Thorax 1993;48:89-90 89 Technical note Thorax: first published as 10.1136/thx.48.1.89 on 1 January 1993. Downloaded from A nomogram for obtaining the of in alveolar air

Amod Karnik, Dilip R Karnad

The relation between the arterial and the (which at 37°C is 47 mm Hg), Fxo2 is the alveolar partial pressure of oxygen (Po2) is fraction of oxygen in inspired air, Paco2 is the used for estimation of venous admixture in partial pressure of carbon dioxide in arterial critically ill patients,' 2 for adjusting the , and R is the . To inspired oxygen concentration to obtain the avoid the tedious task of repeatedly calculat- required arterial Po2 in patients having ing PAO2 by solving the alveolar gas equation, ,;5 as a basis for decid- we attempted to produce a nomogram that ing whether to wean a patient from mechan- takes all these variables into account. Medical Intensive ical ventilation,6 for calculating intrapulmon- Care Unit, Department of ary shunting,7 and for predicting the extent of Medicine, King hypoxaemia during an air flight in patients The nomogram Edward Memorial with chronic obstructive disease.8 The nomogram was drawn on the basis of the Hospital, Parel, Bombay 400 012, India Whereas the arterial Po2 is estimated by blood alveolar gas equation on the assumption that A Karnik gas analysis, the partial pressure of oxygen in the value of R is 0-8. The barometric pressure D R Karnad alveolar gas (PAo2) is obtained from the (PB) is selected on the scale at the bottom of Reprint requests to: alveolar gas equation91O: the left hand portion of the nomogram. A Dr D R Karnad vertical line is plotted upwards from this PB Received 20 August 1991 2 Returned to authors PAO2 = (PB-PH2O)x FIO2- until it intersects the desired FIo2 line. For 18 December 1991 R values of Fio2 that lie between two drawn FIo2 Revised version received 27 January 1992 where PB is the barometric pressure in lines a measured or approximate point is http://thorax.bmj.com/ Accepted 4 February 1992 mm Hg, PH2o is the water vapour pressure selected. From this point a horizontal line is

Nomogram based on the 8 feet alveolar gas equationfor obtaining the alveolar Altitude 0C4 CO 350 400 450 500 550 600 650 700 750 mm Hg above l I partial pressure of oxygen sea level o PI0i-partial 0) co metresometres Alveolar Po2 (PAo2). co N F4 pressure of oxygen in %nl rfn 7Af inspired air; Pacoj- on September 28, 2021 by guest. Protected copyright. partial pressure of carbon dioxide in arterial blood. Correction for the respiratory quotient (R): The correctionfactor is obtained by rounding off 01 Paco2 in kPa to the I nearest whole number and E then dividing this by 10, or E by dividing Paco2 in mm Hg by 10 and 0v rounding this off to the nearest whole number. If R = 0-8, no change in PAo2 is required; if 0 R = 0 7,1 5 times the IC correctionfactor is subtractedfrom the PAo2; 2 if R = 0 9,1 5 times the correction factor is added to PAo2; if R = 1, twice the correction factor is E added to the PAo2- E 0

Barometric pressure Alveolar Po2 I I I I I I mm 8(00 700 600 I Hg 110 50 100 150 200 250 300 350 400 450 mm Hg 90 Karnik, Karnad

plotted, extending rightwards across the nomogram values) was 7 4 mm Hg or 1 kPa vertical scale for the partial pressure of oxygen (95% CI 6-1-8 7 mm Hg or 0 8-1-2 kPa). in inspired air (PIo2, kPa) into the right hand The nomogram is suitable for use up to an portion of the nomogram. The PIo2 value may altitude of 11 000 feet (3353 metres). We Thorax: first published as 10.1136/thx.48.1.89 on 1 January 1993. Downloaded from be read off this vertical (kPa) scale or off the recommend that users draw a vertical line vertical scale on the right in mm Hg if corresponding to the barometric pressure at desired, but is not essential for obtaining the the altitude at which they are working. The PAO2. The horizontal line that is being plotted line for use at sea level is the one already is extended until it intersects the desired drawn at 760 mm Hg. Plotting values on a Paco2 line. A vertical line through this point line that is already drawn increased accuracy. of intersection, drawn upwards (if the Our nomogram therefore gives fairly horizontal line is in the upper half of the accurate values of PAO2 within a reasonable nomogram) or downwards (if the horizontal time. Moreover, its ease of use makes it a line is in the lower half of the nomogram) will convenient alternative to a pocket calculator. give the PAO2 value in kPa and mm Hg. In most clinical circumstances the res- We are grateful to the resident doctors from the department of piratory quotient is assumed to be 0 837; we medicine for their cooperation, and to Peter Gonsalves for therefore assumed this value when preparing helping us to prepare this nomogram. the nomogram. If, however, in a particular patient the respiratory quotient is measured and is not 0-8, a correction is provided (see 1 Nunn JF. Applied respiratory physiology. London: Butter- legend below figure). worths, 1977:274-310. 2 West JB. Ventilation- relationship. Am Rev Respir Dis 1977;116:919-43. 3 Bone RC, Stober G. Mechanical ventilation in respiratory Validation failure. Med Clin North Am 1983;67:599-619. After some practice in using the nomogram 4 Benator FR, Hewlett AM, Nunn JF. The use of isoshunt lines for control ofoxygen therapy. Br J Anaesthesiol 1973; and a non-programmable pocket calculator, 45:711-8. 20 resident doctors were given 10 problems; 5 Gilbert R, Keighley JF. The arterial/alveolar oxygen tension in each case values were provided for PB, FIO2, ratio. An index of applicable to varying inspired oxygen concentration. Am Rev Respir Dis and Paco2, from which the PAO2 had to be 1974;109: 142-5. derived first with the calculator and then with 6 Balk RA, Bone RC. Mechanical ventilation. In: Bone RC, In 1-5 there was a George RB, Hudson LD, eds. Acute respiratory failure. the nomogram. problems New York: Churchill Livingstone, 1987:213-38. constant PB of 760 mm Hg, so these values 7 Shapiro BA, Harrison RA, Walton JR. Clinicalapplication of could be plotted on the vertical line already blood gases. 3rd ed. Chicago: Year Book Medical Pub- drawn on the nomogram at 760 mm In lishers, 1982:209-27. Hg. 8 Apte NM, Karnad DR. Altitude and the arterial to http://thorax.bmj.com/ problems 6-10 the PB varied from 500 to alveolar oxygen ratio (letter). Ann Intern Med 1990; an 112:547-8. 800 mm Hg. These doctors took average of 9 George RB. Pathophysiology ofacute respiratory failure. In: 13 seconds (95% CI 8-07-18-03 seconds) lon- Bone RC, George RB, Hudson LD, eds. Acute respiratory ger over each problem with the nomogram failure. New York: Churchill Livingstone, 1987:1-9. 10 Riley RL, Cournand A. Ideal alveolar air and the analysis of than with the calculator. The mean of the ventilation perfusion relationships in the lung. J Appl residuals (difference between actual and Physiol 1949;1:825-47. on September 28, 2021 by guest. Protected copyright.