Blood gas adventures at various altitudes Friedrich Luft Experimental and Clinical Research Center, Berlin-Buch
Mount Everest 8848 M Any point in bird watching here?
Respiration is gas exchange: the process of moving gas across membranes. Internal respiration: O 2 to cells, CO 2 away, CO 2 /O 2 is R/Q=0.8 Hypoxia: low tissue oxygen 1. Hypoxemic hypoxia (Hb not saturated; carrier) 2. Anemic hypoxia (1 g Hb carries 1.34 ml O 2 ; 15 g Hb carries 20 ml O 2 or 200 ml/l) 3. Circulatory hypoxia (stagnant, cardiogenic shock) 4. Histotoxic hypoxia (as in cyanide poisoning) Hypoxia?
Carrying oxygen in the blood is not the same as hypoxia; rather it is hypoxemia (low O2 in blood). Hypoxemia is what we clinically detect in our measurements. The differential diagnosis is not the same. - 1. Decreased barometric pressure (Reinhold Messner) - 2. Alveolar hypoventilation (not breathing) - 3. True shunt as in anatomic shunt, intrapulmonary fistula - 4. A mismatch between ventilation and perfusion at the alveolar level. Hypoxemia?
V Q Q External and internal respiration Prenzelberg (349 M) and 6000 M Sea level 6000 M Atmospheric (760 mm Hg) Mouth 380 PO 2 160 (21%) 79 (21%) PN 2 600 300 PCO 2 <1 0.4 Alveolar air PAO 2 100 22 PAN 2 572 271 PACO 2 40 40 PAH 2 O 47 47 35 15 35 31 Without hyperventilation
At altitude, when the PaO 2 is <35 mm Hg, the PaCO 2 is 10 mm Hg PaCO 2 mm Hg 40 Defense Zone 30 20 6000 M 10 0 8000 M 20 40 60 80 100 120 PaO 2 mm Hg
The Himalayan goose has been sighted at 11 000 meters No one knows exactly how he does it!!!
How do we measure the amount of gas in the blood or tissue? Gas laws: P x V = nr x T (n moles, T K, R constant [0.082 L/atm/mol/K ]) P1 x V1 = P2 x V2 (Boyle) V1/T1 = V2/T2 (Charles) P1/T1 = P2/T2 (Guy-Lussac) PO 2 160 PN 2 600 PCO 2 <1 PO 2 160 PN 2 600 PCO 2 <1 Partial pressures, water in gas phase, diffusion (Dalton), gas solubility (Henry)
1 g Hb carries 1.34 ml O 2 15 g/dl Hb carries 1.34x15 = 20 ml O 2 20 ml O 2 /100 ml blood (20 vol%) Arterial O 2 content 20 vol% (Mixed) venous O 2 content 15 vol% O 2 consumption 250 ml/min CO = 5 L/min O 2 delivery = 1000 ml/min Body O 2 extraction 25% Extraction Heart 90% Kidneys 50% Skin 1% etc CaO 2 = (1.34 x Hb x SaO 2 ) + (0.003 x PaO 2 ) O 2 150 ml/l O 2 200 ml/l
Thank you Professor Fick Würzburg ca. 1871
Fick s Principle CO 2 elimination Mixed PvCO 2 PaCO 2 CO (5 L/min) = 250 200 ml/min CvCOCaO 2 (540 2 (200 ml/l) ml/l) - CaCO - CvO 2 (500 2 (150 ml/l) ml/l) RQ = 200 CO 2 /250 O 2 = 0.8
Ventilation and perfusion: VA/Q relationship lower lobes under ventilated and over perfused Total VA/Q = 1
West Zones in a ventilated patient with a Swan-Gans catheter Zone 1. PA > Pa > Pc PA Zone Zone 2. Pa > PA > Pc 3. Pa > Pc > PA PA Pa Pcwp Q=0 PA PLA Zone 3 lies under LA
Ventilation/perfusion match and mismatch VA/Q = 1 VA/Q = 0 VA/Q = VA/Q = 0
V/Q Ratio 1 (perfect unit) Dead space increased: Emphysema Heart failure Pulmonary embolism PEEP CO 2 O 2 >1 Asthma Chronic bronchitis Pulmonary edema Pneumonia Atelectasis Capillaries around PE <1 CO 2 O 2 Dead space ventilation PaO 2 PaCO 2 CO 2 Venous admixture PaO 2 PaCO 2 n or O 2 When dead Space >50%
Anatomic shunt and patho physiological shunt CvO 2 Qs = CcO 2 -CaO 2 Qt CcO 2 -CvO 2 Qc CcO 2 Qs CaO 2 Qt (total) = Qc + Qs (Qc = perfect, Qs = shunt) COPD, asthma, pulmonary embolism, acute and chronic heart failure, pneumonia are all VA/Q problems
Sat 100% Cell Sat 75% Arterial saturation O 2 Venous saturation 28 40 100 28 40 100 PO 2 PO 2 O 2 O 2 Hb Hb O 2 O 2 O 2 O 2 O 2 Oxygen demand VO 2 = Q x Hb x 13.4 x (SaO 2 - SvO 2 )
Hemoglobin-oxygen saturation curve Venous blood Arterial blood 2,3-Diphosphoglycerate binds with greater affinity to deoxygenated hemoglobin, thus enhancing the ability of RBCs to release oxygen near tissues that need it most. 2,3-DPG is thus an allosteric effector. Nanga Parbat PaO 2 60 mm Hg, Sat = 90%
Arterial oxygenation Hemoglobin-dissociation curve (Hb is a magnet for O 2 ) Curve shifts to the right with: increased [H], increased PaCO 2, increased T, increased 2,3 DPG Curve shifts to the left with: decreased [H], decreased PaCO 2, decreased T, decreased 2,3 DPG Know the Pa50 (28 mm Hg at 50% Sat) and the mixed venous PvO 2 (40 mm Hg at 75% Sat)
Shifted Hb-O 2 curve to left Hb is more sticky, Shifted Hb-O 2 to right, Hb is less sticky, left is good for lung, right is good for tissue 100 Dissolved O 2 3% Christian Bohr and the Bohr effect
Hypoxemia review to this point: PaO 2 problems Blood O 2 consumption 200 ml/l, 1000 ml/min Hypoxemia is due to: 1. Decreased barometric pressure (Reinhold Messner) 2. Alveolar hypoventilation (not breathing) 3. True shunt as in anatomic shunt, intrapulmonary fistula, capillary membrane (ARDS, rarely pulmonary fibrosis) 4. Shunt effect (VA/Q mismatch) This is most common
Va = alveolar volume, Vt = tidal volume, Vd = deadspace volume RR = respiratory rate, MV = minute ventilation Alveolar ventilation and the PaCO 2 Vt 500 ml Va Vt Vd Va Vd Normal MV 6 L V A = 4 L V D = 2 L VA = VE (1-Vd/Vt) Minute ventilation also called VE (total) ventilation
The hyperbolic relationship between PaCO 2 and the alveolar ventilation. You can now estimate the alveolar ventilation from any PaCO 2 value Please comprehend this!!! More importantly, you now gain great insights into pathophysiological mechanisms relevant to gas and acid-base problems (and also know more than most professors)
Alveolar ventilation: PaCO 2 problems expressed anatomically or - physiologically VE = VA + VD (alveolar ventilation + dead space ventilation) VA = k (CCO 2 x total expired air) PaCO 2 Clearance formula (ClCr = UCrV/PCr) Amount of blood cleared of CO 2 per min is ~ 4 L PaCO 2 can increase from increased production, decreased alveolar ventilation, or increased dead space
Causes of alveolar hypoventilation Coma ALS Myasthenia Polio Trauma Bad COPD Respiratory center Ventilatory motor nerves Neuromuscular junction Ventilatory muscles Thoracic cage Lungs Airways, blood vessels, deadspace VA/Q relationship
Acute respiratory acidosis 7.20 60 ph 7.4 [H] 40 7.60 20 ~0.3 mmol per mm Hg 7.00 100 This pan must change!! HCO 3 24 mmol/l (metabolic) PaCO 2 80 mm Hg (respiratory)
Chronic compensated respiratory acidosis 7.00 100 7.20 60 ph 7.4 [H] 40 7.60 20 HCO 3 37 mmol/l (metabolic) PaCO 2 80 mm Hg (respiratory)
The alveolar gas equation: and the A/a gradient PAO 2 = PB-PH 2 O x (FiO 2 ) - PaCO 2 /RQ PAO 2 = 760-47 x (0.21) - (PaCO 2 /0.8) (760-47)x0.21=150 150 - (1.25xPaCO 2 ) 150-50 = 100 mm Hg PAO 2 = 150 - (1.25xPaCO 2 ) A-a = 100 - PaO 2 in mm Hg (< 10 mm Hg)
Dope addict, found down, T 34 C, BP 90/65 mm Hg, HR 100 min, RR 4 min ph = 7.12 PaCO 2 = 80 mm Hg PaO 2 = 29 mm Hg HCO 3 = 22 mmol/l AG = 18 Acid-base disturbance? Acute or chronic? Structural lung disease present? (A-a gradient) Why the slight increase in AG? Does the HCO 3 fit for solely acute respiratory acidosis? 150-(1.25x80)=50
Smoker with pneumonia 63 year old chronically ill woman admitted at 17:00 on Friday with dx of pneumonia. Smoker, possible chronic bronchitis. 48 hr dyspnea, fever, cough, right sided chest pain on inspiration. No cyanosis, skin warm and dry, Temp. 39 o C, rales bilat. Chest xray alveolar pattern in right middle and lower lobes. WBC 17,500, yellow sputum, Gram stain, + diplococci
Clinical information and blood gas slip FIO 2 0.21 BP 140/100 ph 7.51 P 126/min PaCO 2 28 mm Hg RR 36/min PaO 2 46 mm Hg Hb 7 g/dl What is her HCO 3? What is her SaO 2? What is her A-a gradient? 29 = 24x28 = 23.7 HCO 3 PAO 2 = 150 - (1.25xPaCO 2 ) PAO 2 = 150 (1.25x28) = 150-35 = 115, A-a gradient Is 69 mm Hg. Apparently a major example of admixture
Questions about the last patient What is the O 2 carrying capacity in the blood? (Hb 7 g/dl; SaO 2 80%) 0.8x1.34x7=7.5 ml/100 ml blood What is her cardiac output? High (we hope) What is the acid-base disturbance present here? What is internal and external respiration? What is the definition of respiratory failure and does this patient have it? Partial respiratory insufficiency What is this patient s alveolar ventilation? 40% increase Where is this patient s oxygen saturation curve? Left What should be done therapeutically for this patient? Supplemental O 2 ; address anemia
Blood gases above 8000 M 148 ml/l SAT 52% PO 2 28 mm Hg Chronic respiratory alkalosis; Alv. Vent. 16 L/Min
1657 Otto von Guericke demonstrated the power of atmospheric gases Thank you!