Prolonged Rate of Decay of Arterial P02 Following Oxygen Breathing in Chronic Airways Obstruction* Garl B. Sherter, M.D.;oO Salim M. Jabbour, M.D.;t Daniel M. Kovnat, M.D.;oO and Gordon L. Snider, M.D., F.G.G.P.! Nine patients with moderate to severe chronic obstructive lung disease were grouped according to results of pulmonary function testing. After a short period of 100 percent oxygen breathing, it took on the average 20 minutes (range -18 to 24 minutes) for their partial pressure of arterial oxygen to return to baseline levels. These data
I
t is frequently desirable to discontinue oxygen
therapy and obtain an arterial blood gas measurement that reflects P02 while the patient is breathing room air. The question of how long to For editorial comment, see page 253
wait after discontinuing the oxygen has remained controversial. Howe and colleagues 1 report elsewhere in this issue a study of serial blood gas measurements in patients with varying types of heart disease after discontinuation of supplemental oxygen. Virtually all of their patients had returned to baseline arterial P02 pressure (Pa02) within seven minutes after discontinuing oxygen breathing and most did so within the first five minutes. None of their patients had significant lung disease by history; pulmonary function tests were not performed. Massaro et al 2 noted a variable rate of return of Pa02 to baseline values after oxygen therapy was discontinued in patients with severe chronic obstructive pulmonary disease (COPD) with carbon dioxide retention. The process took up to 30 minutes. We have studied a carefully characterized population with moderate to severe COPD, and observed that decay of Pa02 to initial values after cessation of °From the Department of Medicine, Boston Veterans Administration Hospital and the Pulmonary Medicine Section, Boston University School of Medicine, Boston. o °Instructor in Medicine, Boston University School of Medicine. tResident in Pulmonary Medicine, Boston VA Hospital. tProfessor of Medicine, Boston University School of Medicine. Supported in part by Pulmonary Special Center of Research Grant No. HL-15063 from the National Heart and Lun~ Institute. Manuscript received August 5; accepted October 18. Reprint requests: Dr. Snider, V A Hospital, 150 S. Huntington, Boston 02130
CHEST, 67: 3, MARCH, 1975
suggest that, after discontinuing supplemental oxygen in patients with chronic airways obstmction, more than 25 minutes should elapse if a blood gas measurement is to reflect with certainty conditions during room air breathing.
a short period of 100 percent oxygen breathing required 20 minutes on the average. MATERIALS AND METHODS
Nine men patients ranging in age from 62 to 81 years (mean 67) with moderate to severe stable COPO, who were undergoing routine physiologic measurements requiring an ind\velling arterial cannula, were selected for shldy. All patients had been heavy smokers and had varying combinations of chronic bronchitis and emphysema. Patient No.2 had also had a right pneumonectomy in the past for carcinoma of the lun~. Spirometry \vas performed using a Stead-Wells water displacernent spirometer, and the forced vital capacity (FVC), forced expiratory volume in one second (FEV 1.0) and the maximal nlid-expiratory flow (FEF:.m-75 pt>r('t>n t) were determined. Functional residual capacity (FRC) was determined by the helium dilution method. Physiologic dead space (V J) ) \vas determined and the steady state carbon monoxide diHusing capacity (D L ) was measured using the Filley~ technique. Tests of uneven ventilation included the end-expiratory nitrogen concentration after breathing 100 percent oxygen for 7 minutes4 and a single breath nitrogen \VaShollt test in which the increase in nitrogen concentration benveen 700 ml and 1200 ml of the expirate was measured after a vital capacity breath of 100 percent oxygen. 5 Repeated determinations of resting, steady state, blood gases were made with the patient breathing rooln air until serial values of the Pa02 and partial pressure of arterial CO 2 (PaCO~d did not vary by more than ± 2 torr. One hundred percent oxygen was administered by demand valve and mouthpiece \vith the patient wearing a nose clip. Arterial blood gases were measured after 20 minutes and then every 5 minutes until a ne\v steady state \vas achieved. Patients \vere returned to room air breathing and arterial blood samples were drawn every 2 to 4 minutes until the patient rehlmed to baseline Pa02 values, and for 6 minutes thereafter. RESULTS
The pulmonary function data for all subjects are shown in Table 1. Although the data for patient no. 2 (status post-pneumonectomy) are presented in
PROLONGED RATE OF DECAY OF ARTERIAL Po 2 : CHRONIC AIRWAYS OBSTRUCTION 259
Table I-Pulmonary Funetion Data on Nine Subjech *
Patient No. 1 2** 3 4 5 6
7 R 9 Mean
TLC Litf'rs
TLC % Predicted8
FVC Litf'rs
9.3 2.8 5.3 5.6 6.9 4.3 9.0 6.8 6.6
176 52 109 116 126 91 180 137 136 134
1.9 1.7 1.8 1.5 2.1 1.2 1.8 3.0 1.2
FVC RV/TLCX100 % Predicted8
54 46 55 45 55 36 51 87 35 52
FRC Liters
FEV 100 Liters
FEF26-71CJ,
7.5 1.5 3.7 4.4 5.2 3.3 7.3 4.4 5.5
0.8 1.3 0.6 0.7 1.1 0.4 0.5 0.9 0.4
0.2 1.1 0.2 0.3 1.1 0.2 0.2 0.2 0.1 0.3
80 39 66 73 70 72 80 56 83 69
Lisee
VI> ml 374 324 300 253 «7 362 232 231 264
DL %N 2 %N 2 ml/min/torr 700-1200t 7 mint 10 11 11 9 8 6 5 11 7 8
7.9 3.5 5.5 6.5 5.8 3.5 2.8 5.0 6.0 5.4
9.1 4.0 8.1 16.0 8.0 14.0 4.6 12 15 10.9
Resting Room Air PaD:! pH PaC 02 torr torr 69 69 60 64 59 48 60 71 49 60
47 47 55 47 52 59 46
7.43 7.41 7.44 7.51 7.39 7.37 7.40
40
7.40
59 51
7.42
• Abbreviatioll8 as in text. **Post right pneumonectomy; data not included in meaDS. tN 2 concentration in the 700-1200 ml of the expirate after a vital rApacity of 100 % 02.' tEnd-t'xpiratory N2 concentration after breathing 100 02 for 7 minutes.·
Tables 1 and 2, they are not included in the statistical analysis. Seven of the remaining eight patients had overdistended lungs with a mean percent predicted 6 total lung capacity (TLC) of 134 and a mean residual volume to TLC ratio (RV/TLC x 1(0) of 69. The mean FVC was 52 percent of predicted. Physiologic dead space (V D) and FRC were markedly elevated in all subjects. Expiratory flow obstruction was severe with all patients having a FEV 1.0 at or below 1 liter. The FEF25-75 p .. n't'nt was well below 1 liter per second in all subjects except patient no. 5. Mean resting steady state Dr. was 8 with a range of 5 to 11 ml/min/torr (nonnal > 17). Tests of inspired gas distribution were severely abnonnaI. Mean values for single breath and 7-minute nitrogen washout detenninations were 5.4 percent (normal < 1.5) and 10.9 percent (nonnal <2.5 percent) respectively. Following arterial cannulation, patients quickly came to resting steady state. The variation in resting baseI:ne Pa02 was small with a mean inter-sample difference of 1.0 torr (SD ± 1.0 torr). Seven of the nine subjects had resting room air Pa02 of less than 60 torr. Eight of the nine had elevated PaC02 (> 45 torr) at rest. Table 2-Re.ting Arterial Blood Ga. Determination. (Torr)
Patient Room air 100% O2 No. PaC02 Pa02 Pa02
1 2 3
69 69 60
6
64 5H 48
7
60
8
71
4
5
9
Mean
4H 60
53 51 53 47 52 75 56 41 74 56
260 SHERTER ET AL
518 518 516 506 511 470 310 499 490 478
Pa02 Minutes aftRr discontinuing supplemental O 2 4
153 83 156 131 168 133 146 127 180 149
8
12 16 18 20 22
107 85 7168 92 72 91 73 109 73 89 71 98 74 98 80 86 63 96 74
78 66 67 63 68 66 71 55 67
75 62 66 61 61 63 68 54 64
72 69 -61 60 66 65 58 58 56 53 62 61 68 68 51 51 62 61
On 100 percent oxygen breathing all except subject 7 raised their Pa02 near or above 500 torr (Table 2). Patient 7 had a repeat Pa02 on !OO percent oxygen breathing of 510, and we presume leak of room air into the breathing circuit at the time of original study. Five of the nine subjects increased their PaC0 2 significantly (mean increase 10.2, range 4 to 16 torr) on 100 percent oxygen. In Figure 1, oxygen decay rate is shown as change in Pa02 from baseline (± SD) plotted against time after discontinuation of supplemental oxygen. Initially the Pa02 fell quickly; at four minutes all subjects had
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MINUTES AFTER RETlIHN TO ROOM AIR BHEATHING 1. Change in arterial P02 from baseline value on the ordinate, plotted against tinle on the abscissa. Time is measlued from the discontinuation of suppletnental oxygen. Data are the means of patient no. 1 and patients nos. 3 through 9. The shaded horizontal band represents ± ISD of the mean difference behveen the average of each subject's baseline values and the individual values. FIGURE
CHEST, 67: 3, MARCH, 1975
Pa02 values of less than 200 torr. Most subjects required 20 minutes for their Pa02 to return to baseline although subject 6 required 24 minutes. Subject 2 (status post-pneumonectomy) had the smallest FRC and, despite the presence of severe airways obstruction, returned to baseline in eight minutes. Our subjects had rather uniformly poor pulmonary function tests and all had prolonged oxygen decay rates. There were no significant relationships between any of the pulmonary function parameters and oxygen decay times. DISCUSSION
Howe and colleagues, l using supplemental oxygen via face mask in patients with presumably normal lungs, reported return of oxygen levels to baseline values within seven minutes of resuming room air breathing. In our population of severe COPD patients receiving 100 percent oxygen, the average time for return to baseline was 20 minutes with a range of 18 to 24 minutes. Although we used a high concentration of inspired oxygen, all Pa02 values were 180 torr or less by the fourth minute after return to room air breathing. At four minutes the mean Pa02 was 149 torr as compared with 229 torr at the start of decay in the data of Howe. Thus, the higher initial Pa02 values attained in the present study appear to have contributed little to the much longer decay time in our subjects. Along the same lines, Massaro et al,2 studied six subjects with COPD receiving oxygen via nasal catheter at 1 to 2 liters per minute How. They found that five patients took up to twenty minutes for Pa02 to fall to baseline levels after discontinuing oxygen, but the sixth patient took 40 minutes to return to his resting level. It appears that factors such as severity of airways obstruction, intrapulmonary distribution of inspired air and size of FRC are more important than the inspired oxygen concentration. Patients with COPD have many areas within their lungs with abnonnal ratios of ventilation to perfusion. Overdistended areas that are ventilated poorly
CHEST, 67: 3, MARCH, 1975
with diminished perfusion are cleared of nitrogen slowly on 100 percent oxygen breathing and likewise are cleared of oxygen slowly on return to room air breathing. Although there is a high oxygen gradient and driving pressure between these spaces and the arterial blood, low perfusion limits the amount of oxygen transferred. The high oxygen containing gas in these underperfused spaces can also move by diffusion and collateral ventilation into better perfused adjacent areas of lung. These air spaces are cleared of their high oxygen-containing gas slowly and provide a source or oxygen to the blood for a prolonged period of time. We conclude that in patients with normal or nearnormal lungs, seven minutes may well be long enough for Pa02 values to have returned to baseline level. However, in patients with COPD who are receiving oxygen therapy, one should wait at least 25 minutes after resuming room air breathing if all patients are to have certainly returned to their baseline level. ACKNOWLEDGMENT: We are indebted to John A. Burnett and David P. Smith for their technical assistance. REFERENCES
1 Howe JP III, Alpert JS, Rickman FD, et al: Return of
2 3
4
5
6
arterial Po~ values to baseline after supplemental oxygen. Chest 67 :256-258,1975 Massaro DJ, Katz S, and Luchsinger PC: Effect of various modes of oxygen administration on the arterial gas values in patients with respiratory acidosis. Br Med J 2:627, 1962 Filley GF, MacIntosh DJ, Wright GW: Carbon monoxide uptake and pulmonary diffusing capacity in nonnal subjects at rest and during exercise. J Clin Invest 33:530, 1954 Cournand A, Baldwin ED, Darling RC, et al: Studies on intrapulmonary mixture of gases ( IV. Significance of pulmonary emptying rate and simplified open circuit measurement of residual air). J Clin Invest 20:681, 1941 Comroe JH, Fowler WS: Detection of uneven alveolar ventilation during a single breath of oxygen. A new test of pulmonary disease. Am J Med 10:408, 1951 Baldwin E de F, Coumand A, Richards DW Jr: Pulmonary insufficiency (I. Physiological classification, clinical methods of analysis, standard values in normal subjects). Medicine 27 :243, 1948
PROLONGED RATE OF DECAY OF ARTERIAL Po 2 : CHRONIC AIRWAYS OBSTRUCTION 261