肺内分流zuihou学习资料

1、Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime. 1 1 11 2 2 2 2 3 33 3 4 4 44 5 6 肘静脉、下肢、上腔、右心房、右心室、肺动脉的静脉氧分压？ 静脉采血点的氧分压 血压 Hypoxia occurs more easily than hypercarbia. Why? PaO2的降低 远多于 PaCO2的升高 出着容易进着难 浓度差、物理弥散化学解离性质，气体分压 Pt =+ P1P2P3P4 气体分压 PO2+ PC

2、O2= a constant In the alveolus, the mixture of gasses contains nitrogen, water vapor, trace gasses, oxygen and carbon dioxide. At the end of a breath, the pressure in the alveolus = atmospheric pressure. So. PB=PN2+ PH2O + Ptracegasses+ PO2+ PCO2 Or. PO2+ PCO2= a constant 一个多了另一个就少了 The burly alveol

3、us (high V/Q). The weakling alveolus (low V/Q). A fundamental question: In terms of arterial O2 and CO2 tensions, can the burly alveolus compensate for the weakling alveolus? for PaO2. Yes or No？ for PaCO2. Yes or No？ This basic fact explains a lot. Know it cold. The weakling alveolus (shunt or V/Q

4、mismatch) The burly alveolus Can the burly alveolus compensate for the weakling alveolus? Not for oxygen! The burly alveolus cant saturate hemoglobin more than 100%. SaO2 of equal admixture of burly and weakling alveolar blood = 89% pO2 = 50 mm Hg SaO2 = 75% pO2 = 50 mm Hg SaO2 = 80% SaO2 = 75% SaO2

5、 = 98% pO2 = 130 mm Hg pO2 = 40 mm Hg pO2 = 130 mm Hg pO2 = 40 mm Hg 肌体的 储备和动员 The weakling alveolusThe burly alveolus Can the burly alveolus compensate for the weakling alveolus? Yes, for CO2! The burly alveolus, if it tries real hard, can blow off extra CO2. Pulmonary venous blood pCO2 and PaCO2 =

6、 40 mm Hg pCO2 = 44 mm Hg pCO2 = 44 mm Hg pCO2 = 36 mm Hg pCO2 = 46 mm Hg pCO2 = 36 mm Hg pCO2 = 46 mm Hg 健壮的肺能排出更多的二氧化碳而吸进和运载储备更多的氧 Shunt, or “weakling” (low V/Q) alveolus SaO2 = 75% “Burly” (high V/Q) alveolus SaO2 = 99% Normal alveolus SaO2 = 96% Equal admixture of “weakling” and “burly” alveolar

7、 blood has SaO2 = (75 + 99)/ 2 = 87%. Average alveolar PACO2 = 40 mm Hg. Hence, PaCO2 = 40 mm Hg For CO2, burly alveolus CAN compensate for the weakling alveolus. Weakling alveolus Burly alveolus Normal alveolus Admixture of burly and weakling alveolar blood 二氧化碳的事好解决 氧的事不好办 面积时间溶解缓冲难受都有余地，有通气量就行。 P

8、aO2 is always slightly lower than PAO2？ 问题 什么是肺内分流、肺外分流？ 正常肺内分流多少？ 分流的形式有哪些？ 分流增加的结果？ 分流量如何判断评估测算？ 怎么减少分流？ 1. Gas exchange, 2. A key to lung disorders, 3. Uneven distribution of tidal volume and perfusion, 4. Blood gases, 5. The PO2 - PCO2 diagram, 6. The VA / Q- curve, 7. Blood-R-curves, 8. Dead sp

9、ace, 9. Anatomic venous-to-arterial shunt, 10. Ficks law of diffusion, 11. Single -breath diffusing capacity, 12. Compensation of VA /Q - mismatch, 13. Pulmonary bloodflow, 14. Regional ventilation. . Pulmonary Shunting 肺循环、体循环、冠脉循环 肺内分流量（Qsp,Qs/Qt）概念 每一次右心室搏出的血液均进入肺循环，经过氧合作用后流回左心。生 理条 件下，心排血量（Qt）只有

10、很小部分未经氧合直接回入左心，此部分血量 称为解 剖分流。 在没有房、室间隔或其他心血管缺陷的前提下，生理性的解剖分流由支 气管 动脉的部分血液营养支气管后，血中氧已被消耗，流回入肺静脉，还有少 量冠状 静脉血流通过迷走静脉（Thebesian Vein）也直接回入左心所形成，一 般在5% 以下。 在病理情况下，如因炎性渗出液或水肿液充满肺泡腔或因肺不张肺泡完 全萎 陷时，吸入气完全不能进入该病变区肺泡内，虽然血流仍经过此区域但不 能进行 气体交换，含还原血红蛋白的静脉血直接回入左心，宛如有右至左的分流 存在。 此部分因病理原因引起的分流和解剖分流的总和称为肺内分流（Qs）。 当肺内分流占心排

11、血量成分过大时，将引起低氧血症。此种低氧血症与 上述 V/Q失调所引起的低氧血症有所不同，它不伴有CO2分压的升高，而PA- aO2显 著增加，而且不能因提高吸入气氧浓度使之得到改善。 1. The word “shunt” refers to blood that has not exchanged gases that mixes with blood that has exchanged gases. 2. Sources of shunt: Thebesian circulation that perfuses the left ventricle then dumps into th

12、e left ventricle. Bronchial circulation that perfuses lung tissue and empties into the pulmonary vein. In normal people this accounts for about 2-4% of total blood flow. Perfusing collapsed alveoli or having a hole in the wall of the atria or ventricles will produce a right to left shunt. 左冠状动脉主要供应左

13、心室前部，右冠状动脉主要供应左心室后部和右心室。 左冠状动脉的血液流经毛细血管和静脉后，主要经由冠状窦回流入右心房， 而右冠状动脉的血液则主要经较细的心前静脉直接回流入右心房。 还有一小部分冠脉血液可通过心最小静脉直接流入左、右心房和心室腔内。 Pulmonary Shunting PERFUSION WITHOUT VENTILATION Pulmonary shunt is that portion of the cardiac output that enters the left side of the heart without coming in contact with an a

14、lveolus. “True” Shunt No contact Anatomic shunts (Thebesian, Pleural, Bronchial) Cardiac anomaliesintrapulmonary fistulavascular lung tumors “Shunt-Like” (Relative) Shunt contact but not enough Some ventilation, but not enough to allow for complete equilibration between alveolar gas and perfusion. T

15、rue Shunt Anatomic shunts+Capillary Shunt Alveolar collapse (atelectasis) Alveolar fluid accumulation (pulmonary edema) Alveolar consolidation (pneumonia) Ture Shunts are refractory to oxygen therapy. oxygen therapy will NOT help (at least to the expected degree). 解剖分流 生理情况下，肺内也存在解剖分流(anatomic shunt

16、)，即有一小部分静脉血经支气管 静脉和肺 内动静脉吻合支直接流入肺静脉，以及心内最小静脉直接流至左心，其分流量约占心 输出量 的23。这部分血液未经氧合即流入体循环动脉血中，称之为真性分流(真性静脉 血掺杂， ture venous admixture)。 解剖分流增加的原因可见于：支气管扩张时伴有支气管血管扩张，和肺小血管栓塞时 肺动 脉压增高导致的肺内动静脉短路开放；以及慢性阻塞性肺病时，支气管静脉与肺静脉 之间形 成的吻合支等，都使相当多的静脉血掺人动脉血中。 肺不张或肺实变时，病变肺泡完全无通气功能，但仍有血流，流经该处的血液完全未 进行 气体交换而掺入动脉血中，类似解剖分流。 临床呼

17、吸衰竭的发病机制中，单纯通气不足，单纯弥散障碍，单纯的肺内分流或死腔 通气 增加的情况较少，常常是几个因素共同或相继发生作用。如慢性阻塞性肺病发生呼吸衰 竭的机 制为：支气管炎症、分泌物堵塞等引起气道狭窄或阻塞，而有明显的阻塞性肺通气障 碍； 呼吸肌疲劳所致的呼吸动力减弱，肺组织的炎症、间质和肺的纤维化以及累及胸膜，引 起肺和 胸廓顺应性的降低，导致限制性肺通气障碍；肺泡的纤维化、炎症等引起肺泡膜损伤， 弥散 面积减少和弥散距离增加，导致弥散障碍；由于部分肺泡的通气减少或丧失，造成功 能性分 流增加。由于毛细血管床的破坏，血管的重建使部分肺泡的肺血流明显减少，造成死腔 样通气 增加，从而导致V

18、AQ失调；由于动静脉吻合支的开放等引起真性分流显著增多。 由解剖分流增加引起的换气障碍，其血气变化也仅有PaO2降低。鉴别功能性与真性 分流 的一个有效方法是吸入纯氧，若吸入纯氧30min能提高PaO2，则为功能性分流；而对真 性分流， 则吸入纯氧无明显提高PaO2的作用。 Normal shunt bronchial circulation and Thebesian veins aorta Pulmonary veins Shunt etiologies Normal Bronchial circulation Thebesian veins Intracardiac Tetralogy

19、of Fallot, VSD, etc. Intrapulmonary Bronchial intubation Obesity Cirrhosis Osler-Weber-Rendu 功能性分流 肺内的、肺外的解剖分流 Shunt-Like Effect Blood that does exchange gases with alveolar gases but does not obtain a PO2 that equals that of a normal alveolus Hypoventilation 低通气 Uneven distribution of ventilation 分

20、布不均 Bronchospasm Excessive mucus in the tracheobronchial tree Alveolar-capillary diffusion defects 弥散异常 Pulmonary fibrosis Not enough time for diffusion to occur Readily improved by oxygen therapy 地球上必然的 区别？ 真解剖分流（肺内外） 相对解剖（功能）性分流 （肺内） 分流样效应（肺内） ventilationperfusion balance or imbalance 肺泡与血液之间的气体交换

21、，不仅取决于足够的肺泡通气和有效的气体弥散，还取决于肺泡通 气量与肺血流量的比例配合，即通气血流比值。 正常人平静呼吸时平均肺泡通气量（VA）为4L/min，平均肺血流量（Q）为5L/min，通气血流 （VA/Q）比值为0.8。由于受重力影响气体和血流的分布在肺内各部分并不均匀，直立体位时， 肺通气量和肺血流量自上而下都是递增的，但以血流量的增幅更为明显，因而VA/Q比值肺上部 可高达3.0，而肺底部仅为0.6，但通过自身调节机制，使总的VA/Q保持在最合适的生理比值0.8 通气血流匹配matching VA/QVA/Q比例失调的基本形式 当肺部病变时，由于部分肺泡的通气量不足或血流量减少，使

22、 肺泡 的通气血流比例失调(ventilation-perfusion imbalance)，而引 起气体 交换障碍，这是呼吸衰竭发生的最常见机制。VAQ比例失调，表现 为如 下两种基本形式： （1）部分肺泡通气不足VA/Q比值降低 部分肺泡因阻塞性或限制性通气障碍而引起严重通气不足，但血流 量未相应减少，VA/Q比值下降，造成流经该部分肺泡的静脉血未经充分 氧合便掺入动脉血中，称静脉血掺杂(venous admixtrure)，因为如同动- 静脉短路又称功能性分流(functional shunt)。 正常成人也存在功能性分流仅约占肺血流量的3%，严重的慢性阻 塞性肺病时，可以增至肺血流量的

23、30%50%，从而严重地影响换气功能。 （2）部分肺泡血流不足VA/Q比值升高 肺动脉分支栓塞、炎症，肺动脉收缩，肺毛细血管床大量破坏 可使 流经该部分肺泡的血液灌流量减少，而该部分肺泡的通气相对良好， 使 VA/Q比值明显升高。这使该部分肺泡内的气体未能与血液进行有效 的气 体交换，则使死腔气量增加。 死腔气量包括解剖死腔（指不参与气体交换的气管及支气管管 腔容 积）和肺泡死腔（指有通气而无血流灌注的肺泡容量）。 死腔样通气(dead space like ventilation)指的就是有通 气的肺泡 血流相对地减少，以致于这些肺泡内的气体，得不到充分的利用。 正常 人死腔气量与潮气量之比

24、低于30%，严重肺疾患时可高达60%70%。 VAVAQ Q比例失调的血气变化 肺泡通气与血流比例失调时的血气变化，无沦是部分肺泡通气不足引起的 功能性 分流增加，还是部分肺泡血流不足引起的死腔样通气，均主要引起PaO2 降低， 而PaCO2可正常、降低或升高，这主要由健全肺泡的代偿功能，以及氧与 二氧 化碳解离曲线的特性所决定。 (1)当部分肺泡通气不足，流经该处的血液得不到充分的气体交换，使血液 氧分 压降低，二氧化碳分压升高。健全肺泡代偿性的增加通气量，使流经健全 肺泡的 血液氧分压升高。但由于氧解离曲线S型的特点，氧分压达 100mmHg(13.3kPa) 时，血氧饱和度已高达95以上

25、，已处于S型曲线上端的平坦段，此时， 即使健 全肺泡因通气加强进一步提高了氧分压，但血氧含量的增加也极少，因此 无法代 偿通气不足肺泡所造成的低氧血症。 (2)当部分肺泡血流不足时，流经该处的血液氧分压虽显著增高，同理血氧 含量 的增加也很少。而健全肺泡因血流量增加，使VAQ比值小于正常，流经 此处的 血流量虽多却不能充分氧合，所以造成VAQ比例失调时PaO2和氧含量 都明显 降低。由于二氧化碳解离曲线的特性，当PaCO2在37.5-60 mmHg(58kPa)范围 内，血液二氧化碳含量与PaCO2几乎呈直线关系，代偿性通气增强的肺泡， 血 中的二氧化碳可得以大量排出，使PaCO2保持在正常水

26、平，甚至因代偿过 度， 而致PaCO2低于正常，只有在严重障碍和代偿不足时，PaCO2才会高于 正常。 Intrapulmonary shunt in obesity: When FRC is below closing capacity, perfusion of non-ventilated alveoli is SHUNT. 肥胖低氧的原因？ 手术后病人低氧原因？ 麻醉 体位 手术 吸纯氧 容量分布 The same minute ventilation can cause markedly different amounts of alveolar ventilation, depen

27、ding on tidal volume. 功能残气量下降时肺泡萎陷多 分流增加 Author Samee, S ; Altes T ; Powers P ; de Lange EE ; Knight-Scott J ; Rakes G Title Imaging the lungs in asthmatic patients by using hyperpolarized helium-3 magnetic resonance: assessment of response to methacholine and exercise challenge Journal Title Journa

28、l of Allergy PAO2肺泡气氧分压肺泡气氧分压 呼吸商 肺泡气COCO2 2分压 0.863Vco2 PaCO2= PACO2 = VA PACO2 ：肺泡气CO2分压 Vco2：每分钟CO2 产量 VA ：肺泡通气量 PaCO2：动脉血CO2分压 二氧化碳就是透 公式转换 呼吸商呼吸商(R)= PACO2VA (PiO2-PAO2) VA PiO2-PAO2= PACO2 R PAO2=PiO2- PACO2 R 肺泡气肺泡气O2分压分压 PACO2 PAO2=PiO2 R PAO2 ：肺泡气O2分压 PiO2 ：吸入气O2分压 PACO2 ：肺泡气CO2分压 R：呼吸商 Gas

29、eous Environment Atmosphere: Nitrogen and Oxygen, negligible Carbon Dioxide. Clinical Relevance of Environment Altitude: PO2 depends on PB 低于21% Suffocation: PO2 depends on fractional O2 Oxygen therapy: PO2 depends on fractional O2 PFP IIB OO 22 47() Evaluating FIO2 流量表刻度高吸入氧浓度就高吗？ High flow devices

30、 may not be delivering the FIO2 that is set If the patients total flowrate is exceeding the flow from the oxygen delivery device, the FIO2 will decrease Water in the aerosol tubing will increase FIO2 High flow oxygen delivery systems should be analyzed 管道流量吸入氧浓度常不精确 在呼吸机上比较准确 浓度、流量；压力 Ideal Alveolar

31、 Gas Equation. PP P R PF R R AI A AI OO CO COO 22 2 22 1 Clinically Useful Form: Complete Form: PP P R AI A OO CO 22 2 PAO2 PAO2 = (PBARO - PH2O) x FIO2 (PaCO2/0.8) On FIO2 of less than 60% PAO2 = (PBARO - PH2O) x FIO2 PaCO2 On FIO2 greater than 60% Normal Values: Room Air: 100 104 mm Hg 100% Oxygen

32、: 600 Compare PAO2 to PaO2 Healthy people: PAO2 = PaO2 In an ideal lung, PaO2 and PaCO2 = PAO2 and PACO2. In normal healthy, these values are close but not identical. In disease conditions, the numbers can vary greatly. Two Approaches to Comparison (PAO2 PaO2) difference 减法：差 PaO2 / PAO2 ratio 除法：比

33、A-a Difference PAO2 - PaO2 Normally 5-20 mmHg Values increase with increasing age and the supine position. Because of normal anatomical shunt Ventilation/Perfusion mismatching A-a difference increases with pulmonary disease Problem: Normal range changes on 100% O2 A：理想状态的一腔（肺泡）模型 In a healthy young

34、person, the PAO2 PaO2 is normally with small shunts, the magnitude of venous admixture is proportional to A-a gradient = with larger shunts, the relationship is lost. 2 V/Q scatter 3 Actual alveolar PO2 (PAO2) = due to the non-linear shape of the oxygen dissociation curve, with everything else being

35、 equal = the greater the PAO2, the greater the A-a gradient 4 Cardiac output = cardiac output is inversely proportional to alveolar/arterial O2 content difference, given the same venous admixture BUT, venous admixture also decrease with reduced CO = PaO2 is relatively unchanged 5 Hb concentration =

36、Hb does not influence pulmonary end-capillary/arterial oxgen content difference But increase in Hb would cause small decrease in the tension difference 6 Alveolar ventilation = increased ventilation increase both PAO2 and A-a gradient. When venous admixture 3% AND ventilation 1.5L/min, the higher th

37、e shunt/ventilation, = lower PCO2 = lower CO = greater alveolar/arterial O2 content difference = A-a gradient is greater than the increase in PAO2 = PaO2 can actually decrease with higher ventilation PaO2 /PAO2 a/A ratio 比率 Normally averages just over 0.8 a/A ratio falls with pulmonary disease. Lowe

38、r limit normal: young (room air) :0.74 older (room air) :0.78 Both groups (100% O2):0.82 Normal value is greater than 75% on any FIO2 Example: 100/104 = 96% 96% of oxygen is diffusing across the A-C membrane (A-a) Difference vs. a/A Ratio 哪个相关分流更好？ Normal Normal (A-a) PO2 Difference (mmHg) a/A PO2 r

39、atio Sick Sick PaO2/FIO2 ratio 氧合指数 Normal value is 400 500 Example: 100 mm Hg/.21 = 476 Value between 200 300 = ALI Value less than 200 = ARDS Values less than 200 correlate with a shunt of greater than 20% 肺泡-动脉氧气压力差动脉-肺泡氧气分率氧合指数（Oxygenation index） C(a-v)O2：动脉-静脉氧气含量差 P(A-a)O2 / PaO2 呼吸指数（Respirat

40、ory index） PaO2 / FIO2于1974年由Dr. Horovitz提出，因为计算容易，且与 肺内分 流（Qsp/Qt）的相关性不错，所以临床应用甚广。 P(A-a)O2因加入了吸入氧气分率及动脉二氧化碳压力两指数，可 以分 辨出因通气量过低导至二氧化碳累积而造成的氧合不良，但影响P(A- a)O2 的因素很多，包括吸入氧气分率、通气血流灌注比不配合、肺内分流 及右 向左的心内分流，其中肺内分流又随着各种肺疾状况、病患年龄及不 同的 体位而改变，此外P(A-a)O2也受混合静脉氧气含量的相关因素影响， 如组 织氧气消耗量、心搏出量及血红素量。 一般P(A-a)O2对呼吸常态空气的

41、病患有无氧合障碍相当敏感，但 由于它 与肺内分流间的相关性不佳且受太多非肺因素影响，所以在重症病患 并不 实用。 PaO2 / PAO2及 P(A-a)O2 / PaO2 分别由Dr. Gilbert与Dr. Goldfarb提出。 与肺内分流作相关性分析，PaO2 / FIO2、PaO2 / PAO2与P(A- a)O2 / PaO2 三者较近似（r=0.720.74），P(A-a)O2则稍差（r=0.62）。 Rule Of Thumb “50/50 Rule” Although V/Q imbalances are the most common cause of hypoxemia i

42、n patients with respiratory diseases, physiologic shunting also can occur commonly, esp. in the critically ill. If the Fi02 is 50% P(A-a)O2 = 110 - 95 = 15 mm Hg Despite severe hypoventilation, there is no evidence here for lung disease. Hypercapnia is most likely a result of disease elsewhere in th

43、e respiratory system, either the central nervous system or chest bellows. Alveolar Gas Equation PAO2 = PIO2 - 1.2 (PaCO2) where PIO2 = FIO2 (PB - 47). PAO2 = PIO2 - 1.2 (PaCO2)PAO2 = PIO2 - 1.2 (PaCO2) PAO2 = PIO2 - 1.2 (PaCO2) PAO2 is the average alveolar PO2 PIO2 is the partial pressure of inspire

44、d oxygen in the trachea. PIO2 = FIO2 (PB 47 mm Hg) = 0.21 (760-47) = 150 mm Hg Breathing room air at sea level, PAO2 = 150 1.2 (40) = 150 - 48 = 102 mm Hg Note: FIO2 is fraction of inspired oxygen and PB is the barometric pressure. 47 mm Hg is the water vapor pressure at normal body temperature. Thi

45、s is the abbreviated version of the AG equation, suitable for clinical purposes. Alveolar Gas Equation PAO2 = FIO2 (PB 47 mm Hg) 1.2(PaCO2) In order to bring O2 into the blood, alveolar PO2 (PAO2) has to always exceed arterial PO2 (PaO2). Whenever PAO2 decreases, PaO2 decreases as well. Thus, from t

46、he AG equation: If FIO2 and PB are constant, then as PaCO2 increases both PAO2 and PaO2 will decrease: hypercapnia causes hypoxemia. If FIO2 decreases and PB and PaCO2 are constant, both PAO2 and PaO2 will decrease: suffocation causes hypoxemia. If PB decreases (e.g., for altitude), and PaCO2 and FI

47、O2 are constant, both PAO2 and PaO2 will decrease: mountain climbing causes hypoxemia. What is the alveolar PO2 (PAO2) at sea level* in the following circumstances? a) FIO2 = .21, PaCO2 = 20 mm Hg b) FIO2 = .21, PaCO2 = 60 mm Hg c) FIO2 = .40, PaCO2 = 30 mm Hg *BP = 760 mm Hg Alveolar Gas Equation P

48、AO2 = FIO2 (PB 47 mm Hg) 1.2(PaCO2) What is the alveolar PO2 (PAO2) at sea level* in the following circumstances? a) FIO2 = .21, PaCO2 = 20 mm Hg ANSWER: PAO2 = .21(713) - 1.2(20) = 126 mm Hg b) FIO2 = .21, PaCO2 = 60 mm Hg ANSWER: PAO2 = .21(713) - 1.2(60) = 72 mm Hg c) FIO2 = .40, PaCO2 = 30 mm Hg

49、 ANSWER: PAO2 = .40(713) (1.2)30 = 249 mm Hg *BP = 760 mm Hg Alveolar Gas Equation PAO2 = FIO2 (PB 47 mm Hg) 1.2(PaCO2) P(A-a)O2 P(A-a)O2 is the alveolar-arterial difference in partial pressure of oxygen. It is commonly called the “A-a gradient,” though it does not actually result from an O2 pressur

50、e gradient in the lungs. Instead, it results normal ventilation-perfusion imbalance in the lungs (normal “venous admixture,” about 3% of cardiac output). PAO2 is always calculated, based on FIO2, PaCO2 and barometric pressure. PAO2 = FIO2 (PB 47 mm Hg) 1.2(PaCO2) PaO2 is always measured, on an arter

51、ial blood sample in the blood gas machine. Normal P(A-a)O2 ranges from 5 to 25 mm Hg breathing room air (it increases with age and with FIO2). A higher than normal P(A- a)O2 means the lungs are not transferring oxygen properly from alveoli into the pulmonary capillaries. Except for right to left car

52、diac shunts, an elevated P(A-a)O2 signifies some sort of problem within the lungs that has caused ventilation-perfusion imbalance (increase over the normal venous admixture). Virtually all lung disease lowers PaO2 via the mechanism of increased V-Q imbalance, e.g., COPD, pneumonia, atelectasis, pulm

53、onary edema. a)FIO2 = .21, PaCO2 = 20 mm Hg 112 mm Hg b)FIO2 = .21, PaCO2 = 60 mm Hg 62 mm Hg a)FIO2 = .40, PaCO2 = 30 mm Hg 129 mm Hg * Always a calculation * Always a measurement P(A-a)O2 Alveolar PO2 * PaO2* P(A-a)O2* a)FIO2 = .21, PaCO2 = 20 mm Hg PAO2 = .21(713) - 1.2(20) = 126 mm Hg112 mm Hg 1

54、4 mm Hg (nl.) b)FIO2 = .21, PaCO2 = 60 mm Hg PAO2 = .21(713) - 1.2(60) = 72 mm Hg62 mm Hg 10 mm Hg (nl.) FIO2 = .40, PaCO2 = 30 mm Hg PAO2 = .40(713) (1.2)30 = 249 mm Hg129 mm Hg 120 mm Hg * Always a calculation * Always a measurement P(A-a)O2 Alveolar PO2 * PaO2* P(A-a)O2* SaO2 and oxygen content T

55、issues need a requisite amount of oxygen molecules for metabolism. Neither the PaO2 nor the SaO2 tells how much oxygen is in the blood. How much is provided by the oxygen content, CaO2 (units = ml O2/dl). CaO2 is calculated as: CaO2 = quantity O2 bound + quantity O2 dissolved to hemoglobin in plasma

56、 CaO2 = (Hb x 1.34 x SaO2) + (.003 x PaO2) CaO2 = 15 x 1.34 x .98 + (.003 x 100) CaO2 = 19.7 + 0.3 = 20 ml O2/dl blood Hb = hemoglobin in gm%; 1.34 = ml O2 that can be bound to each gm of Hb; SaO2 is percent saturation of hemoglobin with oxygen; 0.003 is solubility coefficient of oxygen in plasma: 0

57、.003 ml dissolved O2/mm Hg PO2. How much oxygen is in the blood? PaO2 vs. SaO2 vs. CaO2 OXYGEN PRESSURE: PaO2 Since PaO2 reflects only free oxygen molecules dissolved in plasma and not those bound to hemoglobin, PaO2 cannot tell us “how much” oxygen is in the blood; for that you need to know how muc

58、h oxygen is also bound to hemoglobin, information given by the SaO2 and hemoglobin content. OXYGEN SATURATION: SaO2 The percentage of all the available heme binding sites saturated with oxygen is the hemoglobin oxygen saturation (in arterial blood, the SaO2). Note that SaO2 alone doesnt reveal how m

59、uch oxygen is in the blood; for that we also need to know the hemoglobin content. OXYGEN CONTENT: CaO2 Tissues need a requisite amount of O2 molecules for metabolism. Neither the PaO2 nor the SaO2 provide information on the number of oxygen molecules, i.e., how much oxygen is in the blood. (Neither

60、PaO2 nor SaO2 have units that denote any quantity.) Only CaO2 (units ml O2/dl) tells how much oxygen is in the blood; this is because CaO2 is the only value that incorporates the hemoglobin content. Oxygen content can be measured directly or calculated by the oxygen content equation: CaO2 = (Hb x 1.