How to be less invasive

Comparative value of peripheral and central venous pCO2 in predicting normal paCO2 during anaesthesia

The ability of venous pCO2 to predict arterial pCO2 within the normal range was tested by measuring pCO2 in blood sampled simultaneously from a large forearm vein (PER), from the superior vena cava (SVC), and from an artery in 35 anaesthetized patients. The relationship between arterial and both venous pCO2's were studied in a first series of 15 patients (ASA physical status class I-II) anaesthetized with methohexitone, fentanyl, pancuronium and nitrous oxide/oxygen, and in a second series of 20 patients scheduled for cardiac surgery anaesthetized with flunitrazepam, fentanyl, pancuronium and nitrous oxide. A marked correlation was found between arterial and both venous pCO2's samples in the normal patients (a/PER: r = 0.922; a/SVC: r = 0.940); in the patients with abnormal cardiovascular status the correlation observed was less pronounced (a/PER: r = 0.501; a/SVC: r = 0.507). In view of the similar correlation coefficients observed from the PER or SVC blood sampling sites, we conclude that the degree of accuracy of the prediction of paCO2 from the venous pCO2's is not modified by the origin of the venous blood. The differences between the coefficients of correlation found in the normal patients and in those with abnormal cardiovascular function indicate that venous pCO2 as estimate of paCO2 appears useful only in subjects with normal haemodynamic status.

A pocket calculator program for noninvasive assessment of cardiorespiratory function

A pocket calculator program is described which facilitates rapid evaluation of pulmonary function at rest and during exercise. It embodies the Fick Principle applied to carbon dioxide; mixed venous carbon dioxide tension is measured by rebreathing, obviating the need for central venous or pulmonary artery catheterisation. The program is in two parts, Gas Phase and Blood Phase, which may be used separately. The variables calculated include tidal volume, CO2 production, oxygen consumption, respiratory quotient, alveolar ventilation and dead space: tidal volume ratio (Gas Phase); bicarbonate concentration, base excess, veno-arterial CO2 content difference, cardiac output, mixed venous oxygen saturation and pulmonary venous admixture (Blood Phase). Use of the program enables these variables to be calculated rapidly and accurately. Accuracy is improved by the application of equations which correct for alveolar-to-blood tension differences, effects of differing hemoglobin concentrations, arterial oxygen saturations and acid-base abnormalities; these calculations are otherwise very lengthy, time consuming and prone to error. The program allows simple noninvasive measurements to be applied to patients with a wide variety of metabolic, cardiac and pulmonary abnormalities in rest and exercise.

Transcutaneous noninvasive monitoring of carbon dioxide tension

We evaluated the accuracy of a new device for continuous noninvasive measurement of cutaneous PCO2. The Hewlett-Packard capnometer (model 47210/HA) works by means of an infrared transducer applied to the forearm over an area of skin that has been stripped of the stratum corneum. Capnometer transcutaneous carbon dioxide pressure (CPCO2) was compared with arterial carbon dioxide pressure (PaCO2) during 60 simultaneously obtained measurements in 13 hemodynamically stable patients. Each patient was studied for 1 1/2 to 5 hours, and a wide range of PaCO2 values (21 to 82 mm Hg) was represented. The data show a clinically significant relationship whereby PaCO2 = CPCO2 - 4.13, with a SE of +/- 2.19 mm Hg. Clinical usefulness of noninvasive cutaneous CO2 monitoring can be foreseen in patients whose ventilatory support is being tapered, in those with respiratory depression caused by various neuromuscular disorders, and in patients with chronic obstructive pulmonary disease and acute respiratory failure. Our results indicate that continuous transcutaneous CPCO2 measurements are safe and accurate and strongly suggest that they can be of clinical usefulness in a select group of hemodynamically stable patients.