ALVEOLAR/ARTERIAL/VENOUS GAS TENSIONS General -each gas contributes separately to the total gas pressure -partial pressure of each gas is directly proportional to its concentration -air contains 21% oxygen at 760 barometric pressure (sea level) -partial pressure of oxygen at sea level PO2=159.6 mmHg Oxygen alveolar oxygen tension pulmonary end-capillary oxygen tension arterial oxygen tension mixed venous oxygen tension Carbon Dioxide mixed venous carbon dioxide tension alveolar carbon dioxide tension pulmonary end-capillary carbon dioxide tension arterial carbon dioxide tension end tidal carbon dioxide tension Oxygen Alveolar oxygen tension -with every breathe the upper airway humidies oxygen to temp of 37 degree C -with humidification of oxygen the added vapor pressure of water decreases the inspired concentration of oxygen Inspired oxygen concentration before humidification of upper airways PI02 = Pb x FI02 = 159.6 mm Hg Pb = barometric pressure =760 mmHg at sea level FI02=inspired oxygen concentration= 21% =.21 at sea level Inspired oxygen concentration after humidification of upper airways PI02 = (Pb-47mmHg) x FI02 (760-47)mmHg x 0.21 (713 mmHg) .21 149.7 mmHg ALVEOLAR OXYGEN TENSION PA02= PI02 - PaCO2/R 149.7 mmHg - ( 40mmHg/.8) 149.7 mmHg - 50 mmHg 99.7 mmHg PaCo2 is measured by ABG R= respiratory quotient. normal =0.8 Pumlonary end-capillary oxygen tension (Pc' 02) -practical reference Pc' 02 is identical to PA02 -the PA02 - Pc' 02 gradient is miminal Factors affecting Pc' 02 -rate of oxygen diffusion across the alveolar-capillary membrane (large capillary surface area promotes oxygen diffusion) -pulmonary capillary blood flow -transit time = pulmonary capillary blood volume x cardiac output (nl. =0.8 sec; 70ml/ 5000ml/min) -enhanced oxygen binding to hemoglobin * Rate Limiting Factor of Oxygen Transfer from Alveolar gas to blood is binding of oxygen to hemoglobin Pulmonary diffusing capacity reflects -capacity of oxygen to pass through the alveolar-capillary membrane -permeablitliy of oxygen to pass through the alveolar-capillary membrane -assesment of pulmonary blood flow -oxygen uptake is generally limited by pulmonary blood flow rather than oxygen diffusion across the alv-cap membrane ex. exercise is healthy individuals at high altitudes ex. patients with extensive destruction of the alveolar-capillary membrane DLCO = carbon monoxide uptake/ PACO decreased DLCO: indicated imediment of gas transfer across the alveolar-cap. membrane Causes: -abnormal V/Q ratios -destruction of the gas alveolar-capillary membrane -extemely short capillary transit times -can exacerbated with increases of oxygen consumption and cardiac output ex. exercise Arterial oxygen tension (Pa02) -not calculated -measured at room air with ABG -normal A-a gradient < 15 mmHg -normal A-a gradient upto 20-30 mmHg with increasing age may be normal estimated Pa02 = 102 - age/3 -nl Pa02 btn 60-100 mm Hg -decreases in Pa02 may be from increases in closing capacity compared with FRC Pa02< 60 mmHg = hypoxemia Most common mechanism for hypoxemia: increased A-a gradient. Factors affecting A-a gradient -degree of right to left shunting: direct relationship -amount of V/Q scatter -mixed venous oxygen tension: indirect relationship low cardiac outputs tends to accentuate the effects shunts have on Pa02 high cardiac outputs tends to increase venous admixture by increasing mixed venous oxygen tension Mixed venous oxygen tension -nl value Pv02 = 40 mmHg -represents overall balance between oxygen consumption and oxygen delivery -obtained from pulmonary artery cathetor mixed venous sample contains blood drained from -superior vena cava -inferior vena cava -right side of heart Carbon Dioxide metabolic byproduct of aerobic metabolism occuring in the mitochondria continual small gradients of CO2 tensions -from mitochondria to cell cytoplasm -cell cytoplasm to extracellular fluid -extracellular fluid to venous blood -venous blood to alveoli -alveoli to expired air for elimination Mixed venous carbon dioxide tension (Pv'CO2) -nl value = 46 mmHg -consist of CO2 from various tissues each with varying metabolic activity ex. skin tissue has lower metabolic activity therefore releases lower CO2 ex. heart tiss. has higher metabolic activity therefore releases more CO2 Alveolar carbon dioxide tension -represents the balance between total C02 production and alveolar ventilation -balance between production and elimination -more dependent on alveolar ventilation than C02 production due to buffering capacity of increased C02 production PaC02 = VC02/ VA VC02 = C02 production VA = Alveolar ventilation steady-state: C02 production = C02 elimination imbalances : C02 production > C02 elimination increases total body C02 content ex.hypoventilation hypoperfusion Pulmonary end-capillary carbon dioxide tension -generally identical to PAC02 -diffusion rate of C02 across the alveolar-capillary membrane is 20 times greater than oxygen Arterial carbon dioxide tension (PaC02) -measured with ABG -nl values 38+/- 4 mmHg -nl value of 40 mmHg accepted for practical use -even amongst moderate to severe disturabances usually fail to alter PaC02 due to buffering capacity increased PaC02 -low V/Q ratios decreased PaC02 -high V/Q ratios Arterial-alveolar C02 gradients are not common but may occur in: -presence of marked V/Q abnormalities -venous admixture > 30 % End-tidal carbon dioxide tension (ETC02) -clinically used to estimate PaC02 -gradient represents dilution of alveolar gas with C02 free gas from alveolar dead space PAC02-PETC02 gradient < 5 mmHg