A R T E R I A L O X Y G E N T E N S I O N D U R I N G A N A E S T H E S I A

JULIE KNUDSEN

It has been found that an increase in the alveolar-arterial P o z gradient is inevitable during anaesthesia both with spontaneous and artificial respiration (Nunn et al. (1965)5, Sykes et al. (1965)9). On the other hand, it has been shown that controlled hyperventilation with large tidal volumes (Sykes et al. ( 1965)g), intermittent hyperinflations (Bendixen et al. (1964)') and high mean pressure in the respiratory cycle (Frumin et al. (1959)5, Bergman (1963)2) may diminish the shunt effect in question.

The present investigation was performed in order to find out whether the degree of hyperventilation and the technique of ventilation commonly used in nitrous-oxide-oxygen-curare anaesthesia administered in a non-rebreathing circuit is able to minimise the shunt effect and elevate the alveolar oxygen tension (PA@ sufficiently to result in a normal arterial oxygen tension (Pa& in the majority of patients, when about 21 per cent oxygen in inhaled gas is administered.

As normal PaOz is in this study regarded PaOz values close to the pre- operative PaOz found in the patient in question or close to the PaOz to be expected according to the age of the patient.

M A T E R I A L A N D M E T H O D S

In 42 patients from 26 to 76 (mean 55) years of age selected at random, PaOz was measured during anaesthesia and operation. None of the patients showed clinical signs of cardiopulmonary disease, but in five patients ECG showed left heart strain and/or myocardial degeneration, and in three patients radiological examination of the chest revealed slight pleural effusion. Twenty- nine of the patients underwent breast operations, seven upper abdominal operation, four lower abdominal operations, one nephrectomy, and one was operated on the neck for a diverticulum of the oesophagus.

From the Department of Anaesthesia, the Finsen Institute, Copenhagen, Denmark.

549

The patients were premedicated with Nembutal 100 mg and Scopolamine 0.3 mg. Anaesthesia was induced with Thiopentone 200400 mg, intubation performed after curarisation, the anaesthesia continued with nitrous-oxide- oxygen in a non-rebreathing circuit (Ruben valve) and the muscular relaxation maintained with d-tubocurarine. The respiration was controlled at rates of 14 to 18 per minute, in five patients by means of a respirator (Anaestalung), the rest of the patients being manually ventilated.

The respiratory minute volumes were chosen beforehand paying regard to the height and weight of the patient, and varied from 754 to lO+$l per minute. The flowmeter setting remained unchanged throughout the anaesthesia. In six cases it was 6 1 N,O + 1% 1 0,, in 28 cases 7 1 N,O + 2 1 0,, in six cases 7 1 N,O + 2 1 0, and in two cases 8 1 N,O + 2 1/3 1 O,, the oxygen percentage in the inhaled air (FIo,) thus varying from 20 to 22 per cent. The oxygen tension in the inhaled gas (PIo,) was measured in 30 cases simultaneously with the arterial sampling and found to vary from 157 to 188 mm Hg (mean 171 mm Hg).

During anaesthesia an arterial sample was drawn from the radial artery from more than 1/2 hour up to 2 hours after the start of ventilation. In 19 of the 42 patients a sample was taken from the femoral artery immediately before induction of anaesthesia, and 1-2 hours after a premedication, which probably does not influence Pao, (Pierce et al. (1965)'j).

P a 0 2 in blood was measured polarographically with Radiometer P o , electrodes (E 5044 and E 5046). As fluctuations of 2-4 mm Hg were found in series of measurements from the same blood sample, generally 4-6 measure- ments were performed, the average being used as the final result.

RESULTS

It was found that PaO, during anaesthesia in 35 out of the 42 cases rose or was unchanged as compared with pre-operative Pao,, or was above what could be expected according to the age of the patients as illustrated in figure 1. The regression line of P a 0 2 against age shown in the figure was calculated from the pre-operative Pao, values measured in this study. I t is found to be inter- mediate between the curve found by Loew et al. (1962)6 based on 390 healthy adults and the curve found by Raine and Bishop (1963)9 based on 70 subjects.

In four cases the Pao, fell as compared with pre-operative Pa02, and in another three cases it was found to be below the regression line.

Table 1 shows the 19 Pao, values measured pre-operatively and the cor- responding values measured during anaesthesia. I t is seen that the values found during anaesthesia are, on an average, 10 mm higher than the average of pre- operatively measured Pao,.

550

9- I50

125

loo

75

mmHg

p-

+

-

&

35 45 55 65 75

FIG. 1.-Arterial P o , plotted against age. PaO, measured during anaesthesia in patients, who had no pre-operative PaO, measure-

o PaO, measured during anaesthesia in patients in whom the PaO, was measured prior

+ PaO, measured prior to anaesthesia. - PaO,/age regression line calculated from the pre-operative PaO, values nicasured.

AGE - YEARS

ment.

to anaesthesia.

PaCO, measured in 30 cases simultaneously with Pa02 during anaesthesia ranged from 20.5 mm Hg to 32.5 mm Hg (mean 25.5 mm Hg). The arterial oxygen saturation (Saoz) measured in seven cases, with P a 0 2 ranging from 61-88 mm Hg during anaesthesia, ranged from 93.5 to 103 per cent (mean 99 per cent).

D I S C U S S I O N

In four of the 42 patients observed, Pa02 had decreased in relation to pre-operative PaOz, maximally by 8 mm Hg. One of these four patients was obese, one obviously given a too small minute volume considering her height and weight, and a third had an upper abdominal operation and was mechanic- ally ventilated. (Totally five out of the seven upper abdominal cases in this series showed an increased PaOz during anaesthesia and operation.) I t is likely that obese patients should be given a higher P r o z to maintain normal PaOz,

55 1

TAULE 1.

Arterial PO, (mm Hg) measured prc-operatively and during anaesthesia in 19 patients.

Age of patients PaO, PaO, (Yeam) pre-operatively during anaesthesia

26. . . . . . . . . . . . . . . . 41. . . . . . . . . . . . . . . . 43. . . . . . . . . . . . . . . . 45. . . . . . . . . . . . . . . . 46. . . . . . . . . . . . . . . . 46 ................ 46 ................ 48 . . . . . . . . . . . . . . . . 48 . . . . . . . . . . . . . . . . 51 . . . . . . . . . . . . . . . . 55. . . . . . . . . . . . . . . . 58 . . . . . . . . . . . . . . . . 61 . . . . . . . . . . . . . . . . 64 . . . . . . . . . . . . . . . . 65 . . . . . . . . . . . . . . . . 65 . . . . . . . . . . . . . . . . 67 ................ 68 . . . . . . . . . . . . . . . . 69 . . . . . . . . . . . . . . . .

104 90 92 87 96 79 90 85 79 83 74 77 79 73 64 76 78 79 78

149 104 112 109 I04 79

123 95 72 9-1 81 71 91 65 83 87 7 7 72 78

Average PaO,. . . . . 82 92

and the same is possibly the case with patients undergoing upper abdominal operations. Whether mechanical or manual ventilation is best suited to mini- mise the shunt effect during anaesthesia seems to need further investigation.

I t should be noticed that three of the four patients in whom Pa02 decreased, had their pre-operative Pa02 below the regression line. However, three other patients having an even lower pre-operative PaOz showed an unchanged or markedly increased Pa02 during anaesthesia. The question whether a decrease in PaOz during anaesthesia generally occurs in the patients who before have the lowest PaOz seems to need further investigation, but it is obvious that the phenomenon is most often seen in elderly patients.

Of the other three patients with Pa02 below the regression line during anaesthesia, a 64-year-old male undergoing a short procedure for gastrostomy, showed a P a 0 2 of 57 mm Hg, the lowest value found in the series. Since there was no pre-operative value in this case, the Pa02 was measured.26 hours aftrr operation, realising that it might still be influenced by the procedure. The patient, who was found clinically all right, had then a P a 0 2 of 55 mm, and a PaCO, of 37 mm Hg. Probably the normal P a 0 2 of this patient is below the

552

value indicated by the regression line. The same might be the case with the last two patients. These patients were both 71 years old, and the PaOz values found were 61 mm and 64 mm Hg. The first patient had a short anaesthesia for removal of an axillary gland. In the second, the PaOz measured on the fifth day after mastectomy was found to be 67 mm Hg.

I t was found by Raine and Bishop (1963)', Ulmer and Reichel (1963)'O and Loew and Thews (1962)4 that PaOz in healthy adults a t rest shows wide individual variations, and that normal PaOz is relatively low in old age.

Whether the normal PaOz is also the optimal one, is open to discussion, but it does not seem reasonable that the PaOz in healthy adults should neces- sarily be higher than usual during anaesthesia, where the oxygen consumption is decreased unless other factors are impairing the tissue oxygenation at the same time.

The study of this limited series did not provide evidence in support of the assumption that an oxygen percentage in the inhaled gas even as low as 21 does not ensure normal PaOz in the majority of patients under the conditions of the study. This does, of course, not imply that normal PaOz should be aimed at in all cases. In patients, e.g. with low blood volume or other cardio- vascular impairments, it is probably advisable to aim at a PaOz somewhat above normal levels. This is found in elderly patients to require a F I o z above 21 per cent.

I t should be taken into consideration that the use of more than 30 per cent 0, in the inhaled gas makes it necessary to supplement the effect of the N,O, e.g. with Halothane or Pethidine. As long as it is not a generally accepted routine to administer oxygen in the postoperative period, this supplementation may enlarge the known (Gordh et al. (1958)4, Nunn (1965)6) decrease in PaOz in the postoperative period, especially after long-lasting operations. This might be of importance during the first postoperative hours when the patient is probably more sensitive to hypoxia because of the increasing metabolic rate at this time.

S U M MARY

Arterial oxygen tension was measured in 42 patients during anaesthesia and closed-chest operations. The patients were curarised and hyperventilated with 20-22 per cent 0, in N,O in a non-rebreathing circuit. The ventilation was mainly manual at rates of 14-18, the minute volume being about 9 1. In almost half of the cases PaOz was measured prior to anaesthesia. The PaOz values found during anaesthesia was in 35 of the 42 patients above or equal to the pre-operative PaOz or normal PaOz for the age of the patient. Of the remaining seven patients, four revealed a decrease of maximally 8 mm Hg as compared with pre-operative values, and in three the PaOz was, though lower

553

than normal for the age, probably close to the usual PaOz of the patients in question.

It was thus found that about 21 per cent 0, in the inhaled gas under the conditions of this study was able to maintain normal arterial PaOz according to the age in the majority of the patients.

REFERENCES

1. BENDIXEN, H. H., BULLWINKEL, B., HEDLEY-WHYTE, J., and LAVER, M. B.: Atelectasis and shunting during spontaneous ventilation in anesthetized patients. Ancsfhesiologv. 1964, 25, 297.

2. BERGMAN, N. A. : Effect of different pressure breathing patterns on alveolar-arterial gradients in dogs. 3. Appl. Physiol. 1963, 18, 1049.

3. FRUMIN, J. M., BERGMAN, N. A., HOLADAY, D. A., RACKOW, H., and SALANITRE, E.: Alveolar-arterial 0, differences during artificial respiration in man. 3. Appl. Physiol. 1959, 14, 694.

4. GORDH, T., LINDERHOLM, H., and NORLANDER, 0.: Pulmonary function in relation to anaesthesia and surgery evaluated by analysis of oxygen tension of arterial blood. Acfa anacsfh. Scandinav. 1958, 2, 15.

5. LOEW, P. G., and THEWS, G. : Die Altersabhangigkeit des arteriellen Sauentoffdruckes bei der berufstatigen Bevolkerung. Klin. Wchnrchr. 1962, 40, 1093.

6. NUNN, J. F.: Influence of age and other factors on hypoxaemia in the postoperative period. Lancet. 1965, 11, 466.

7. NUNN, J. F., BERGMANN, N. A., and COLEMAN, A. J.: Factors influencing the arterial oxygen tension during anaesthesia with artificial ventilation. Brit. 3. Anaesfh. 1965, 37, 898.

8. PIERCE, JOHN A., and GAROPALO, N. L. : Preoperative medication and its effect on blood gases.3.A.M.A. 1965, 194,487.

9. RAINE, J. M., and BISHOP, J. M.: A-a differences in 0, tension and physiological dead space in normal man. 3. Appl. Physiol. 1963, 18,284.

10. SLATER, E. M., NILSSON, S. E., LEAKE, D. L., PARRY, W. L., LAVER, M. B., HEDLEY- WHYTE, J., and BENDIXEN, H. H.: Arterial oxygen tension measurements during nitrous-oxide-oxygen anaesthesia. Ancsthsiology. 1965, 26, 641.

11. SYKES, M. K., YOUNG, W. E., and ROBINSON, B. E.: Oxygenation during anaesthesia with controlled ventilation. Brit. 3. Anacsth. 1965,37,314.

12. ULMER, W. T., and ~ I C H E L , G.: Untersuchungen uber die Altersabhangigkeit der alveolaren und arteriellen Sauerstoff- und Kohlensauredrucke. Klin. Wchnrchr. 1963, 41, 1.