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The hypoxaemic patient with a normal chest radiograph
Published in Paul F. Jenkins, Making Sense of the Chest X-ray, 2013
Going back to the example of the young woman, we are right to infer that a paCO2 of 3 kPa would have resulted in a pAO2 of more than 12.5 kPa and that her paO2 should have been correspondingly higher if lung function had been normal. Unlike CO2, the transfer of oxygen is never perfect (because of an unavoidable degree of ventilation–perfusion mismatch) and this means that there is always an alveolar–arterial oxygen difference (pAO2 – paO2), which increases with age. Rather than guessing, we should be precise about the paO2 that can be expected for a particular paCO2 (which reflects the degree of VA) and the first step is to calculate pAO2. This is straightforward because pAO2 is easy to calculate using the modified alveolar gas equation as follows:
Gas Exchange in the Lungs
Published in Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal, Principles of Physiology for the Anaesthetist, 2020
Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal
Oxygen moves passively across a partial pressure gradient throughout the body. A partial pressure gradient exists across every point where oxygen is transferred: from the atmosphere to the alveoli and to arteries, capillaries, tissues and mitochondria (where the gradient is maintained by oxygen consumption during oxidative phosphorylation). The oxygen cascade describes the sequential falls in oxygen partial pressure from atmosphere to cellular mitochondria (Figure 17.6): Air. The O2 concentration in air is 21% (i.e. 21 kPA or 160 mmHg).Trachea. Humidification of inspired air occurs in the upper respiratory tract. Water vapour in the trachea exerts a saturated vapour pressure (SVP) of 6.3 kPa or 47 mmHg.Alveolus. By the time the oxygen has reached the alveolus, the has fallen to about 15 kPa or 100 mmHg. The is a balance between two processes: O2 delivery into the alveolus by alveolar ventilation and the removal of oxygen from the pulmonary capillaries by oxygen consumption. Using the alveolar gas equation: for a person breathing room air where the SVP H2O is 47 mmHg at 37°C, or Arterial blood. The of arterial blood is 13.3 kPa or 95 mmHg. The is slightly lower than caused by shunts, ventilation–perfusion mismatch, diffusion defects and venous oxygen tension.Capillary. Capillary is in the order of 6–7 kPa or approximately 40 mmHg.Mitochondria. Oxygen then diffuses into cells in the capillary bed. The in the mitochondria is 1–5 kPa or 4–22 mmHg depending on its metabolic activity.
A randomized controlled comparison of three modes of ventilation during cardiopulmonary bypass on oxygenation in pediatric patients with pulmonary hypertension undergoing congenital heart surgeries
Published in Egyptian Journal of Anaesthesia, 2022
Ahmed Ali Gado, Salwa Mohamed Hefnawy, Ashraf M Abdelrahim, Mostafa Abdel Wahab Abdel Aziz Alberry, Mai A. El Fattah Madkour
Following cardiopulmonary bypass,all groups received intermittent positive pressure ventilation with FiO2 0.5, VT 7–10 ml/kg, and ventilation frequency adjusted according to the age of the patients to maintain end-tidal carbon dioxide tension 35–45 mmHg. Positive end-expiratory pressure (PEEP) was not used, either before or after cardiopulmonary bypass. Pulmonary function outcomes were recorded after induction and after weaning from cardiopulmonary bypass and before transferring to ICU as the following parameters: alveolar-arterial partial pressure oxygen difference (AaDO2) with each blood gas sampling according to the alveolar gas equation [16];
Severe, transient pulmonary ventilation-perfusion mismatch in the lung after porcine high velocity projectile behind armor blunt trauma
Published in Experimental Lung Research, 2020
David Rocksén, Ulf P. Arborelius, Jenny Gustavsson, Mattias Günther
Mechanisms related to immediate and severe hypoxia after BABT were determined by dynamic respiratory changes in the exposed (right) and the contralateral (left) lung. FiO2 was kept at 21% and alveolar minute ventilation was equal in exposed animals and controls. Alveolar oxygen pressure (PAO2) was therefore assumed to equal PaO2. The alveolar gas equation calculates PAO2 from PIO2, PaCO2, and FiO2 with a constant R for the respiratory exchange ratio, defined as the ratio between the amount of CO2 and O2 exchange. R was not assumed at a specific level (normally 0.8) due to the heterogenous BABT-injury.19 SaO2 was assumed at 100% in the lungs and CcO2 was assumed to equal alveolar O2 content.