causes increased dead space?
The commonest causes of increased alveolar deadspace are
airways disease–smoking, bronchitis, emphysema, and asthma
. Other causes include pulmonary embolism, pulmonary hypotension, and ARDS.
How tidal volume is calculated?
To determine the adequate tidal volume (Vt) to deliver during protective ventilation, it is necessary to calculate the patient's PBW. This is accomplished by using the Devine's formulas (
3
) adjusted by gender:
Male: PBW = 50 + 0.91 × (height in cm–152.4) Kg
. Female: PBW = 45.5 + 0.91 × (height in cm–152.4) Kg.
What can affect tidal volume?
Measurement of tidal volume can be affected (usually overestimated) by
leaks in the breathing circuit
or the introduction of additional gas, for example during the introduction of nebulized drugs.
Is tidal volume affected by frequency of breathing?
| Rest Exercise | Tidal volume 0.5 litres 3 litres | Minute ventilation 6 litres per minute 90 litres per minute |
|---|
What frequency defines high frequency modes of ventilation?
High-frequency ventilation (HFV) generally is defined as
60 to 3000 breath cycles/min
but has been technically defined by the FDA as a rate exceeding 150 breath cycles/min.
How does an oscillator vent work?
HFOV utilizes
active inspiratory and expiratory phases to produce small tidal volumes
, usually equal to or less than dead space [4, 5, 48]. The rapid respiratory rate helps maintain alveolar ventilation, while the lungs maintain inflation through a constant mPaw [5, 7, 11, 13, 23, 26, 39].
How do you use HFOV?
Start at a frequency of 10 Hz and a Power of 3.0 to 5.0. Initial MAP 4 cm above MAP while on CMV. Check CXR 2 hrs after converting to HFOV, then adjust MAP to achieve optimal lung volume (9-10 ribs expanded). If not oxygenating, increase MAP by 2 cm every hour until oxygenation improves.
