Tissue Oxygenation, Hypoxemia and Shunting PDF

Tissue Oxygenation, Hypoxemia RTT125 and Shunting Learning Objectives and Reading Resources ▪ CLO: 3.0 ▪ Reading Resources ▪ Clinical Blood Gases: Chapter 9, 11 ▪ Respiratory Care Anatomy and Physiology: 7, 12 The tissues require 4 things to properly oxygenate: Ad...

Tissue Oxygenation, Hypoxemia RTT125 and Shunting Learning Objectives and Reading Resources ▪ CLO: 3.0 ▪ Reading Resources ▪ Clinical Blood Gases: Chapter 9, 11 ▪ Respiratory Care Anatomy and Physiology: 7, 12 The tissues require 4 things to properly oxygenate: Adequate PaO2 Tissue Oxygenation Perfusion Requirements Hemoglobin A place for O2 to be utilized Types of Hypoxia Hypoxemic Cardiogenic Anemic Histotoxic Mechanisms of Hypoxemia ▪ Can you think of clinical examples for each of the mechanisms of hypoxemia? Classic Shunt Equation Modified Shunt Equation P(A-a)O2 PaO2/PAO2 PaO2/FiO2 Assessment of Physiologic Shunting Assessment of Physiologic Shunting Classic Shunt Equation ▪ The gold standard in the measurement of the efficiency of oxygen uptake by the lungs. ▪ Requires mixed venous blood samples via a pulmonary artery catheter (Swan-Ganz). 𝑄𝑠𝑝 𝐶𝑐𝑂2 − 𝐶𝑎𝑂2 = 𝑄𝑡 𝐶𝑐𝑂2 − 𝐶𝑣𝑂2 Assessment of Physiologic Shunting Modified Shunt Equation ▪ In the absence of true mixed venous values, a modification to the classic equation can be made. 𝑄𝑠𝑝 𝐶𝑐𝑂2 − 𝐶𝑎𝑂2 = 𝑄𝑡 𝐶𝑐𝑂2 − 𝐶𝑎𝑂2 + (𝐶𝑎𝑂2 − 𝐶𝑣𝑂2) ▪ CaO2 – CvO2 = 3.5 – 5 vol % ▪ This value is dependent on the cardiac output and the extraction ratio. ▪ Significance of Shunt: ▪ 30% = severe intrapulmonary disease. Life threatening and in need of PPV and PEEP or possibly HFO, ECMO Assessment of Physiologic Shunting P(A-a)02 ▪ Commonly referred to at the A-a gradient. ▪ A measurement of the efficiency of oxygen loading. With a perfect blood and alveolar match, the A-a gradient would be insignificant. Correspondingly, an increase in A-a gradient suggests an increase in physiologic shunt. 𝑃 𝐴 − 𝑎 𝑂2 = 𝑃𝐴𝑂2 − 𝑃𝑎𝑂2 𝑃𝑎𝐶𝑂2 ▪ Clinical alveolar air equation: 𝑃𝐴𝑂2 = 𝑃𝐵 − 47 𝐹𝑖𝑂2 − 0.8 𝑃𝑎𝐶𝑂2 𝑃 𝐴 − 𝑎 𝑂2 = 𝑃𝐵 − 47 𝐹𝑖𝑂2 − − 𝑃𝑎𝑂2 0.8 Assessment of Physiologic Shunting PaO2/PAO2 ▪ A measurement of oxygen transfer across the A-C membrane and expressed as a percentage. PaO2/PAO2 = The percentage of successful oxygen transfer. 𝑃𝑎𝑂2 𝑃𝐴𝑂2 ▪ Lower limit of normal: ▪ 0.75 or 75% ▪ A low percentage (e.g., 30%) indicates poor transfer and increased shunting. Assessment of Physiologic Shunting PaO2/FiO2 ▪ An index of pulmonary oxygen exchange efficiency. ▪ The easiest calculation to make at the bedside, ignoring changes in PaCO2. 𝑃𝑎𝑂2 𝐹𝑖𝑂2 ▪ A normal FiO2 (≈0.2) and a normal PaO2 of 80-100 mmHg provides us with a range of normal: ▪ Approximately 400-500 mmHg ▪ A PaO2/FiO2 value lower than 200 mmHg most often indicates a shunt greater than 20% ▪ Estimation of expected PaO2:  ▪ The amount of oxygen unloaded from the blood at the level of the tissues. Volume a-v difference (VO2) compared to the delivered amount (DO2). Oxygen 𝐶𝑎𝑂2 − 𝐶𝑣𝑂2 Extraction 𝑂2 𝐸𝑅 = 𝐶𝑎𝑂2 Ratio ▪ Normal: 5 𝑣𝑜𝑙% 20 𝑣𝑜𝑙% = 25% 𝑆𝑎𝑂2 −𝑆𝑣𝑂2 Estimation: 𝑂2 𝐸𝑅 = 𝑆𝑎𝑂2 Fick Equation ▪ Shows the relationship between cardiac minute output (Q), arteriovenous oxygen content difference C(a-v)O2 and oxygen consumption (VO2). 𝑉𝑂2 = 𝑄 𝑥 𝐶 𝑎 − 𝑣 𝑂2 ▪ Delivery of O2 DO2 = 𝑄 𝑥 𝐶𝑎𝑂2 𝑥 10 (𝑑𝐿 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟) ▪ Normal O2 delivery per minute is about 1000 mL. With a normal extraction of 25%, this means that 250 mL is utilized in a normal person at rest. ▪ When would someone be at risk of not having enough O2 delivery? ▪ Example: a patient in the ICU has a cardiac output of 2.4L/min, PaO2 of 90mmHg, SpO2 of 95%, Hb of 15 vol%. Is he adequately oxygenated? Effects of Cardiac Output on Shunting ▪ Arterial blood is a mixture of oxygenated and shunted blood. Effects of Cardiac Output on Shunting ▪ Effects of decreased cardiac output on PaO2 with normal physiologic shunting. Effects of Cardiac Output on Shunting ▪ The effect of substantial physiologic shunting with normal cardiac output. Effects of Cardiac Output on Shunting ▪ The effect of decreased cardiac output on PaO2 with increased physiologic shunting. Effects of Cardiac Output on Shunting ▪ The effects of increased cardiac output on PaO2 with increased physiologic shunting. Clinical Implications PaO2 is widely held as a measurement of oxygen loading efficiency or to measure the presence of shunting. An increase in PaO2 while FiO2 is stable is usually attributed to an improvement in lung function (decreased shunt). 19 Other Considerations ▪ Remember, the content of oxygen in the venous mixture is not only due to cardiogenic factors. Other important variables can all impact the arterial venous difference, such as: ▪ Hemoglobin quantity (e.g., anemia) ▪ Metabolism ▪ Abnormal systemic circulation distribution ▪ Tissue O2 utilization Shunt Example: Shunt Practice: A 36-year-old woman is admitted to the ICU with bilateral pneumonia and placed on a ventilator with FiO2 of 1.0. ▪ ABG = 7.39/42/145 (pH/CO2/O2) ▪ SpO2 = 99% ▪ etCO2 = 30mmHg ▪ PvO2 = 34mmHg SvO2= 63% ▪ Hb = 13gm/dl ▪ C.O = 3.5L/min ▪ Pb = 760mmHg ▪ Calculate the classic shunt equation, oxygen consumption, A-a gradient, and the extraction ratio, and comment on the end tidal CO2 value. ▪ Please submit a copy of your answers to the ‘Shunt Calculations’ dropbox folder in iLearn by September 26th.