Determination of Skin Blood Flow by 133Xe Washout and by Heat Flux from a Heated tc-Po2 Electrode

¹³³Xe washout measurements were used to determine cutaneous and subcutaneous blood flow beneath a specially designed double-thermostated tc-Po₂ electrode. The skin blood flow was determined using thermal methods based on reduced heat dissipation during blood flow cessation. A total of 20 measurements were performed on two healthy volunteers, using the volar side of the right forearm as the experimental area. Cutaneous as well as subcutaneous blood flow increased with increasing electrode temperature. The cutaneous blood flow increased from 12.3 ± 1.3 ml (100 g)^-1-min^-1 (37C) to 49.1 ± 5.4 ml (100 g)^-1.min^-1 (45C) and the subcutaneous values from 20.9 ± 0.2 ml (100 g)^-1 -min^-1 to 57.3 ± 0.5 ml (100 g)^-1 -min^-1. Preheating of the measuring area or injection of papaverine as blood flow accelerator did not increase the maximum blood flow values. A considerable inter-individual difference between cutaneous and subcutaneous blood flow was observed, but in spite of that a good overall correlation between the ¹³³Xe washout measurements and the two thermal flow measurements was found (r = 0.932 and 0.945, respectively). It is concluded that in some cases, but not always, measurements of tc-Po₂ at electrode temperatures of 45C take place on a maximally perfused skin and that it is possible to determine skin blood flow by means of determinations of the heat dissipated from the tc-Po2 electrode to the underlying skin.     

Estimation of Blood Flow in Transcutaneous PO2 Measurements

A method for estimation of the cutaneous blood flow in transcutaneous Po₂ measurements is presented. Recordings of electrode and skin temperature make it possible to compute the effect dissipated to the circulating blood. Eighteen measurements were performed on three healthy volunteers at electrode temperature settings from 37.0°C to 45.0°C. The blood-flow estimates ranged from 0.07 to 0.19 ml·cm^-2·min^-1. At an electrode temperature of 45.0°C the investigations showed a tc-Po₂ value as low as 7 mmHg (0.9 kPa) which, however, corresponded well to the lowest blood-flow estimate determined. The temperature-corrected (37°C) a-Po₂-tc-Po₂ gradient ranged from 50 mmHg to 95 mmHg (6.7–12.6 kPa). The investigations confirm the importance of simultaneous determinations of cutaneous blood flow, capillary temperature and cutaneous oxygen consumption in order to describe the connection between arterial and cutaneous oxygen tension. The cutaneous blood How seems in this connection to be the most important parameter.     

Dermal Heat Transport Analysis for Transcutaneous O2 Measurement

Heat from a transcutaneous oxygen electrode is transmitted locally to the blood beneath it causing a shift in the HbO₂ dissociation curve. This increases the local PO₂, and allows a measurable PO₂, at the skin surface. The temperature effect on the HbO₂, curve must be accounted for in in vivo calibration of Ptco₂, data. To do this, the capillary blood temperature beneath the electrode must be known. A heat balance is written around the capillary blood with heat being conducted in from the electrode and carried out by two means: conduction to deep tissue; and transport away by the flowing capillary blood. The following equation is the steady state solution of the heat transport problem:
T₈ = ±
where Z = ± = 0.17
T₈, = capillary blood temperature
T₁ = electrode temperature
T_o = body temperature
ρ = blood density
P = cutaneous perfusion
δ = dermal capillary depth
k = thermal conductivity of skin
C± = heat capacity of blood
This solution shows the capillary blood temperature may be calculated if the T₁ and T_o are measured and the physiologic constants in 2 are known. 2 is a dimensionless heat transport number which represents the relative importance of perfusion to conduction effects on the deterring T₈, and may be used as a data correlating parameter. Z = 0.17 is obtained using literature values for the physiological constants. This analysis used in conjunction with a mass transport analysis for oxygen will produce a theoretically based correlation scheme for in vivo calibration of heated transcutaneous oxygen electrodes.     

Sweep Gas Flowrate and C02 Exchange in Artificial Lungs

Abstract A simple analysis and graphic result are presented for characterizing the dependence of CO₂ exchange on the sweep gas (ventilating gas) flowrate in artificial lungs. The analysis requires no knowledge of the device-specific mass transfer characteristics of an artificial lung, nor does it require detailed mathematical modeling or computer simulation. Rather, it uses appropriate normalization to establish generic features of the gas flow dependency of CO₂ exchange that are applicable to all artificial lung devices. Principal results are that the transition from relatively gas flow-sensitive to gas flow-insensitive CO₂, exchange occurs at sweep gas flowrates of approximately 40–60 times the CO₂ exchange rate. Achieving a CO₂ exchange rate within 85% of maximal (for a given oxygenator and blood-side conditions) requires a sweep gas flowrate of no less than approximately 50 times the nominal CO₂ exchange rate. When the sweep gas flowrate is less than 20 times the CO₂ exchange rate, CO₂ exchange is highly gas flow dependent and less than one-half the maximal possible rate.     

Reliability of CO2 measurements from the airway by a pharyngeal catheter in unintubated, spontaneously breathing subjects

Although several short communications have appeared describing attempts to record the concentrations of carbon dioxide (CO₂) from the unintubated airway by a catheter placed in the nose, so far only few reports have documented the reliability of the method. To evaluate the reliability of CO₂ measurements by a catheter in the open, unintubated airway during spontaneous respiration, a 12 CH PVC catheter was forwarded through the nostril to the hypopharynx and connected to a capnograph in nine healthy volunteers. Another capnograph was connected to a tightly fitting face mask and simultaneous CO₂ recordings were attained from the two parts of the airway during normoventilation, hyperventilation and rebreathing. A corresponding blood sample was drawn from the radial artery for blood gas analysis. The configurations of the capnograms recorded from the pharyngeal catheter were similar to those recorded from the face mask. The results were analysed by a multifactor analysis of variance. The carbon dioxide tension ( p CO₂) was significantly influenced by degree of ventilation ( P <0.0001), subject ( P <0.0001), measurement site ( P =0.030) and interaction subject-ventilation ( P =0.015). In spite of the significant influence of the measurement site, the difference between end tidal carbon dioxide tension ( P CO₂(ET)) and carbon dioxide tension in arterial blood ( P CO₂(a)) was small. The mean differences between paired measurements ( p CO₂(ET)- p CO₂(a)) were -0.10 kPa±0.41 kPa (mean±SD) for the catheter and -0.20 kPa ±0.43 kPa for the face mask. The study demonstrates that reliable recordings of CO₂ concentrations during spontaneous respiration can be obtained by a thin catheter positioned in the hypopharynx.     

Changes in CMRO2, EEG and concentration of etomidate in serum and brain tissue during craniotomy with continuous etomidate supplemented with N2O and fentanyl

Fourteen patients with supratentorial cerebral tumours were anaesthetized with continuous etomidate infusion (30 or 60 μg kg^-1 min^-1) supplemented with N₂O 67% and fentanyl. Peroperatively cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMR_O2) were measured twice by the Kety and Schmidt method. Simultaneously with the CBF measurements, blood for serum etomidate was sampled and EEG was recorded in 2-min periods in 12 patients. In 10 patients a brain biopsy for etomidate was taken peroperatively and correlated with the other data. The results indicate a dose-dependant increase in scrum etomidate and brain tissue etomidate, a decrease in CMR_o2 and suppression of EEG activity. In individual studies an increase in serum etomidate or a decrease in CMR_o2 correlated to a suppression of the EEG activity, and vice versa . However, the wide variations in these relationships within and between patients make any conclusion regarding CMR_o2 impossible from the EEG recording, infusion rate of etomidate or scrum concentration of etomidate.     

French national quality control for pH and blood gases: Ten years experience

A national quality control program for blood gas analysers has been established in France since 1983. Buffer solutions are used at two levels of pH, Po₂, Pco₂ for testing 950 laboratories using 22 different models. These annual programs inform about the accuracy and the precision of the instruments. They also make the user conscientious to different procedures of quality control. Information is also obtained on the yearly turnover of the types of analysers.     

Influence of Changes in Arterial PCO2 on Cerebral Blood Flow and Cerebral Energy State during Hypothermia in the Rat

In order to study the relationship between arterial P_CO2 and cerebral blood flow (CBF) in hypothermia, the body temperature of artificially ventilated rats was decreased to 22C, and changes in CBF were evaluated from arteriovenous differences in oxygen content (AVD_O2) at Pa_CO2 values of 15, 30, 40 and 60 mm Hg. The results were compared to those obtained at normal body temperature (37C) over the Pa_CO2 range 15–60 mm Hg. Separate experiments were performed to evaluate CBF and CMR_O2 at 22C and a Pa_CO2 of 15 mm Hg, using an inert gas technique for CBF. The tissue contents of phosphocreatine, ATP, ADP, AMP and lactate were measured in hypothermic animals at Pa_co2values of 15, 30 and 60 mm Hg.
The results showed that changes in CBF were of the same relative magnitude in hypothermia and normothermia when Pa co2 was increased from about 35 to about 60 mm Hg. However, with a decrease in Pa_CO2 the reduction in CBF was much more pronounced in hypothermia, and at Pa_CO2 15 mm Hg CBF was less than 20 % of the value measured in normothermic and normocapnic animals. The results of the metabolite measurements gave no evidence of tissue hypoxia in spite of the pronounced reduction in CBF. Although the results demonstrate that the brain of a hypothermic animal is protected against the harmful effects of a lowered CBF, it may not warrant recommending hyperventilation in clinical cases of hypothermia, especially not in patients with arteriosclerosis or cerebrovascular diseases.     

In Vivo vs. In Vitro Response Time of Trancutaneous Po2 Electrodes A comparison of four devices in newborn infants

Four devices for transcutaneous P_o2 (tcP_o2) monitoring have been applied simultaneously in 16 infants. Both during a maximal change in Pa_o2 and during physiological P_o2 variations, the in vivo response time of the electrodes did not show the differences observed in vitro.
We compared A, a prototype of the electrode by Huch, Lübbers and Huch (25 μm Teflon membrane); B, the commercial version of A by Hellige-Draeger (25 μm Teflon); C, the Radiometer TCM I oxygen monitor (25 μm polypropylene); and D, the Roche macrocathode electrode (6 μm Mylar), at 44C.
In vitro the 50% response times were 2.9 ( A ), 4.4 ( B ), 3.7 ( C ), and 7.4 ( D ) sec. The rates of tcP_o2 changes at the midpoint of the response curves were 3.8 ( A ), 2.0 ( B ), 3.0 ( C ), and 1.7 ( D ) kPa/sec.
In vivo during a sudden change from hyperoxaemia (Fi_o2 1.0) to normoxaemia the respective rates were 0.6 ( A ), 0.8 ( B ), 1.1 ( C ) and 1.0 ( D ) kPa/sec. The in vivo 50% response times were 53.3 ( A ), 51.1 ( B ), 46.2 ( C ), and 45.3 ( D ) sec. The lag time between Pa_o2 and tcP_o2 was about one third of this overall response time.
The response to more physiological variations of Pa_o2 (periodic breathing) was not different among the tested electrodes in terms of damping and of delay of the tcP_o2 deflections. In a steady state the correlation of tcP_o2 44C vs Pa_o2 was close ( r = 0.98) with all devices up to 6.1 kPa (456 torr).     

Cutaneous Monitoring of Systemic Pco2 on Patients in the Respiratory Intensive Care Unit being Weaned from the Ventilator

A procedure for measuring the partial pressure of CO₂ in equilibrium with the epidermis was developed at Hewlett-Packard Laboratories. It consists of determining by infra-red absorption techniques the concentration of CO₂ inside a small (50 μl) chamber applied hermetically over and around an epidermal window (2.25 cm²) stripped of its stratum corneum or horny layer. We have applied the procedure to 25 patients in the Respiratory Intensive Care Unit at Stanford Hospital. Only patients scheduled for weaning from the ventilator were selected for the study. The epidermal windows were on the medial aspect of the forearm and were monitored for 3–6 h. Arterial blood samples were periodically withdrawn from the catheterized radial artery and were analysed by conventional means for subsequent comparison with the cutaneous measurements which were recorded in real time at the rate of one a minute. The range of arterial P_co2 values that were measured varied from 3.33 to 9.30 kPa (25 to 70 mmHg) and correlated well with the corresponding cutaneous P_co2 values that were typically higher than the arterial values by 0.7 kPa (5.2 mmHg) with a standard deviation of 0.2 kPa (1.5 mmHg). Some typical recordings of cutaneous P_co2 are shown and discussed.     

Intrathoracic and pulmonary blood volume during CO2-pneumoperitoneum in humans

Background : Induction of CO₂-pneumoperitoneum may have significant effects on systemic and pulmonary haemodynamics. We hypothesized, that intrathoracic (ITBV) and pulmonary blood volume (PBV) are affected during intra-abdominal CO₂-insufflation, which may be pronounced by positional changes of the patient.
Methods : Sixteen anaesthetized patients were studied before, during and after CO₂-pneumoperitoneum for laparoscopic cholecystectomy. A dye indicator technique was used to assess ITBV and PBV. In addition, gas exchange and haemodynamics were recorded.
Results : In the supine position, induction of CO₂-pneumoperitoneum had no effects on ITBV, PBV and cardiac output. Mean systemic arterial pressure increased from 10.9±1.5 kPa (82±11 mmHg) to 12.7±1.5 kPa (95±11 mmHg, P<0.01). In the reverse Trendelenburg position ITBV decreased from 19.8±5.1 ml . kg^-1 to 16.7±3.7 ml . kg¹ ( P <0.05) during CO₂-insufflation, but increased to control values after 20 min. PBV decreased from 4.2±1.2 ml . kg^-1 to 3.4±1.1 ml . kg^-1 (P<0.05) and remained decreased during CO₂-pneumoperitoneum. Calculated venous admixture was unchanged throughout the study. Deflation of CO₂-pneumoperitoneum increased ITBV (22.4±5.2 ml . kg^-1, P<0.05) and cardiac output above control values.
Conclusions : In anaesthetized-paralyzed patients in the reverse Trendelenburg position intra-abdominal CO₂-insufflation is associated with significant alterations of ITBV and PBV. The release of CO₂-pneumoperitoneum is associated with a re-distribution of blood into the thorax.     

A Transcutaneous Po2 Electrode Incorporating a Thermal Clearance Local Blood Flow Sensor

Several possible methods for estimating and recording skin blood flow by thermal clearance are presented. The method chosen for trial with an O₂ electrode was to thermoregulate an annular anode which covered the cathode, surrounding it like a heat shell, and to measure the cooling of the cathode by blood flow, by implanting a thermistor near the surface of the cathode. This device is far less affected by ambient temperature than is simple measurement of heating power in an uninsulated tcP_o2 electrode.     

Respiratory Depressant Action of Tilidine during N2O+O2, Anaesthesia

The respiratory depressant actions of pethidine and tilidine during anaesthesia were compared in 18 surgical patients anaesthetized with N₂O + O₂ after thiopental induction. Five minutes after thiopental, 0.5 mg/kg pethidine or 1.5 mg/kg tilidine were each given intravenously to six patients, the remaining six patients serving as controls.
Minute ventilation, respiratory rate, end-tidal CO₂ and Pco₂ from arterialized venous blood were measured up to 30 min. Pethidine caused the following maximal changes: 0–0.98±0.24 (s.e. mean) 1/min, rate -.5.5 ± 0.7/min, C0_2ET+0.7±0.1 vol % and Pco₂ + 5.7±1.1 mm Hg. These changes occurred within 10 xnin of the injection.
In terms of the above parameters, tilidine caused at least as pronounced a respiratory depression as pethidine. The peak effect of tilidine, however, could not be measured with certainty, since the respiratory depression first became apparent 15 min after the injection, and then increased throughout the study period. The long onset time of tilidine explains our previous failure to demonstrate tilidine-induced respiratory depression.     

Postoperative analgesia by nicomorphine intramuscularly versus high thoracic epidural administration: Effects on ventilatory and airway occlusion pressure responses to CO2

In this study the effects of nicomorphine, administered either intramuscularly or by high thoracic epidural route, on the ventilatory and airway occlusion pressure response to CO₂ were investigated and compared. Twenty-four patients scheduled for thoracic surgery were allocated randomly to postoperative pain relief by i.m. nicomorphine or by high thoracic epidural nicomorphine. The ventilatory response to 5% carbon dioxide was measured in all patients: first 1 day before operation, secondly on the first day after surgery immediately before nicomorphine administration and finally after the administration, at the moment when no further rise in end-tidal Pco₂ (P_etco₂) was measured. Respiratory response was assessed in two ways, by measuring minute ventilation (Ve) and mouth occlusion pressure (p_0.1). There was a significant depression in ventilatory response to CO₂ in the intramuscular group ( P = 0.03) due to nicomorphine as assessed by the slope of V_e vs P_etCO₂ No significant depression was found in the epidural group, irrespective of measurement of V_E, or P_0.1. No significant shift of apnoeie threshold-P_ETco₂ was observed in either group.     

Problems of Calibration and Stabilization of tcPo2 Electrodes

The stirring effect factor Ø, used to correct tcP_o2 readings for the gradient of P_o2 induced in skin by electrode O₂ consumption, was measured with 20 μm cathode electrodes at 44C for various membrane, spacer and electrolyte combinations. Two in vitro models closely resemble skin Ø values: (1) 50% ethylene glycol/water equilibrated with air at 44C, compared to air above this liquid; (2) a mock skin consisting of 25 μm Teflon on foam rubber, in air, dry. Ø varied from 1.01 with 22 μm polypropylene over a 12 μm cellophane spacer, with either aqueous or nonaqueous, chloride-free alkaline electrolyte, to more than 1.10 when 25 μm Teflon was tested without a spacer. Ø may be predicted: Ø = 1+ 0.44 i/(M + 10C + 20) where i is pA/mmHg electrode sensitivity, and M and C are membrane and cellophane thickness, μm. Pressure sensitivity, tested against foam rubber, was minimized by use of a spacer, and often increased with age of Teflon membrane. Drift may result from water vapour movements across membranes if calibration media have P(H20) either too high or too low. Minimum drift on skin was seen after electrode equilibration with P(H₂0) = 58 mmHg. Drift on skin is also caused by a slow effect of P_co2 on reference electrode potential, and this is minimized by buffering electrolyte with K₂B₄O_7.     

The 600 Gram CO2 Absorption Canister: An Experimental Study

Model investigations were performed on a canister containing 600 g of Sodasorb indicator soda lime in a transparent lime compartment used in the Hafnia circle. The CO₂ input was 300 ml per min. Carbon dioxide will be efficiently eliminated from the gas mixture conveyed to the patient connection in this system for at least 4 h, irrespective of the rate of fresh gas flow. There was proportionality between the fresh gas inflow and the duration of effective CO₂ elimination. The relative efficacy of the CO₂ absorption was investigated at different levels in the canister. The temperature in the axis of the lime compartment was 43–52° C, and the temperature of the gas mixture conveyed to the patient attachment 30–35° C.
The colour shift in the lime charge was a reliable indicator of the efficiency of CO₂ elimination. The canister should be replaced when the colour shift has progressed to 2/3 of the lime charge.
The canister contains no disposable parts except for the lime charge. All the components of the circle system should be taken apart and cleaned after each anaesthesia.     

Decrease in plasma volume from intra-abdominal trauma in rats is prevented by H1 but not by H2 histaminergic blockade

Intra-abdominal surgery causes plasma extravasation which, in the rat, is prevented by combined histaminergic H₁ and H₂ blockade. We evaluated the relative importance of H₁ and H₂ blockade in this situation.
Cloralose-anaesthetized Wistar rats were subjected to a standardized abdominal trauma. Mean arterial pressure (MAP), heart rate (HR) and haematocrit (Hct) were monitored and plasma volume (PV) was determined before and 1 h after the trauma.
Compared to non-traumatized rats (n=8), the trauma decreased PV and increased Hct (n=8). MAP decreased during the actual trauma, whereas HR remained stable throughout. In animals given cimetidine 25 mg kg^-1 i.v. just prior to the trauma (n=8), PV, Hct and MAP changed, as in the non-blocked rats. In contrast, pyrilamine, 10 mg kg^-1 completely prevented the decrease in PV and the increase in Hct (n=7), but not the decrease in MAP.
The findings suggest that a histaminergic HI mechanism is of importance for the plasma loss elicited by intra-abdominal trauma.     

From in vitro to in vivo monitoring

In vitro monitoring is inherently invasive with discrete measurements on blood samples and the results are often delayed an hour or more when the analyses are performed in the central laboratory. The delay may be greatly reduced if the analyses are performed near the patient. In vivo monitoring may be non-invasive and may provide continuous real-time data but the accuracy usually does not match that of in vitro measurements. In vivo monitoring therefore finds its application in the detection of trends of change, and it is needed only for quantities that change rapidly and unpredictably and where a suitable therapeutic action is available. In critically ill patients, this applies to the arterial p O₂, p CO₂, and pH, and the mixed venous p O₂. Ideal in vivo monitoring techniques are not available for all these quantities. In the newborn, the arterial p O₂ may be monitored with a transcutaneous p O₂ electrode. In the adult, the arterial p O₂ may be monitored indirectly by monitoring the arterial oxygen saturation with a pulse oximeter and the mixed venous p O₂ by monitoring the mixed venous oxygen saturation with a catheter tip sensor. The arterial p CO₂ may be monitored with a transcutaneous p CO₂ electrode or by capnography, i. e., by monitoring the end-expiratory p CO₂. Other in vivo monitoring techniques such as gastric tonometry for the gastric mucosal pH and thoracic impedance measurement have found some routine application, whereas near-infrared spectrometry for oxy- and deoxyhaemoglobin in the brain, and magnetic resonance spectroscopy for tissue ATP are at the stage of research and development.     

Alfentanil and Skeletal Muscle Circulation, Oxygen Consumption and P50

The preservation of blood flow to skeletal muscles has low priority in the intact organism. If cardiovascular function is disturbed, for example by anesthetic drugs, skeletal muscle circulation diminishes or stops. Skeletal muscle surface pH (m-pH) is a sensitive indicator of muscle cell oxygenation and a fall in m-pH therefore provides an early warning of deterioration in overall cardiovascular performance. In the present study we investigated the peripheral effects of a new short-acting fentanyl derivative, alfentanil. Twelve dogs were anesthetized with a bolus injection of alfentanil 0.16 mg-kg^-1 i.v. M-pH was recorded continuously, while total body oxygen consumption, oxygen transport and P50 were calculated. No changes were found. In the second part of the study, we pretreated six of the dogs with the "calcium antagonist" verapamil 0.5 mg-kg^-1, while the other six dogs served as controls. After a rechallenge dose of alfentanil, we again found the peripheral perfusion sufficient to meet the oxygen demand of the muscles. Side-effects to alfentanil were a decrease in Pao₂, due to an increase in pulmonary shunting of venous blood, and an increase in Paco₂. The changes in pulmonary ventilation-perfusion relationships were, however, not of a magnitude that should cause concern when alfentanil is used in normal subjects.     

Clinical relevance of the B12/N2O interaction A report of a seminar

The interaction of nitrous oxide and vitamin B₁₂ and its implications are not the exclusive territory of any one discipline. The initial discovery was by a chemist but it is of obvious relevance to anaesthetists and intensivists; some complications are neurological others haematological. The interaction provides an extremely important research tool as the first easily available B₁₂-deficient animal model. Finally there are implications for exposure to contaminated atmospheres in hospitals and in industry.