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The relationship of conjunctival and arterial blood gas oxygen measurements
Resuscitation, 18 (1989) 31-36 Elsevier Scientific Publishers Ireland Ltd.
31
THE RELATIONSHIP OF CONJUNCTIVAL GAS OXYGEN MEASUREMENTS
SHERMAN “Department
PODOLSKY’, of Emergency
JOHN WERTHEIMERb Medicine
AND ARTERIAL
BLOOD
and SUSAN HARDING”
and bDepartment of Internal Medicine, Division
of
Cardiology, Albert Einstein Medical Center, York and Tabor Roads, Philadelphia, PA 19141 KJ23.A.I (Received January 22nd, 1988) (Revision received November lst, 1988) (Accepted November 30th, 1988)
SUMMARY
Conjunctival oxymetry (Cjo,) measures peripheral tissue oxygen at the conjunctival level. CjO, changes can indicate pulmonary or circulatory conditions leading to shock. Literature review does not define ‘normal’ CjO,/ABG PaO, ratios. We designed a study to measure these ratios. Twenty-two healthy patients undergoing cardiac catheterization had simultaneous Pcjo, and PaO, measurements completed. The range of conjunctival oxygen measurements was from 34 to 68 mmHg with a mean of 50.5 mmHg. The PaO, readings ranged from 65 to 93 mmHg with a mean of 77.1 mmHg. The average PcjO,/Pao, ratio was 0.656 with a range of 0.47-0.93. Thus the Pcjo, is on average 66% of the arterial blood gas PaO,. This ratio of 0.66 can serve as a base for further clinical studies in which Pcjo, is looked at in patients with pulmonary or circulatory illnesses or injuries. Key
words: Conjunctival oxygen measurements -
Normal values
INTRODUCTION
Conjunctival oxymetry is a method of monitoring peripheral oxygen at the level of the palpebral conjunctiva [l]. These measurements reflect peripheral tissue oxygen perfusion [2-51. Changes in perfusion or ventilation, such as blood shunting or pulmonary defects, can be reflected as changes in conjunctival oxygen measurements [6-81. In order to assess critically ill patients with this technique, it is necessary to know conjunctival oxygen values of uncompromised patients and the correlation between conjunctival oxygen and arterial blood gas measurements. Literature 0 1989 Elsevier Scientific Publishers Ireland Ltd. 0300-9572/89/$03.50 Printed and Published in Ireland
32
reviewed does not reveal studies performed which correlate Cjo, and arterial blood gas oxygen measurements in non-compromised hospitalized human subjects. Thus we designed a study to measure these correlations. MATERIALS
Inclusion criterk
All patients over the age of 18 undergoing elective cardiac catheterization (CC) at the Albert Einstein Medical Center Northern Division CC laboratory were eligible for inclusion. Reasons for CC included investigation of known or suspected coronary, valvular, or myocardial disease. The specific intent was to include patients without current hemodynamic compromise who were undergoing elective CC, a population not acutely ill in whom arterial blood gases are routinely obtained. Exclusion
criteria
Exclusion criteria were the following: pat.ients with artificial eyes, patients with known eye infections or diseases, and patients with abnormal eye shapes. These criteria were noted by examining the patient for eye disorders prior to device insertion. The examination was accomplished with a pen light. Also excluded were patients with an allergy to polymethacrylate. METHODS
Patients eligible for inclusion were identified the day before CC by a physician member of the CC team. The study was described to the patient and informed consent was obtained. The following day, the patient was medicated with 25 mg of diphenhydramine and 5 mg of diazepam orally and was taken to the CC laboratory where the patient was again examined for eye anomalies prior to device insertion. If the eye had no known or detected abnormalities, the conjunctival oxymeter (CO) (Orange Medical, Irvine, CA) was inserted by a physician or respiratory therapist. Prior to placement the CO sensor was zeroed with electrolyte solution. This zeroing assured appropriate responses to oxygen changes. The CO was placed on the left sclera such that the oxygen electrode rested against the upper eyelid conjunctiva’s capillary bed. The sensor was then allowed to equilibrate for at least 15 min prior to the first reading. After equilibration, PcjO, was displayed as PO, in mmHg. All patients were breathing room air. Following sensor placement, cardiac catheterization was performed under sterile conditions using local anesthesia, via the femoral vessels. Recording of the CjO, reading was done during simultaneous sampling of aortic blood for arterial blood gas measurements.
33 RESULTS
Twenty-two patients participated in this study. There were 15 males and 7 females. The range of conjunctival oxygen measurements was from 35 to 68 mmHg with a mean of 50.5 mmHg f 10.2 (Table Il. The arterial blood gas PaO, readings ranged from a low of 65 to a high of 93 with a mean of 77.1 f 8.2 mmHg (Table Il. The average PcjO,/PaO, ratio is 0.656 + 0.15 with a range of 0.47-0.93 (Table Il. Thus it appears that the Pcjo, is on average 66% of the arterial blood gas Pao,. DISCUSSION
In the evaluation of patients with potentially critical illnesses or injuries, early changes in vital signs suggestive of shock, such as tachycardia or hypotension, may be equivocal or absent. The traditional determination of TABLE
I
Pcjo,, Pao,, Pcjo,/Pcjo, AND SEX CARDIAC CATHETERIZATION
1 2 3 4 ,; 7 3 9 10 11 12 13 14 15 16 l’? 18 19 20 21 22 Mean with S.D.
IN 23 NON-COMPROMISED
PATIENTS
UNDERGOING
Pcjo,
Pao,
PcjoJPao,
67 60 48 45 47 38 36 40 47 51 35 42 61 59 54 53 48 35 68 67 53 58
72 75 71 65 75 80 65 75 77 82 72 89 87 69 69 93 84 66 84 81 91 74
0.93 0.80 0.67 0.69 0.62 0.48 0.55 0.54 0.61 0.62 0.49 0.47 0.70 0.85 0.78 0.57 0.57 0.53 0.81 0.83 0.58 0.78
M M M F M M M M M F F M F M F M M F M M F M
50.5 + 10.2
77.1 + 8.2
0.656 + 0.13
15 Males 7 Females
Ratio
Sex
34
vital signs, including determination of orthostatic changes, is frequently not sensitive enough to reliably discern occult bleeding or early hypovolemia and concomitant inadequate tissue perfusion. if directly measureable, has been Measurement of tissue oxygenation, proposed as a sensitive alternative. The transcutaneous oxygen electrode, a portable non-invasive method of measuring tissue oxygen tension, operates by heating the skin around an oxygen sensor, thus enhancing dermal capillary flow. An electrode measures the partial pressure of oxygen as it diffuses through a gel contact medium. However, the heating element can cause local burns and different transcutaneous oxygen readings can be elicited secondary to variable skin thickness. Conjunctival oxygen monitoring is a method of measuring tissue oxygenation non-invasively which does not entail the above problem. Conjunctival oxymetry measures tissue oxygen at the paipebral conjunctiva. The conjunctiva is supplied with blood flow from the palpebral branches of the ophthalmic artery, a branch of the internal carotid artery, and superficial cutaneous branches of the facial artery. Because the conjunctiva is only 4 cells thick, it has been suggested that Pcjo, is approximately equal to peripheral tissue PO,. Conjunctival Pao, is measured by an electrode placed against the upper eyelid conjunctiva. The sensor consists of an eye-sized ring conformer with a miniaturized Clark-type platinum cathode silver/chloride anode oxygen electrode. This sensor also contains a miniaturized thermistor which measures the conjunctival temperature. The ring conformer is made from a polymethacrylate material, the same material used to make hard contact lenses. The electrode is positioned to measure oxygen diffusion as blood is delivered to the conjunctival capillaries. The reduced oxygen produces an electrical current which is transferred to a monitor where it is converted to a voltage. Once the sensor is in place, the monitor reads out the voltage as a partial pressure of oxygen in mmHg on a liquid display panel. The monitor responds to conjunctival oxygen changes in approximately 45 s. The objective of our study was to determine conjunctival oxygen readings in a hemodynamically stable population of hospitalized patients. Out-patients were considered but rejected because of the obvious difficulty in having volunteers undergo arterial blood gas determinations. The elective cardiac catheterization population, who routinely have ABG determinations, was then chosen. Review of the literature for specific work correlating ABG, Pao, and PcjO, measurements showed a number of interesting observations. Kwan and Fatt in 1971 were the first researchers to develop a Clark type oxygen electrode which could measure palpebral conjunctival oxygen tensions [I]. They measured both conjunctival oxygen tension and arterial oxygen tensions in adult rabbits who inspired various oxygen mixtures varying from 10 to 100% oxygen. They noted that on room air the mean conjunctival oxygen PO, was 70 Torr and the mean arterial PO, was 93
35
Torr. This gave a Pcjo,/Pao, index of 0.75. Unfortunately, they only made a few measurements on room air, with the majority of measurements made at oxygen tensions >21%. Absolute numbers are not presented, but a graph of their results suggests a linear correlation between Pcjo, and Pao, when the F,o, was greater than room air and in particular when the mean arterial PaO, went above 100 [l]. Limitations of this research that make it difficult to extrapolate their findings to noncompromised humans are the use of rabbits and that the majority of their readings were done at F,O, values greater than room air. In 1984, Fink and Ray studied the relationship between arterial blood gas PO, and Pcjo, in dogs which underwent both arterial blood gas and conjunctival oxygen monitoring and found a mean Pcjo, or 56 + 19 Torr and a mean Pao, of 95 f 20 Torr when the Fio, was 21% (i.e. room air) [8]. This gave a Pcjo,/Pao, ratio of 0.59. This was done on room air but was performed in dogs. In 1983 Isenberg and Shoemaker studied the correlation between conjunctival oxygen readings and arterial blood gases in 19 adult patients who were undergoing surgery [9]. They found that the ratio of the conjunctival and arterial blood gas measurements was approximately 0.48 2 15.9. Again, the applicability of these findings to non-anesthesized, nonintubated patients not inspiring supernormal 0, concentrations is uncertain. In 1985, Isenberg and Green correlated mean conjunctival oxygen tensions with age in 101 healthy adult subjects [lo]. Their studies suggested that a normal conjunctival oxygen is 58 f 14 Torr. However, they did not correlate this with arterial blood gas Pao,. An actual index between the Pcjo, and ABG Pao, is necessary. Unfortunately, the Isenberg and Green study did not make this correlation [lo]. In our study the mean Pcjo, was 50.5 Torr and the mean ratio of Pcjo, to Pao, was 0.66. The Pcjo, is less than in Isenberg and Shoemaker’s studies which may be explained by the mildly sedated state of the patients. Although, the amount of sedation given to this group of patients is small, and patients so sedated are considered to be at resting baseline. Most recently Hess et al studied the relationship between conjunctival PO, and arterial PO, in 16 healthy volunteers under hypoxic, normoxic, and hyperoxic states [ll]. The mean Pcjo, was 66, mean Pao, 99 and mean Pcjo,/PaO, ratio 0.68. Under normoxic conditions, the mean Pcjo,/PaO, ratio was 0.64. This is quite close to our value of 0.656 and demonstrates a similarity between healthy volunteers and not acutely ill hospitalized patients. The PcjoJPao, ratio of 0.66 is of potential use in the clinical setting. If a patient with multiple trauma has a normal arterial blood gas but a low Pcjo, and therefore a low Pcjo,/Pao, ratio, we might then predict that this patient is hypovolemic with peripheral vasoconstriction. This has been suggested by Abraham and Oye who showed that a PcjO,/Pao, ratio of 0.57 or less can be used as a 100%1 sensitive of blood volume loss secondary to trauma [6]. Uses of the ‘normal’ ratio might also be found in non-invasive
36
monitoring of patients who have pulmonary dysfunction. Here one would expect to see a Pcjo, and Pao, correlation. In summary our study measured the Pcjo,/Pao, ratio in 22 volunteers who underwent cardiac catheterization. We found that this ratio was approximately 0.66. This may be of clinical value and serve as a takeoff to further clinical studies in which Pcjo, is looked at in patients with both pulmonary and/or circulatory illnesses. REFERENCES 1
6 7
8
9 10 11
M. Kwan and I. Fatt, A noninvasive method of continuous arterial oxygen tension estimation from measured palpebral conjunctival oxygen tension, Anesthesiology, 35 (19711 309-314. M. Langham, Utilization of oxygen by the component layers of the living cornea, J. Physiol., 117 (19521 461-470. T. Guderson, Trans. Am. Ophthal. Sot., 36 (1938) 207. R.M. Hill and I. Fatt, Oxygen depletion of a limited reservoir by human conjunctiva, Nature, 200 (19631 1011-1012. I. Fatt and T.A. Deutseh, The relation of conjunctival PO, to capillary bed PO,, Crit. Care Med., 11 (19831 446-448. E. Abraham, R.K. Oye and M. Smith, Detection of blood volume deficits through conjunctival oxygen tension monitoring, Crit. Care Med., 12 (1984) 931-934. W.C. Shoemake, S. Fink and W. Ray et al., Effect of hemorrhagic shock on conjunctival and transcutaneous oxygen tensions in relation to hemodynamic and oxygen transport changes, Crit. Care Med., 12 (19841949-952. S. Fink, C.W. Ray, S. McCartney et al., Oxygen transport and utilization in hyperoxia and hypoxia: Relation of conjunctival and transcutaneous oxygen tensions to hemodynamic and oxygen transport variables, Crit. Care Med., 12 (1984) 943-948. S.J. Isenberg and W.C. Shoemaker, The transconjunctival oxygen monitor, Am. J. Ophthalmol., 95 (19831 803-806. S.J. Isenberg and B.F. Green, Changes in conjunctival oxygen tension and temperature with advancing age, Crit. Care Med., 13 (19851 683-685. D. Hess, C. Evans, K. Thomas et al., The relationship between conjunctival PO, and arterial PO,in 16 normal persons, Resp. Care, 31 (19861 191-197.
31
THE RELATIONSHIP OF CONJUNCTIVAL GAS OXYGEN MEASUREMENTS
SHERMAN “Department
PODOLSKY’, of Emergency
JOHN WERTHEIMERb Medicine
AND ARTERIAL
BLOOD
and SUSAN HARDING”
and bDepartment of Internal Medicine, Division
of
Cardiology, Albert Einstein Medical Center, York and Tabor Roads, Philadelphia, PA 19141 KJ23.A.I (Received January 22nd, 1988) (Revision received November lst, 1988) (Accepted November 30th, 1988)
SUMMARY
Conjunctival oxymetry (Cjo,) measures peripheral tissue oxygen at the conjunctival level. CjO, changes can indicate pulmonary or circulatory conditions leading to shock. Literature review does not define ‘normal’ CjO,/ABG PaO, ratios. We designed a study to measure these ratios. Twenty-two healthy patients undergoing cardiac catheterization had simultaneous Pcjo, and PaO, measurements completed. The range of conjunctival oxygen measurements was from 34 to 68 mmHg with a mean of 50.5 mmHg. The PaO, readings ranged from 65 to 93 mmHg with a mean of 77.1 mmHg. The average PcjO,/Pao, ratio was 0.656 with a range of 0.47-0.93. Thus the Pcjo, is on average 66% of the arterial blood gas PaO,. This ratio of 0.66 can serve as a base for further clinical studies in which Pcjo, is looked at in patients with pulmonary or circulatory illnesses or injuries. Key
words: Conjunctival oxygen measurements -
Normal values
INTRODUCTION
Conjunctival oxymetry is a method of monitoring peripheral oxygen at the level of the palpebral conjunctiva [l]. These measurements reflect peripheral tissue oxygen perfusion [2-51. Changes in perfusion or ventilation, such as blood shunting or pulmonary defects, can be reflected as changes in conjunctival oxygen measurements [6-81. In order to assess critically ill patients with this technique, it is necessary to know conjunctival oxygen values of uncompromised patients and the correlation between conjunctival oxygen and arterial blood gas measurements. Literature 0 1989 Elsevier Scientific Publishers Ireland Ltd. 0300-9572/89/$03.50 Printed and Published in Ireland
32
reviewed does not reveal studies performed which correlate Cjo, and arterial blood gas oxygen measurements in non-compromised hospitalized human subjects. Thus we designed a study to measure these correlations. MATERIALS
Inclusion criterk
All patients over the age of 18 undergoing elective cardiac catheterization (CC) at the Albert Einstein Medical Center Northern Division CC laboratory were eligible for inclusion. Reasons for CC included investigation of known or suspected coronary, valvular, or myocardial disease. The specific intent was to include patients without current hemodynamic compromise who were undergoing elective CC, a population not acutely ill in whom arterial blood gases are routinely obtained. Exclusion
criteria
Exclusion criteria were the following: pat.ients with artificial eyes, patients with known eye infections or diseases, and patients with abnormal eye shapes. These criteria were noted by examining the patient for eye disorders prior to device insertion. The examination was accomplished with a pen light. Also excluded were patients with an allergy to polymethacrylate. METHODS
Patients eligible for inclusion were identified the day before CC by a physician member of the CC team. The study was described to the patient and informed consent was obtained. The following day, the patient was medicated with 25 mg of diphenhydramine and 5 mg of diazepam orally and was taken to the CC laboratory where the patient was again examined for eye anomalies prior to device insertion. If the eye had no known or detected abnormalities, the conjunctival oxymeter (CO) (Orange Medical, Irvine, CA) was inserted by a physician or respiratory therapist. Prior to placement the CO sensor was zeroed with electrolyte solution. This zeroing assured appropriate responses to oxygen changes. The CO was placed on the left sclera such that the oxygen electrode rested against the upper eyelid conjunctiva’s capillary bed. The sensor was then allowed to equilibrate for at least 15 min prior to the first reading. After equilibration, PcjO, was displayed as PO, in mmHg. All patients were breathing room air. Following sensor placement, cardiac catheterization was performed under sterile conditions using local anesthesia, via the femoral vessels. Recording of the CjO, reading was done during simultaneous sampling of aortic blood for arterial blood gas measurements.
33 RESULTS
Twenty-two patients participated in this study. There were 15 males and 7 females. The range of conjunctival oxygen measurements was from 35 to 68 mmHg with a mean of 50.5 mmHg f 10.2 (Table Il. The arterial blood gas PaO, readings ranged from a low of 65 to a high of 93 with a mean of 77.1 f 8.2 mmHg (Table Il. The average PcjO,/PaO, ratio is 0.656 + 0.15 with a range of 0.47-0.93 (Table Il. Thus it appears that the Pcjo, is on average 66% of the arterial blood gas Pao,. DISCUSSION
In the evaluation of patients with potentially critical illnesses or injuries, early changes in vital signs suggestive of shock, such as tachycardia or hypotension, may be equivocal or absent. The traditional determination of TABLE
I
Pcjo,, Pao,, Pcjo,/Pcjo, AND SEX CARDIAC CATHETERIZATION
1 2 3 4 ,; 7 3 9 10 11 12 13 14 15 16 l’? 18 19 20 21 22 Mean with S.D.
IN 23 NON-COMPROMISED
PATIENTS
UNDERGOING
Pcjo,
Pao,
PcjoJPao,
67 60 48 45 47 38 36 40 47 51 35 42 61 59 54 53 48 35 68 67 53 58
72 75 71 65 75 80 65 75 77 82 72 89 87 69 69 93 84 66 84 81 91 74
0.93 0.80 0.67 0.69 0.62 0.48 0.55 0.54 0.61 0.62 0.49 0.47 0.70 0.85 0.78 0.57 0.57 0.53 0.81 0.83 0.58 0.78
M M M F M M M M M F F M F M F M M F M M F M
50.5 + 10.2
77.1 + 8.2
0.656 + 0.13
15 Males 7 Females
Ratio
Sex
34
vital signs, including determination of orthostatic changes, is frequently not sensitive enough to reliably discern occult bleeding or early hypovolemia and concomitant inadequate tissue perfusion. if directly measureable, has been Measurement of tissue oxygenation, proposed as a sensitive alternative. The transcutaneous oxygen electrode, a portable non-invasive method of measuring tissue oxygen tension, operates by heating the skin around an oxygen sensor, thus enhancing dermal capillary flow. An electrode measures the partial pressure of oxygen as it diffuses through a gel contact medium. However, the heating element can cause local burns and different transcutaneous oxygen readings can be elicited secondary to variable skin thickness. Conjunctival oxygen monitoring is a method of measuring tissue oxygenation non-invasively which does not entail the above problem. Conjunctival oxymetry measures tissue oxygen at the paipebral conjunctiva. The conjunctiva is supplied with blood flow from the palpebral branches of the ophthalmic artery, a branch of the internal carotid artery, and superficial cutaneous branches of the facial artery. Because the conjunctiva is only 4 cells thick, it has been suggested that Pcjo, is approximately equal to peripheral tissue PO,. Conjunctival Pao, is measured by an electrode placed against the upper eyelid conjunctiva. The sensor consists of an eye-sized ring conformer with a miniaturized Clark-type platinum cathode silver/chloride anode oxygen electrode. This sensor also contains a miniaturized thermistor which measures the conjunctival temperature. The ring conformer is made from a polymethacrylate material, the same material used to make hard contact lenses. The electrode is positioned to measure oxygen diffusion as blood is delivered to the conjunctival capillaries. The reduced oxygen produces an electrical current which is transferred to a monitor where it is converted to a voltage. Once the sensor is in place, the monitor reads out the voltage as a partial pressure of oxygen in mmHg on a liquid display panel. The monitor responds to conjunctival oxygen changes in approximately 45 s. The objective of our study was to determine conjunctival oxygen readings in a hemodynamically stable population of hospitalized patients. Out-patients were considered but rejected because of the obvious difficulty in having volunteers undergo arterial blood gas determinations. The elective cardiac catheterization population, who routinely have ABG determinations, was then chosen. Review of the literature for specific work correlating ABG, Pao, and PcjO, measurements showed a number of interesting observations. Kwan and Fatt in 1971 were the first researchers to develop a Clark type oxygen electrode which could measure palpebral conjunctival oxygen tensions [I]. They measured both conjunctival oxygen tension and arterial oxygen tensions in adult rabbits who inspired various oxygen mixtures varying from 10 to 100% oxygen. They noted that on room air the mean conjunctival oxygen PO, was 70 Torr and the mean arterial PO, was 93
35
Torr. This gave a Pcjo,/Pao, index of 0.75. Unfortunately, they only made a few measurements on room air, with the majority of measurements made at oxygen tensions >21%. Absolute numbers are not presented, but a graph of their results suggests a linear correlation between Pcjo, and Pao, when the F,o, was greater than room air and in particular when the mean arterial PaO, went above 100 [l]. Limitations of this research that make it difficult to extrapolate their findings to noncompromised humans are the use of rabbits and that the majority of their readings were done at F,O, values greater than room air. In 1984, Fink and Ray studied the relationship between arterial blood gas PO, and Pcjo, in dogs which underwent both arterial blood gas and conjunctival oxygen monitoring and found a mean Pcjo, or 56 + 19 Torr and a mean Pao, of 95 f 20 Torr when the Fio, was 21% (i.e. room air) [8]. This gave a Pcjo,/Pao, ratio of 0.59. This was done on room air but was performed in dogs. In 1983 Isenberg and Shoemaker studied the correlation between conjunctival oxygen readings and arterial blood gases in 19 adult patients who were undergoing surgery [9]. They found that the ratio of the conjunctival and arterial blood gas measurements was approximately 0.48 2 15.9. Again, the applicability of these findings to non-anesthesized, nonintubated patients not inspiring supernormal 0, concentrations is uncertain. In 1985, Isenberg and Green correlated mean conjunctival oxygen tensions with age in 101 healthy adult subjects [lo]. Their studies suggested that a normal conjunctival oxygen is 58 f 14 Torr. However, they did not correlate this with arterial blood gas Pao,. An actual index between the Pcjo, and ABG Pao, is necessary. Unfortunately, the Isenberg and Green study did not make this correlation [lo]. In our study the mean Pcjo, was 50.5 Torr and the mean ratio of Pcjo, to Pao, was 0.66. The Pcjo, is less than in Isenberg and Shoemaker’s studies which may be explained by the mildly sedated state of the patients. Although, the amount of sedation given to this group of patients is small, and patients so sedated are considered to be at resting baseline. Most recently Hess et al studied the relationship between conjunctival PO, and arterial PO, in 16 healthy volunteers under hypoxic, normoxic, and hyperoxic states [ll]. The mean Pcjo, was 66, mean Pao, 99 and mean Pcjo,/PaO, ratio 0.68. Under normoxic conditions, the mean Pcjo,/PaO, ratio was 0.64. This is quite close to our value of 0.656 and demonstrates a similarity between healthy volunteers and not acutely ill hospitalized patients. The PcjoJPao, ratio of 0.66 is of potential use in the clinical setting. If a patient with multiple trauma has a normal arterial blood gas but a low Pcjo, and therefore a low Pcjo,/Pao, ratio, we might then predict that this patient is hypovolemic with peripheral vasoconstriction. This has been suggested by Abraham and Oye who showed that a PcjO,/Pao, ratio of 0.57 or less can be used as a 100%1 sensitive of blood volume loss secondary to trauma [6]. Uses of the ‘normal’ ratio might also be found in non-invasive
36
monitoring of patients who have pulmonary dysfunction. Here one would expect to see a Pcjo, and Pao, correlation. In summary our study measured the Pcjo,/Pao, ratio in 22 volunteers who underwent cardiac catheterization. We found that this ratio was approximately 0.66. This may be of clinical value and serve as a takeoff to further clinical studies in which Pcjo, is looked at in patients with both pulmonary and/or circulatory illnesses. REFERENCES 1
6 7
8
9 10 11
M. Kwan and I. Fatt, A noninvasive method of continuous arterial oxygen tension estimation from measured palpebral conjunctival oxygen tension, Anesthesiology, 35 (19711 309-314. M. Langham, Utilization of oxygen by the component layers of the living cornea, J. Physiol., 117 (19521 461-470. T. Guderson, Trans. Am. Ophthal. Sot., 36 (1938) 207. R.M. Hill and I. Fatt, Oxygen depletion of a limited reservoir by human conjunctiva, Nature, 200 (19631 1011-1012. I. Fatt and T.A. Deutseh, The relation of conjunctival PO, to capillary bed PO,, Crit. Care Med., 11 (19831 446-448. E. Abraham, R.K. Oye and M. Smith, Detection of blood volume deficits through conjunctival oxygen tension monitoring, Crit. Care Med., 12 (1984) 931-934. W.C. Shoemake, S. Fink and W. Ray et al., Effect of hemorrhagic shock on conjunctival and transcutaneous oxygen tensions in relation to hemodynamic and oxygen transport changes, Crit. Care Med., 12 (19841949-952. S. Fink, C.W. Ray, S. McCartney et al., Oxygen transport and utilization in hyperoxia and hypoxia: Relation of conjunctival and transcutaneous oxygen tensions to hemodynamic and oxygen transport variables, Crit. Care Med., 12 (1984) 943-948. S.J. Isenberg and W.C. Shoemaker, The transconjunctival oxygen monitor, Am. J. Ophthalmol., 95 (19831 803-806. S.J. Isenberg and B.F. Green, Changes in conjunctival oxygen tension and temperature with advancing age, Crit. Care Med., 13 (19851 683-685. D. Hess, C. Evans, K. Thomas et al., The relationship between conjunctival PO, and arterial PO,in 16 normal persons, Resp. Care, 31 (19861 191-197.