Evaluation of a single transcutaneous PO2-PCO2 sensor in adult patients

We evaluated a new transcutaneous gas monitor designed to measure simultaneously transcutaneous oxygen (PtcO2) and carbon dioxide (PtcCO2) tensions. A total of 514 simultaneous transcutaneous and arterial gases were obtained in 47 adult ICU patients. Mean PtcCO2 was close (SEE less than 4 torr) to mean PaCO2, but mean PtcO2 was considerably less than mean PaO2. However, PtcO2 changes larger than 15 torr virtually always indicated respective increases or decreases in PaO2. Similarly, PtcCO2 changes larger than 5 torr almost invariably indicated a parallel change in PaCO2. From this study we conclude that monitoring of transcutaneous gases yields reliable trend information on arterial gases and that it is a valuable noninvasive adjunct in the monitoring of gas exchange in adult patients.     

Transcutaneous monitoring during high-frequency jet ventilation

Simultaneous transcutaneous PO2 (PtcO2) and PCO2 (PtcCO2) recordings, done on a patient during high-frequency jet ventilation (HFJV) in thoracic surgery, reflected exactly PaO2 and PaCO2 changes induced by surgical manipulation and by ventilator setting alterations. PtcO2 and PtcCO2 monitoring with a single sensor was valuable during HFJV for the early detection of acute changes in the efficacy of gas exchange and for the correction of the ventilator setting.     

Factors affecting heated transcutaneous PO2 and unheated transcutaneous PO2 in preterm infants

The authors evaluated transcutaneous PCO2 (PtcCO2) and PO2 (PtcO2) electrodes in 25 infants. Their diagnosis were severe hyaline membrane disease (HMD) (18), aspiration syndrome (3), severe hydrops, (3) persistent fetal circulation (6), and the others, congenital pneumonia, congenital plural effusion, pulmonary hemorrhage. In most all, the cardiovascular system was compromised, i.e., PDA with congestive heart failure and shock. PtcO2 electrode was heated to 43.5 degrees C while PtcCO2 electrode was not heated. Simultaneous arterial blood pressure (ABP), pH, arterial blood gases were obtained with the transcutaneous gas measurements. The data were analyzed first dividing all the paired arterial and transcutaneous gas tensions into those with and without cardiovascular drugs (dopamine, isoproterenol), and second, the paired values were divided into those taken (a) during severe acidosis (pH less than 7.25), (b) hypotension (less than 2 SD) of normal, and (c) hypotension and acidosis. These data show: (1) the unheated PtcCO2 and heated PtcO2 accurately correlated with the simultaneous arterial measurements: (2) PtcCO2 reflects tissue PCO2; (3) drugs affect both the PtcCO2 and PtcO2; (4) elevated PtcCO2 dissociating from the simultaneous PaCO2 in neonates with cardiovascular compromise results from decreased tissue perfusion. These data suggest that transcutaneous gas sensors perform dual functions; first, as gas monitors in patients without cardiovascular alterations, and second, in patients with cardiovascular compromise, PtcCO2 reflected tissue perfusion and PtcCO2 monitored oxygen delivery to the tissues.     

Transcutaneous O2 and CO2 monitoring of high risk surgical patients during the perioperative period

The usefulness of noninvasive transcutaneous oxygen (PtcO2) and carbon dioxide (PtcCO2) sensors as well as invasive monitoring of flow and oxygen transport were evaluated in the perioperative period of a small series of high risk surgical patients. We used the pattern of physiological events preceding intraoperative death as the criteria for evaluation of the relative usefulness of these variables. Cardiac output (CO), oxygen delivery (DO2), and O2 consumption (VO2) provided the earliest warning of impending circulatory deterioration and were most useful during critical nonlethal circulatory episodes; these were closely paralleled by the PtcO2 index (PtcO2/PaO2); the PtcCO2 was less sensitive. Heart rate (HR) and mean arterial pressure (MAP) were highly variable with frequent changes unrelated to change in flow and O2 transport.     

Discrepancies between transcutaneous and end-tidal carbon dioxide monitoring in the critically ill neonate with respiratory distress syndrome

PaCO2, transcutaneous PCO2 (PtcCO2), and end-tidal PCO2 (PetCO2) measurements were studied in 12 critically ill neonates. PtcCO2 was measured using a combination CO2/O2 sensor during the routine care of these patients. End-tidal sidestream sampling was performed during blood gas measurement as dictated by the patient's clinical condition. There was a linear correlation between PtcCO2 and PaCO2 (n = 51, r = .71, slope = 0.90). PetCO2 and PaCO2 did not correlate as well (n = 51, r = .52, slope = 0.42). Acidosis negatively affected the correlation between PtcCO2 and PaCO2. When pH was greater than 7.30, r = .75 and slope = 1.28 (n = 38), whereas when pH was less than 7.30, r = .62 and slope = 0.73 (n = 13). The presence or absence of a metabolic acidosis did not have a significant effect on the slopes obtained. PtcCO2 monitoring using combined sensors is a useful and practical means of monitoring in the neonatal ICU, although acidosis affects the ability to correlate transcutaneous and arterial values. End-tidal sidestream measurements are not as clinically useful because they vary due to different ventilation/perfusion relationships in the sick neonate.     

Transcutaneous PCO2 monitoring on adult patients in the ICU and the operating room

Studies were performed on 44 patients who were monitored continuously with transcutaneous carbon dioxide (PtcCO2) sensors. The patients were monitored intermittently with arterial and mixed venous blood gases and full hemodynamic and oxygen transport data. Twenty of the studies were performed intraoperatively. A total of 411 data sets revealed a correlation coefficient, r, between arterial and transcutaneous PCO2 of 0.80 when the patients were not in low flow shock, i.e., cardiac index (CI) greater than 1.5 L/min x M2. On the basis of these data, the authors have found the normal arterial-transcutaneous carbon dioxide gradient, delta CO2, (delta CO2 = PtcCO2 -- PaCO2) to be 23 +/- 11 torr. The PtcCO2 monitor was found to be a valuable trend monitor of arterial CO2 tensions of adults during adequate cardiac function in the ICU and the operating room. Twenty-four data sets were collected while 3 patients were monitored during severe shock (CI less than 1.5 L/min x M2). PtcCO2 trended inversely with changes in CI during shock and did not follow PaCO2 (r = --0.85). During shock, delta CO2 = 61 %/- 25 torr. The severity of shock could be roughly determined by comparing the PtcCO2 values with arterial CO2 tensions.     

Evaluation of noninvasive measurements of oxygenation in stable infants

The accuracy with which transcutaneous measurements of oxygen tension reflect PaO2 in older infants has recently been questioned. We therefore examined the effect of maturation, i.e., age or skinfold thickness, on the accuracy of transcutaneous oxygen tension (PtcO2) and oxygen saturation (StcO2) measurements in 19 infants (age 1 to 61 wk) undergoing elective cardiac catheterization. Twenty-seven simultaneous arterial and transcutaneous measurements revealed a good correlation between PtcO2 and PaO2 (r = .91, slope .77, intercept 3.23 torr). The mean arterial-transcutaneous PO2 difference of 10 torr (range - 15 to 35) was independent of age but was significantly correlated with skinfold thickness (r = .45, p less than .05). There was also a good correlation between StcO2 and SaO2 (r = .95, slope .65, intercept 27.8%). The mean arterial-transcutaneous oxygen saturation of 1.4% (range - 17.3 to 14) was unaffected by age or skinfold thickness. However, neither PtcO2 or StcO2 measurements were accurate in patients with severe hypoxemia; StcO2 consistently overestimated the SaO2 when the SaO2 was below 70%. Thus, in this study the discrepant PtcO2 measurements in older infants were due to increasing skinfold thickness rather than age. PtcO2 monitoring still has an important role in oxygen monitoring and together with StcO2 provides valuable information on oxygenation.     

The transcutaneous oxygen challenge test: a noninvasive method for detecting low cardiac output in septic patients

The transcutaneous partial pressure of oxygen (PtcO?) index has been used to detect low-flow state in circulatory failure, but the value of the transcutaneous oxygen challenge test (OCT) to estimate low cardiac output has not been thoroughly evaluated. The prospective observational study examined 62 septic patients requiring PiCCO-Plus for cardiac output monitoring. Simultaneous basal blood gases from the arterial, central venous catheters were obtained. Cardiac indices were measured by the transpulmonary thermodilution technique at the same time, then the 10-min inspired 1.0 fractional inspired oxygen concentration (FIO?) defined as the OCT was performed. Transcutaneous partial pressure of oxygen was measured continuously by using a noninvasive transcutaneous monitor throughout the test. The values for arterial pressure of oxygen (PaO?) were examined on inspired of 1.0 FIO?. We calculated the PtcO? index = (baseline PtcO?/baseline PaO?), 10-min OCT (10 OCT) = (PtcO? after 10 min on inspired 1.0 O?) - (baseline PtcO?), and the oxygen challenge index = (10 OCT) / (PaO? on inspired 1.0 O? - baseline PaO?). Patients were divided into two groups: a normal cardiac index (CI) group with CI of greater than 3 L/min per m (n = 41) and a low CI group with CI of 3 L/min per m or less (n = 21). The 10 OCT and the oxygen challenge index predicted a low CI (≤ 3 L/min per m) with an accuracy that was similar to central venous oxygen saturation, which was significantly better than the PtcO? index. For a 10 OCT value of 53 mmHg, sensitivity was 0.83; specificity, 0.86; a positive predictive value, 0.92; and a negative predictive value, 0.72 for detecting CI of 3 L/min per m or less. We propose that the OCT substituted for the PtcO? index as an accurate alternative method of PtcO? for revealing low CI in septic patients.     

Transcutaneous and liver surface PO2 during hemorrhagic hypotension and treatment with phenylephrine

Transcutaneous PO2 (PtcO2) and liver surface PO2 (PIO2) were measured in six mongrel dogs during hemorrhagic shock, normotensive shock, and volume resuscitation. Normotension was produced during extreme hypovolemia by an infusion of phenylephrine. PtcO2 and PlO2 were compared to each other and to hemodynamic and oxygen transport variables. PtcO2 and PlO2 correlated well with cardiac index (CI) r = .71 and .86, respectively; n = 60) and with each other (r = .79; n = 60). Heart rate, mean arterial pressure (MAP), and PaO2) correlated less with PtcO2 or PlO2. During the normotensive shock period, PtcO2, PIO2, CI, oxygen delivery (DO2), and oxygen consumption (VO2) were all severely decreased, while PaO2 and MAP were normal and lactic acid concentrations were elevated. It was concluded that PtcO2 follows changes in PlO2 during hypotensive and normotensive low cardiac output shock in mongrel dogs. Low PtcO2 values are associated with low values of PlO2, DO2, VO2, and rising lactic acid concentrations in dogs. These animal data imply that low PtcO2 values encountered in clinical monitoring during anesthesia and surgery may correspond to decreased blood volume, blood flow, and PlO2.     

Comparison of arterial blood gas with continuous intra-arterial and transcutaneous PO2 sensors in adult critically ill patients

We compared the partial pressure of oxygen directly via a continuous intra-arterial probe (PiaO2) and indirectly using a transcutaneous device (PtcO2) with simultaneously obtained arterial blood PaO2. The PiaO2 values were measured using a bipolar oxygen sensor placed through an 18-ga arterial catheter. The PtcO2 values were measured using a transcutaneous O2-CO2 sensor placed on the abdomen. Seven critically ill, hemodynamically stable, ventilator-dependent adult patients were studied. Measurements were obtained at varying concentrations (0.25 to 1.0) of inspired oxygen after a 10-min stabilization. A total of 78 simultaneous values were obtained; by linear regression: PiaO2 = 0.91 PaO2 + 1.39 (r = .98, standard errors of the estimate [SEE] = 18.6); PtcO2 = 0.39 PaO2 + 36.2 (r = .89, SEE = 14.1). To assess these instruments as trend monitors, we compared the changes in simultaneous PaO2, PiaO2, and PtcO2 values; by linear regression: delta PiaO2 = 0.90 delta PaO2 + 3.88 (r = .96, SEE = 27.7); delta PtcO2 = 0.43 delta PaO2 + 5.6 (r = .94, SEE = 15.2). We conclude that, although these instruments correlate highly with the PaO2, the SEE was substantial and therefore may limit their clinical reliability in adults. Any acute or clinically significant change in PiaO2 or PtcO2 should be confirmed with a blood gas PaO2.     

Effect of hypercarbia and shock on transcutaneous carbon dioxide at different electrode temperatures

Transcutaneous CO2 tension (PtcCO2) was measured with heated and nonheated transcutaneous carbon dioxide electrode sensors during hypercarbia and standardized hypovolemic shock in anesthetized dogs. The 95% response times of the PtcCO2 electrode, measured during step increases in FICO2 at four electrode temperatures: 37, 39, 41, and 44 degrees C, were 15, 7.5, 5, and 3.5 min, respectively. During experiments with normal cardiac output, the PtcCO2 correlated well with PaCO2 (r = 0.96). Three PtcCO2 electrode temperatures were tested for response to hypovolemic shock. Data from all PtcCO2 electrodes failed to correlate with PaCO2 during shock. PtcCO2 values rose as cardiac output decreased; there was a good negative correlation (r = -0.95). The 37 and 42 degrees C electrodes were affected by shock at cardiac index values below 2 L/min.M2; but the 44 degrees C probe was not affected until a cardiac index of 1.5 L/min.M2 was reached. Values from the 44 degrees C electrode rapidly returned to preshock values after fluid resuscitation. The response to resuscitation lagged in the 37 and 42 degrees C electrodes and did not return the preshock values until the cardiac index had been normalized for more than 15 min. It was concluded that a PtcCO2 electrode heated to 44 degrees C would be more useful in adult patients because of its faster response to hypercarbia, shock, and resuscitaton. The PtcCO2 values are higher than the corresponding PaCO2 values but they correlate well until the cardiac index (CI) falls below 1.5 L/min.M2; then PtcCO2 values have a strong negative correlation with the corresponding CI values.     

Transcutaneous PO2 monitoring during sodium nitroprusside infusion

To determine the effect of vasodilator-induced hypotension on the relationship between transcutaneous PO2 (PtcO2) and PaO2, six mongrel dogs and 20 patients were monitored before and during infusion of sodium nitroprusside (SNP), with PtcO2 sensors heated to 45 degrees C. First, SNP infusion was used to lower the mean arterial pressure (MAP) of anesthetized dogs, in steps of approximately 25%, to 49 +/- 3 (SD) mm Hg. There were no significant changes in cardiac output, oxygen consumption, PtcO2, PaO2, or PtcO2 index (PtcO2/PaO2). Twenty patients were monitored during general anesthesia. Fifteen patients were being treated for acute hypertension with SNP, and in five patients hypotension was induced with SNP to minimize blood loss. There was a significant decrease in MAP, PaO2, and PtcO2, and a significant increase in alveolar-arterial PO2 difference during SNP infusion. PtcO2 index did not change significantly from its preinfusion value in either group.     

Comparison of noninvasive measurements of carbon dioxide tension during withdrawal from mechanical ventilation

End tidal CO2 tension (PetCO2) and transcutaneous CO2 tension (PtcCO2) were compared with arterial CO2 (PaCO2) before and after withdrawal of mechanical ventilation in 20 patients predisposed to hypercarbia. With stable PaCO2 during mechanical ventilation, the correlation coefficient (r) between PaCO2 and PetCO2 was .9, and between PaCO2 and PtcCO2, .87. PtcCO2 considerably overestimated PaCO2 in three patients who were receiving dopamine. After withdrawal of mechanical ventilation, changes in PaCO2 were closely paralleled by changes in PetCO2 and PtcCO2 (r = .82 and .86, respectively). Nine of 20 patients had an increased PaCO2 of 10 torr or greater. In eight of these, PetCO2 and PtcCO2 rose by at least 5 torr, and in seven, the rise in PetCO2 and PtcCO2 was within 5 torr of the rise in PaCO2. During mechanical ventilation, PetCO2 and PtcCO2 estimated stable PaCO2 with sufficient accuracy for clinical use, except in patients with cutaneous vasoconstriction. After withdrawal of mechanical ventilation, changes in PetCO2 and PtcCO2 were predictive of important PaCO2 increases, warranting continued exploration and evaluation as to their use in monitoring patients predisposed to hypercarbia.     

Transcutaneous carbon dioxide monitoring during neonatal transport

We studied the value of transcutaneous carbon dioxide (PtcCO2) monitoring during neonatal transport. Thirty-two neonates with respiratory distress were alternately enrolled in an experimental group (results of PtcO2 and PtcCO2 available for clinical management) and a control group (results of only PtcO2 available). Although differences were not significant, infants in the experimental group had more changes in the intermittent mandatory ventilation (IMV) settings during transport, and more such infants arrived at the receiving hospital with acceptable pH and PCO2 values. On arrival at the receiving hospital, two patients in the control group had acidosis and hypercarbia and were placed on IMV immediately on arrival. No such patients were encountered in the experimental group. For patients needing IMV during transport, the percentage of study time spent with PtcCO2 measurements in the normal range (35 to 45 torr) was greater for the experimental group (p less than .02). Continuous PtcCO2 monitoring during transport offers the opportunity to further decrease the risks of transporting a critically ill neonate.     

The significance of hypoxemia with low inspired O2 fraction before extubation

Arterial and transcutaneous O2 (PtcO2) and CO2 (PtcCO2) tensions, arterial O2 saturations (SaO2) and P50 values were measured in 47 patients before extubation. In order to unmask ventilation to perfusion (VA/Q) inequality, all variables were obtained without CPAP and with FIO2 of 0.40 as well as with CPAP of 5 cm H2O and FIO2 of 0.40, 0.35, 0.30, 0.25, and 0.21. Eighty to 90% of the patients had PaO2/FIO2 lower than 300 torr and no significant difference in PaO2 or SaO2 was found between those who were successfully extubated (group S, n = 38) and those who required reintubation (group R, n = 9). On the other hand, the patients in group R had significantly lower P50 values, and their PtcO2 values decreased at a greater incline with the lowering of FIO2 than those in group S. Pulmonary dysfunction does not solely explain the need for reintubation in group R. It is obvious that arterial hypoxemia may become more dangerous when the patient has a low P50, anemia, or hypermetabolism. Because PtcO2 seems to uncover these factors, it is a valuable method for predicting the patient's condition before extubation.     

Changes in cardiac output after acute blood loss and position change in man

Thoracic bioimpedance cardiac output (Qtbi) was measured at 1-min intervals in 27 volunteers before, during, and after withdrawing 500 ml (3.7 to 8.5 ml/kg; mean 5.8) of blood. The effects of passive leg raising (PLR) and standing on Qtbi were measured before and after blood withdrawal. Arterial oxygen saturation (SaO2), transcutaneous oxygen tension (PtcO2), mean arterial BP (MAP), and heart rate (HR) were also measured before and after blood withdrawal. Thoracic bioimpedance cardiac index (CI) decreased 18% (0.8 +/- 0.1 L/min.m2, p less than .0001) and stroke volume index (SI) decreased 22% (14.8 +/- 2.7 ml/beat.m2, p less than .0001) after blood withdrawal. HR, MAP, SaO2, and PtcO2 were not significantly different after blood withdrawal. Before blood withdrawal PLR increased CI 6.8% (0.3 +/- 0.1 L/min.m2, p less than .0001); after blood withdrawal PLR increased CI 11.1% (0.4 +/- 0.1 L/min.m2, p less than .0001). PLR can increase stroke volume and cardiac output in hypovolemic humans.     

Simultaneous monitoring of transcutaneous blood gases and heart-rate variability in neonates

Instantaneous heart rate, indices of long-term and short-term heart-rate variability (HRV), and transcutaneous O2 (PtcO2) and CO2 (PtcCO2) tensions were recorded simultaneously on 164 occasions in 16 neonates. There was significant inverse correlation between PtcCO2 and both HRV indices, while no linear correlation was detected between HRV and PtcO2. The heart rate was positively related to PtcCO2 and inversely correlated with PO2. It is suggested that increasing PCO2 decreases medullary pH, thus increasing heart rate and decreasing HRV. We conclude that each of these monitoring variables is unique: the transcutaneous measurements display the efficiency of respiration, whereas the heart-rate patterns reflect the dynamic condition of the autonomic nervous system.     

Use of transcutaneous O2 monitoring in the intraoperative management of severe peripheral vascular disease

Transcutaneous O2 (PtcO2) and CO2 (PtcCO2) monitoring has been used in infants, in critically ill adults, and more recently, in peripheral vascular disease. The present report compares values of centrally placed chest (PtcO2 and PtcCO2) sensors with values of peripherally placed calf (Ptc'O2 and Ptc'CO2) sensors in a patient with severe peripheral vascular disease during performance of an axillofemoral bypass graft. The calf Ptc'O2 values may be expressed as a ratio of their corresponding PaO2 values or as a percentage of the chest PtcO2, i.e., calf Ptc'O2/chest PtcO2 X 100. The ratio reflects local tissue perfusion in the face of fluctuating PaO2 and central PtcO2 values. The data demonstrate that PtcO2 sensors reflect tissue blood flow and oxygenation and, therefore, are useful measures of tissue perfusion, especially during limb revascularization.     

Cardiopulmonary and intracranial pressure changes related to endotracheal suctioning in preterm infants

Although endotracheal (ET) suctioning is performed frequently in sick newborn infants, its effects on cardiorespiratory variables and intracranial pressure (ICP) have not been thoroughly documented in neonates greater than 24 h who were not paralyzed while receiving mechanical ventilation. This study evaluates these changes in preterm infants who required ventilatory assistance. We measured transcutaneous PO2 and PCO2 (PtcO2 and PtcCO2, respectively), intra-arterial BP, heart rate, ICP, and cerebral perfusion pressure (CPP) before, during, and for at least 5 min after ET suctioning in 15 low birth weight infants less than 1500 g and less than or equal to 30 days of age. One infant was studied twice. A suction adaptor was used to avoid disconnecting the patient from the ventilator and to attempt to minimize hypoxemia and hypercapnia during suctioning. The patients were studied in the supine position and muscle relaxants were not used. PtcO2 decreased 12.1% while PtcCO2 increased 4.7% 1 min after suctioning; however, greater increases in mean BP (33%) and ICP (117%) were observed during suctioning. CPP also increased during the procedure. ICP returned to baseline almost immediately, whereas BP remained slightly elevated 1 min after suctioning. Our findings demonstrate that ET suctioning significantly increases BP, ICP, and CPP in preterm infants on assisted ventilation in the first month of life. These changes appear to be independent of changes observed in oxygenation and ventilation.