What is dead space?
  • Dead space is the fraction of tidal volume which does not participate in gas exchange.
What are the components which make up dead space?
  • Dead space is composed of apparatus dead space and physiological dead space.
  • Physiological dead space is composed of anatomical dead space and alveolar dead space
What is apparatus dead space?
  • Apparatus dead space is the dead space in an artifical breathing circuit.
  • For an intubated patient, the apparatus dead space is represented by the volume of the endotracheal tube, tube connectors, HME and the Y-piece of the ventilator circuit
  • For a patient  ventilated non-invasively, the apparatus dead space is the volume of the NIV mask which is occupied by gas
What happens to the total dead space when the patient is mechanically ventilated?
  • Apparatus dead space can increase the total dead space:
    • Mechanical ventilation using a large NIV mask
    • Large circuit components, eg. a big HME
  • Apparatus dead space can also reduce the total dead space
    • Use of ETT (smaller volume than the upper airway)
    • Tracheostomy (bypass the upper airway altogether)
How do you measure physiological dead space?
  • Using the Enghoff modification of the Bohr equation:
    Enghoff modification of the Bohr equation for physiological dead space
  • I.e. it is the fractional difference between arterial CO2 and expired CO2
  • The Enghoff modification was to use  arterial CO2 (PaCO2) instead of alveolar CO2, because arterial CO2 is the mixed product of all the lung units and therefore should theoretically be representative of the average CO2 of all the alveoli put together.
What is anatomical dead space?
  • Anatomical dead space is "the volume of the air passages through which the gas is conducted to the alveoli", and "the volume of gas exhaled before the CO2 concentration rises to its alveolar plateau" (Nunn's; 8th ed)
  • Alternatively: "Anatomical dead space is the portion of dead space which is external to the alveoli, consisting of mainly conducting airways, and represented by Phase I and half of Phase II in the single-breath nitrogen washout test"
How do you measure anatomical dead space?
  • Using the Fowler method:
    Fowler's dead space measurement - nitrogen over time
     
  • The subject takes a breath of 100% oxygen.
  • Oxygen floods into the respiratory tract, completely displacing the other gases from the respiratory tract, so that at the end of inspiration the anatomical dead space is completely filled with oxygen.
  • Then, the subject exhales.
    • Phase I: dead space, i.e. pure oxygen.
    • Phase II: half dead space, half alveolar ventilation
    • Phase III: alveolar ventilation
      Fowler's method - nitrogen concentration over volume
What factors influence the volume of the anatomical dead space?
  • Those factors are:
    • Body size and age (2ml/kg ideal body weight; higher in the infant)
    • Posture (slightly smaller with the subject in a supine position)
    • Airway manoeuvres: a jaw thrust and chin lift adds an extra 40ml)
    • Lung volume: because as the lung inflates, neighbouring alveoli will pull on the smaller airways by traction, and increase their diameter. 
    • Bronchospasm (this is probably only imaginary, but it is mentioned in textbooks)
    • Tidal volume: anatomical dead space appears to decrease in proportion to the tidal volume.
How does tidal volume affect anatomical dead space?
  • When measured by Bohr's method, the anatomical dead space appears to decrease in proportion to the tidal volume.
  • The threshold for this is around 350ml
  • With low enough volumes, anatomical dead space appears to be zero ml
  • This is because of laminar flow and expiratory gas mixing
    • With low flow rates, there is less turbulent flow in the airways, and a relatively small central column of gas can move in and out of the alveoli while the peripheral gas in the conducting airways remains undisturbed, clinging to their walls
    • because of the relatively slow movement of gas out of the alveoli, there is more chance for the alveolar gas to mix (by diffusion) with the gas of the conducting airways, making the exhaled gas concentration more homogeneous and alveolar-like
What is alveolar dead space?
  • "the part of the inspired gas that passes through the anatomical dead space to mix with gas at the alveolar level, but which does not take part in gas exchange." (Nunn's)
  • It is the difference between physiological dead space and anatomical dead space.
  • Wests' Zone 1 contain alveolar dead space.
  • Under normal circumstances, this volume is minimal. 
How do you measure alveolar dead space?
  • Alveolar dead space can be calculated from the difference between the physiological and anatomical dead space volumes. 
What factors increase the alveolar dead space?
  • Poor cardiac output: eg.haemorrhagic shock
  • Parenchymal lung disease: destructive processes such as emphysema 
  • High positive pressure ventilation: increasing the positive pressure pushes blood out of the well-ventilated regions and therefore increases the alveolar dead space
  • Pulmonary vascular occlusion: eg. venous thromboembolism
  • Posture: decreases when patient is supine
  • Extremes of gravity or acceleration: eg. in weightlessness, it decreased by 30% 
  • Open chest and one lung ventilation, if the lung in the open hemithorax is not deflated
How might shunt mimic dead space?
  • The presence of a large enough shunt can give rise to the appearance of increased physiological dead space.
  • Where shunt is substantial, it can create the illusion of alveolar dead space because it increases the difference between the alveolar and arterial CO2.
  • This is because with a large enough shunt, even CO2 will not be cleared effectively 
  • This will behave very much like dead space, in the sense that increasing the ventilation will have little additional effect on decreasing the PaCO2.
  • This requires a large shunt: with a normal cardiac output, a shunt fraction of 60% would be expected to produce an alveolar dead space fraction of around 21%
What happens when your dead space is suddenly increased?
  • The effect on gas exchange is the same as the effect of decreasing the tidal volume
    • Decreased CO2 clearance
    • Decreased oxygenation due to increased alveolar CO2 
  • Decreased efficiency of ventilation
    • For any given minute volume, CO2 clearance will be decreased
    • Thus, there will be increased minute volume requirements
    • Thus, work of breathing is increased

References

Fowler, Ward S. "Lung function studies. II. The respiratory dead space." American Journal of Physiology-Legacy Content 154.3 (1948): 405-416.

Bohr, Christian. "Ueber die Lungenathmung 1." Skandinavisches Archiv für Physiologie 2.1 (1891): 236-268.

Haldane, John. "The action of carbonic oxide on man." The Journal of physiology 18.5-6 (1895): 430-462.

Astrom, E., et al. "Partitioning of dead space--a method and reference values in the awake human." European Respiratory Journal 16.4 (2000): 659-664

Klocke, Robert A. "Dead space: Simplicity to complexity." Journal of Applied Physiology 100.1 (2006): 1-2.

Wagner, Peter D. "Causes of a high physiological dead space in critically ill patients." Critical Care 12.3 (2008): 148.

Hedenstierna, G., and B. Sandhagen. "Assessing dead space. A meaningful variable?." Minerva anestesiologica 72.6 (2006): 521-528.

Enghoff, H. "Volumen inefficax." Bemerkungen zur Frage des schädlichen Raumes. Uppsala Läkareforen Forhandl 44 (1938): 191-218.

Verscheure, Sara, et al. "Volumetric capnography: lessons from the past and current clinical applications." Critical Care 20.1 (2016): 184.

Fletcher, R., et al. "The concept of deadspace with special reference to the single breath test for carbon dioxide." British journal of anaesthesia 53.1 (1981): 77-88.

Nunn, J. F., and M. H: son Holmdahl. "Henrik Enghoff and the volumen inefficax." Upsala journal of medical sciences 84.2 (1979): 105-108.

Enghöff, H. "Volumen inefficax. Bemerkungen zur Fragedes shädlichen Raumes." Uppsala Läkfür. Fürh 44 (1938): 191-218.

Tang, Yongquan, Martin J. Turner, and A. Barry Baker. "A new equal area method to calculate and represent physiologic, anatomical, and alveolar dead spaces.Anesthesiology: The Journal of the American Society of Anesthesiologists 104.4 (2006): 696-700.

Arnold, John H. "Measurement of the alveolar deadspace: are we there yet?." Critical care medicine 29.6 (2001): 1287-1288.

Hedenstierna, G., and B. Sandhagen. "Assessing dead space. A meaningful variable?." Minerva anestesiologica 72.6 (2006): 521-528.

Numa, Andrew H., and C. J. Newth. "Anatomic dead space in infants and children." Journal of Applied Physiology 80.5 (1996): 1485-1489.

Gogniat, Emiliano, et al. "Dead space analysis at different levels of positive end-expiratory pressure in acute respiratory distress syndrome patients." Journal of critical care 45 (2018): 231-238.

Riley, R. L., et al. "Effect of posture on pulmonary dead space in man." Journal of applied physiology 14.3 (1959): 339-344.

Rea, H. H., et al. "The effects of posture on venous admixture and respiratory dead space in health." American Review of Respiratory Disease 115.4 (1977): 571-580.

Brewer, Lara M., Joseph A. Orr, and Nathan L. Pace. "Anatomic dead space cannot be predicted by body weight." Respiratory care 53.7 (2008): 885-891.

Nunn, J. F., E. J. M. Campbell, and B. W. Peckett. "Anatomical subdivisions of the volume of respiratory dead space and effect of position of the jaw." Journal of Applied Physiology 14.2 (1959): 174-176.

Nunn, J. F., and D. W. Hill. "Respiratory dead space and arterial to end-tidal CO2 tension difference in anesthetized man." Journal of Applied Physiology 15.3 (1960): 383-389.

Kerr, A. A. "Dead space ventilation in normal children and children with obstructive airways diease." Thorax 31.1 (1976): 63-69.

Johns, David P., et al. "Airway distensibility in normal and asthmatic subjects and partitioning of the Fowler dead space." Applied physiology, nutrition, and metabolism 31.4 (2006): 460-466.

Thin, A. G., et al. "Effect of respiratory rate on airway deadspace ventilation during exercise in cystic fibrosis." Respiratory medicine 98.11 (2004): 1063-1070.

Steenblock, U., H. Mannhart, and G. Wolff. "Effect of hemorrhagic shock on intrapulmonary right-to-left shunt (QS/QT) and dead space (VD/VT)." Respiration 33.2 (1976): 133-142.

Tusman, Gerardo, Stephan H. Böhm, and Fernando Suarez-Sipmann. "Dead space during one-lung ventilation." Current Opinion in Anesthesiology 28.1 (2015): 10-17.

Helmy, Tamer Abdallah, A. A. Aglan, and Mohamed Abdelghany Ragab. "Alveolar dead space in mechanically ventilated COPD patients and its impact on weaning." Am. J. Res. Commun 3.8 (2015).

Goldhaber, Samuel Z., and C. Gregory Elliott. "Acute pulmonary embolism: part I: epidemiology, pathophysiology, and diagnosis." Circulation 108.22 (2003): 2726-2729.

Burki, Nausherwan K. "The dead space to tidal volume ratio in the diagnosis of pulmonary embolism.American Review of Respiratory Disease 133.4 (1986): 679-685.

Riley, R. L., et al. "Effect of posture on pulmonary dead space in man." Journal of applied physiology 14.3 (1959): 339-344.

Severinghaus, J. W., and M. Stupfel. "Alveolar dead space as an index of distribution of blood flow in pulmonary capillaries." Journal of Applied Physiology 10.3 (1957): 335-348.

Bjurstedt, H., et al. "Effects of posture on alveolar‐arterial CO2 and O2 differences and on alveolar dead space in man." Acta physiologica Scandinavica 54.1 (1962): 65-82.

Prisk, G. Kim, et al. "Pulmonary gas exchange and its determinants during sustained microgravity on Spacelabs SLS-1 and SLS-2." Journal of Applied Physiology 79.4 (1995): 1290-1298.

Von Nieding, G., et al. "Effect of acceleration on distribution of lung perfusion and on respiratory gas exchange." Pflügers Archiv 342.2 (1973): 159-176.

Lohser, Jens, and Seiji Ishikawa. "Physiology of the lateral decubitus position, open chest and one-lung ventilation." Principles and Practice of Anesthesia for Thoracic Surgery. Springer, New York, NY, 2011. 71-82.

Tusman, Gerardo, Stephan H. Böhm, and Fernando Suarez-Sipmann. "Dead space during one-lung ventilation." Current Opinion in Anesthesiology 28.1 (2015): 10-17.

Wagner, Peter D. "Causes of a high physiological dead space in critically ill patients." Critical Care 12.3 (2008): 148.

Niklason, Lisbet, Johannes Eckerström, and Björn Jonson. "The influence of venous admixture on alveolar dead space and carbon dioxide exchange in acute respiratory distress syndrome: computer modelling." Critical care 12.2 (2008): R53.

Koulouris, N. G., et al. "Noninvasive measurement of mean alveolar carbon dioxide tension and Bohr's dead space during tidal breathing.European Respiratory Journal 17.6 (2001): 1167-1174.

Rossier, P. H., and A. Bühlmann. "The respiratory dead space." Physiological reviews 35.4 (1955): 860-876.

Zuntz, Nathan. Physiologie der Blutgase und des respiratorischen Gaswechsels: Von N. Zuntz. FCW Vogel, 1882.