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Tidal volume

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TLCTotal lung capacity: the volume in the lungs at maximal inflation, the sum of VC and RV.
TVTidal volume: that volume of air moved into or out of the lungs during quiet breathing (TV indicates a subdivision of the lung; when tidal volume is precisely measured, as in gas exchange calculation, the symbol TV or VT is used.)
RVResidual volume: the volume of air remaining in the lungs after a maximal exhalation
ERVExpiratory reserve volume: the maximal volume of air that can be exhaled from the end-expiratory position
IRVInspiratory reserve volume: the maximal volume that can be inhaled from the end-inspiratory level
ICInspiratory capacity: the sum of IRV and TV
IVCInspiratory vital capacity: the maximum volume of air inhaled from the point of maximum expiration
VCVital capacity: the volume of air breathed out after the deepest inhalation.
VTTidal volume: that volume of air moved into or out of the lungs during quiet breathing (VT indicates a subdivision of the lung; when tidal volume is precisely measured, as in gas exchange calculation, the symbol TV or VT is used.)
FRCFunctional residual capacity: the volume in the lungs at the end-expiratory position
RV/TLC%Residual volume expressed as percent of TLC
VAAlveolar gas volume
VLActual volume of the lung including the volume of the conducting airway.
FVCForced vital capacity: the determination of the vital capacity from a maximally forced expiratory effort
FEVtForced expiratory volume (time): a generic term indicating the volume of air exhaled under forced conditions in the first t seconds
FEV1Volume that has been exhaled at the end of the first second of forced expiration
FEFxForced expiratory flow related to some portion of the FVC curve; modifiers refer to amount of FVC already exhaled
FEFmaxThe maximum instantaneous flow achieved during a FVC maneuver
FIFForced inspiratory flow: (Specific measurement of the forced inspiratory curve is denoted by nomenclature analogous to that for the forced expiratory curve. For example, maximum inspiratory flow is denoted FIFmax. Unless otherwise specified, volume qualifiers indicate the volume inspired from RV at the point of measurement.)
PEFPeak expiratory flow: The highest forced expiratory flow measured with a peak flow meter
MVVMaximal voluntary ventilation: volume of air expired in a specified period during repetitive maximal effort

Tidal volume (symbol VT or TV) is the lung volume representing the normal volume of air displaced between normal inhalation and exhalation when extra effort is not applied. In a healthy, young human adult, tidal volume is approximately 500 mL per inspiration or 7 mL/kg of body mass.[1]

Mechanical ventilation

Tidal volume plays a significant role during mechanical ventilation to ensure adequate ventilation without causing trauma to the lungs. Tidal volume is measured in milliliters and ventilation volumes are estimated based on a patient's ideal body mass. 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.

Ventilator-induced lung injury such as ALI/ARDS can be caused by ventilation with very large tidal volumes in normal lungs, as well as ventilation with moderate or small volumes in previously injured lungs, and research shows that the incidence of ALI increases with higher tidal volume settings in nonneurologically-impaired patients. [2]

Initial settings of mechanical ventilation:

For patient without preexisting lung disease

Protective lung Ventilation strategies should be applied with Vt 6ml/kg to 8ml/kg with RR = 12 to 20 and an average starting target minute ventilation of 7 l/min

For patients with chronic obstructive lung disease

Protective lung volumes apply 6ml/kg to 8ml/kg with a rate high enough for proper alveolar ventilation but does not create or aggravate intrinsic peep

Acute respiratory distress syndrome

Protective Lung Ventilation Strategies apply. Vt 6 to 8 ml/kg or as low as 5ml/kg in severe cases. Permissive hypercapnia can be employed in an attempt to minimize aggressive ventilation leading to lung injury. Higher peeps are often required however not all ARDS patients require same peep levels. Patient should be started on 6 ml/kg and peep increased till plateau pressure is 30 cmH20 in most severe cases.

References

  1. ^ Beardsell, I et al: MCEM Part A:MCQs, page 33, Royal Society of Medicine Press, 2009
  2. ^ Gajic, Ognjen; Saqib Dara; Jose Mendez; Abedola Adensanya; Emir Festic; Sean Caples; Rimki Rana; Jennifer StSauver; James Lymp; Bekele Afessa (2004). "Ventilator-associated lung injury in patients without acute lung injury at the onset of mechanical ventilation". Critical Care Medicine- Baltimore. 32 (9): 1817–1824.

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