Charles K. Waterson

Cardiac output measurements. A review of current techniques and research

Cardiac output is the volume of blood ejected by the heart per unit time. It is a useful measurement in that it can be used to evaluate overall cardiac status in both critically ill patients and patients with suspected cardiovascular disease. An ideal cardiac output measurement system would have automated continuous output capability, be minimally invasive, accurate, fast, small, low cost and clinically adaptable. This paper presents a theoretical and practical description of the variety of clinical techniques in use today and lists their advantages and shortcomings with respect to the ideal system. Included are the Fick method, indicator dilution techniques, velocity measurements and transthoracic impedance and combined Doppler ultrasound as noninvasive techniques. In addition, several experimental methods are described along with their desirable features and possible constraints. These include intravascular heating/recording, thermistor tracking of cardiac output, ejection fraction measurements and magnetic susceptability plethysmography.

Hemodynamic effects of high-frequency jet ventilation

The hemodynamic effects of high-frequency jet ventilation (HFJV) and conventional ventilation were compared in normovolemic and functionally hypovolemic dogs. In normovolemic animals, no differences in hemodynamic function were found among spontaneous ventilation, conventional ventilation, and HFJV. When venous return was impaired by 15 cm H2O PEEP, cardiac index and stroke index were 25% higher with HFJV than with conventional ventilation (P < 0.05). In another study with PEEP, conventional ventilation was compared to spontaneous ventilation, HFJV synchronized to five different parts of the cardiac cycle, and asynchronous HFJV. Heart rate was 15% lower and mean arterial pressure was 26% lower with conventional ventilation than with HFJV modes (P < 0.05). There were no differences between synchronous and asynchronous HFJV. These results indicate that hemodynamic dysfunction may be less likely with HFJV than conventional ventilation. No advantage of synchronizing jet pulsations to a specific part of the cardiac cycle could be demonstrated.

Comparison of high-frequency jet ventilation with conventional mechanical ventilation in saline-lavaged rabbits

A surfactant-depletion lung-injury model was produced in 37 New Zealand white rabbits by saline lavage. During the next 2 to 3 h, rabbits were ventilated with conventional mechanical ventilation (CMV, group 1), high-frequency jet ventilation (HFJV, group 2), or CMV for 1 h followed by HFJV for 2 h (CMV/HFJV, group 3). Survival until planned termination of the protocol was 56%, 77%, and 63% in groups 1, 2 and 3, respectively. Causes of early demise were usually pneumothorax or metabolic acidosis. There were no statistically significant differences among the groups with respect to survival, incidence of pneumothorax or metabolic acidosis. Arterial oxygenation was more efficient with HFJV (group 2) (P[A-a]O2 = 372 ± 51 torr [mean ± SE] at 2 h) than with CMV (group 1) (P[A-a]o2 = 512 ± 18 torr at 2 h, p < .01). Furthermore, oxygen gas exchange in 3 of 5 group 3 rabbits improved after institution of HFJV. In contrast to previous findings with high-frequency oscillation (HFO), there were no qualitative histologic differences between lungs ventilated with HFJV vs. CMV. Thus, although HFJV produced more efficient gas exchange in this model, it did not improve pulmonary pathology. HFO may be preferable to HFJV in infant respiratory distress syndrome.

Jet pulse characteristics for high-frequency jet ventilation in dogs.

High-frequency jet ventilation (HFJV) delivers pulsed gas streams to the airway either via small diameter catheters placed inside or via a lumen contained within the wall of a tracheal tube (1-3). Pulsation of the HFJV source gas is typically produced by either electronic-controlled solenoid valves or fluidic systems that can provide precise regulation of the pulsed gas stream (1-7). Frequencies may be independently prescribed or electronically coupled to the cardiac QRS complex (5, 6). HFJV reportedly provides efficient ventilation at peak airway pressures and tidal volumes significantly less than those required for conventional ventilation (8). Effectiveness of HFJV has been correlated with high flow amplitude, high initial inspiratory flow, and sufficient expiratory time (I/E) 2 0.3) (7). The purpose of this study was to investigate highfrequency jet characteristics upon ventilation efficiency using constant gas inlet pressures in dogs. Effects of the solenoid-controlled independent variables (pulse duration, frequency, and wave shape) were observed upon dependent variables [airway peak pressure, positive end-expiratory pressure (PEEP),

Airway pressure as a measure of gas exchange during high-frequency jet ventilation

Airway pressure during high-frequency jet ventilation (HFJV) reflects safety, ventilator performance, and gas exchange. The value of airway pressure as a monitoring and control variable for predicting the effectiveness of gas exchange was examined in 2 studies using healthy dogs. In the first study, HFJV was delivered to the airway via an extra lumen in the wall of an endotracheal tube, at a frequency of 150 cycle/min and 30% inspira-tory time. Airway pressures (peak, mean, trough) were measured at various locations, from 5 cm below to 30 cm above the jet port. Pressures measured above the jet were misleading, but the proper measurement distance below the jet remains uncertain. The second study used the same ventilator settings but varied the airway pressure difference between peak and end-expiratory pressures (2, 4, or 6 cm H2O), and either the mean airway pressure (6 or 10 cm H2O) or the positive end-expiratory pressure (0, 5, 10, or 15 cm H2O). The airway pressure difference correlated strongly with efficiency of gas exchange for both CO2 elimination and oxygenation. Mean and end-expiratory pressures showed little influence over moderate ranges, but use of 15 cm H2O of PEEP decreased efficiency of both CO2 elimination and oxygenation, presumably due to increased dead space because of lung overdistension. We conclude that the airway pressure difference, measured as far distal in the airway as is safe and practical, can be useful for monitoring and controlling HFJV.

Clinical factors influencing the selection of high-frequency jet ventilators.

Criteria for selection of high-frequency ventilators, and in particular high-frequency jet ventilators are not significantly different from those for conventional mechanical ventilators. Selection is based upon the design principles and performance characteristics of the ventilator and successful clinical applications that establish clearly its safety and efficacy. The final choice is also influenced by the physical status of the patient, potential physiologic advantages and disadvantages, the necessary requirements of the clinical situation, and the capability of providing adequate oxygenation and ventilation.

Airway movement in dogs during high-frequency jet ventilation.

Cine tantalum bronchograms were recorded from 7 pentobarbital-anesthetized dogs during spontaneous ventilation (SV), high-frequency jet ventilation (HFJV) at 3 frequencies, and intermittent positive-pressure ventilation (IPPV) at 3 combinations of tidal volume (VT) and rate. During SV and the 3 IPPV conditions, the percent inspiratory increase in the diameter of airways greater than 3 mm was the same as in airways less than 3 mm. With HFJV, the percent increase in the diameter of airways greater than 3 mm was twice that of smaller airways. Increases in airway diameter are proportional to transmural, and hence intraluminal airway pressure. These data, therefore, indicate that the contribution of intraluminal pressure changes to intrapulmonary gas transport in small airways during HFJV is less than with either SV or IPPV, and that mechanisms responsible for intrapulmonary gas transport in small conducting airways during HFJV are different than those associated with either SV or IPPV.

Effects of High Frequency Jet Ventilation Design and Operational Variables upon Arterial Blood Gas Tensions

High frequency jet ventilation (HFJV) is but one mode of high frequency ventilation (HFV) that has been utilized successfully to provide respiratory support. In HFJV, a small pulsating jet of gas flowing from a regulated high pressure source is introduced into the airway. Pulsations result from precise regulation of the gas stream by either fluidic or electromechanical control systems.

A flueric respiratory and anesthetic gas analyzer

A unique fluidic-electronic system utilizing flueric jet-edge resonator oscillator sensors has been developed to continuously measure concentrations of oxygen, carbon dioxide, halothane, enflurane, and nitrous oxide in respiratory and anesthetic gases. The sensing unit consists of two flueric jet-edge resonator oscillators operating in parallel. At a constant geometry, flow rate, and temperature, the frequency generated by these oscillators is a function of the molecular weight and specific heat of the gases flowing through them. Oscillator frequencies are detected by pressure transducers. These transducer signals are processed and converted electronically to a.d.c. voltage which is calibrated and displayed in units of constituent concentrations per voltage. The change in frequency of the sensing unit is linear for CO2 and O2 in air and for CO2, N2O, halothane, and enflurane in oxygen. The sensitivity of the sensors is approximately 65 Hz/%CO2 and 20 Hz/%O2 when referenced against air. For these same sensors, sensitivities for CO2, N2O, halothane, and enflurane are approximately 55 Hz/%CO2, 45 Hz/%N2O, 680 Hz/% halothane, and 790 Hz/%enflurane when referenced against oxygen. Time response for the sensor system is 450±10 msec from zero to 90% full scale.

Effect of alterations in frequency, inspiratory time, and airway pressure on gas exchange during high frequency jet ventilation in dogs with normal lungs

High frequency jet ventilation (HFJV) is becoming increasingly useful for providing respiratory support in patients with normal lungs during operative procedures, and also has been advocated as a technique for ventilating patients during cardiopulmonary resuscitation. We studied the effect of frequency, percent inspiratory time (I/E ratio), peak airway pressure, and airway pressure difference (peak-PEEP) during HFJV as operational variables on the efficacy of gas exchange in dogs with normal lungs. We observed that at a constant peak airway pressure and percent inspiratory time, PaCO2 generally increases as frequency rises above 100/min. In contrast, PaCO2 generally decreases as percent inspiratory time is reduced at a constant frequency and peak airway pressure. In addition, increasing peak airway pressure and airway pressure difference are associated with lower levels of PaCO2. Arterial oxygenation was adversely affected by frequencies above 300/min, but was otherwise not influenced by alterations in frequency, percent inspiratory time, or airway pressure.