Dead space of the respiratory system refers to the space in which oxygen (O2) and carbon dioxide (CO2) gasses are not exchanged across the alveolar membrane in the respiratory tract. Anatomic dead space specifically refers to the volume of air located in the segments of the respiratory tract that are responsible for conducting air to the alveoli and respiratory bronchioles but do not take part in the process of gas exchange itself. These segments of the respiratory tract include the upper airways, trachea, bronchi, and terminal bronchioles. Alveolar dead space, on the other hand, refers to the volume of air in alveoli that are ventilated but not perfused, and thus gas exchange does not take place.
Physiologic dead space (VDphys) is the sum of the anatomic (VDana) and alveolar (VDalv) dead space.
VDphys = VDana + VDalv (L)
Dead space ventilation (VD) is then calculated by multiplying VDphys by respiratory rate (RR).
VD = VDphys x RR (L/min)
Total ventilation (VE) is, therefore, the sum of alveolar ventilation (Valv) and VD.
VE = Valv + VD (L/min)
Enghoff's equation compiles these variables with PaCO2, tidal volume (TV), and expired CO2 (PECO2). It is then implied that VDphys/VT is representative of the portion of a tidal volume that does not participate in gas exchange.
VDphys/VT = (PaCO2 - PECO2)/PaCO2
Dead space has particular significance in the concept of ventilation (V) and perfusion (Q) in the lung, represented by the V/Q ratio. Alveoli with no perfusion have a V/Q of infinity (Q=0), whereas alveoli with no ventilation have a V/Q of 0 (V=0). Therefore, in situations (i.e., V/Q =infinity) in which the alveoli are ventilated but not perfused, gas exchange cannot occur, such as when pulmonary embolism increases alveolar dead space.
The function of a seemingly wasteful design for ventilation that includes dead space is as follows:
Alveolar dead space typically is negligible in a healthy individual. Anatomic, and therefore physiological, dead space normally is estimated at 2mL/kg of body weight and comprises 1/3 of the TV in a healthy adult patient; it is even higher in pediatric patients. Effectively, 1/3 of a TV of inhaled air is rebreathed due to dead space. At the end of expiration, the dead volume consists of a gas mixture high in CO2 and low in O2 compared to ambient air.
End-expiratory dead volume: 5-6% CO2, 15-16% O2; Ambient air: 0.04% CO2, 21% O2
Numerous physiologic factors can influence dead space:
In patients with disease-free lungs who are undergoing general anesthesia for procedures non-affective of the thoracic cavity or diaphragm, dead space and compliance of the lungs has enabled physicians to tailor patients' PEEP to optimal levels, with the reasoning that the point of minimum dead space with maximum compliance represents the point at which the maximum amount of alveoli are opened for ventilation. Increasing VD, however, can signify that alveoli may be over-distending from overly-aggressive ventilation parameters. Lung recruitment maneuvers in adjunct to PEEP in mechanical ventilation has been shown to significantly increase functional residual capacity, compliance, and PaO2 with decreases in dead space compared to PEEP alone.
Dead space can be affected by various clinical scenarios:
Estimation of the dead space can be of significant value in clinical situations for diagnostic, prognostic, and therapeutic value. Dead space is an integral part of volume capnography, which measures expired CO2 and dead space (VDphys/VT) on a breath-by-breath basis for efficient monitoring of patient ventilation. Despite that the VDphys/VT ratio measured by Enghoff's equation is adversely affected by pulmonary shunting in ARDS, VDphys/VT has been shown to be a significant predictor of mortality during early-phase acute respiratory distress syndrome (ARDS), and increases in the VDphys/VT ratio correlated with poorer patient outcomes. Measurement of this dead space provides a quantifiable indicator of overall lung function for physicians to assess throughout the course of ARDS patients' hospital course. PEEP, an integral part of ARDS ventilation management, can be titrated to a patient's specific need based off of capnography and dead space monitoring, but this finding has not been consistently shown in multiple studies.
Physicians with patients suspected of pulmonary embolism can use dead space and capnography findings to exclude the diagnosis with elevated D-dimer, a sensitive but not specific test for an embolism. Furthermore, capnography can be used for periodic monitoring of thrombolysis treatment in pulmonary embolism by trending changes in dead space measurements. Dead space and capnography can prove to be useful tools, minimizing unnecessary tests by ruling out pulmonary embolism with simple capnography measurements.
Clearance of the anatomic dead space is believed to play a significant role in the use of nasal high flow cannulas. It is believed that high nasal flow allows dead space to be cleared more rapidly and subsequently decreasing the portion of dead space that is rebreathed, increasing alveolar ventilation.
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