Scott A. Sasse

Variability of cardiac output over time in medical intensive care unit patients

ObjectivesTo determine the amount of spontaneous variability of cardiac output over time in critically ill patients, and to determine the effect of mechanical ventilation on cardiac output variability over time. DesignCase series. SettingMedical intensive care unit in a Veterans Affairs Medical Center. PatientsTwenty-two patients with indwelling pulmonary artery flotation catheters were studied. Two patients were studied twice. InterventionsDuring a 1-hr time period in which no interventions were required or made, thermodilution cardiac output was determined at baseline and then every 15 mins for 1 hr. At each time point, five individual cardiac output measurements were made and a mean was computed. The covariables of heart rate, respiration rate, mean arterial pressure, mean pulmonary arterial pressure, pulmonary artery occlusion pressure, and temperature were also recorded at each time point. Measurements and Main ResultsThe variability of the five cardiac output measurements made at each time point was expressed by calculating for each patient a coefficient of variation of the measurements. The overall mean coefficient of variation of the measurements was 5.8%. The variability of the cardiac output measurements over time was expressed by calculating for each patient a coefficient of variation over time. The overall mean coefficient of variation over time was 7.7%. A subgroup of 15 “covariable stable‘’ patients (defined as those patients with covariables within ±5% of the mean covariable values during the hour) had a mean coefficient of variation over time of 6.4%, whereas “covariable unstable‘’ patients (with >±5% changes in any covariable) had a mean coefficient of variation over time of 9.9% (p < .05). Patients breathing spontaneously had a mean coefficient of variation over time of 10.1%, whereas mechanically ventilated patients had a mean coefficient of variation over time of 6.3% (p < .05). ConclusionsThe spontaneous variability of cardiac output should be considered when interpreting two cardiac output determinations made at separate times. Due to spontaneous variability alone, a patient with a baseline cardiac output of 10.0 L/min would be expected (95% confidence interval) to have a cardiac output range of 9.2 to 10.8 L/min if covariables were stable, and a range of at least 8.8 to 11.2 L/min if covariables were unstable. Patients who were mechanically ventilated displayed less variability than patients who were breathing spontaneously. (Crit Care Med 1994; 22:225–232)

Oxygen Fick and modified carbon dioxide Fick cardiac outputs

Objective: To compare cardiac outputs estimated from the classical oxygen Fick and modified CO2 Fick methods with thermodilution cardiac output. The modified CO2 Fick cardiac output was obtained by replacing the oxygen uptake (Vo2) in the Fick equation with the CO2 production (Vco2) divided by either an assumed or measured value of the respiratory exchange ratio or with an independently determined constant (Crit Care Med 1991; 19:1270‐1277). Design: Criterion standard study. Setting: The medical and surgical intensive care unit (ICU) in a Veterans Affairs Medical Center. Patients: A total of 17 patients (26 studies) and 11 surgical patients (13 studies), predominantly mechanically ventilated using the intermittent mandatory ventilation mode, were studied over a period of 4.3 hrs. Measurements: A respiratory gas exchange monitor was used to measure Vo2, Vco2, and respiratory exchange ratio at 3‐min intervals. Calculations were performed with arterial and venous oxygen saturations measured with both a laboratory cooximeter and bedside pulse and venous reflectance oximeters. In the oxygen Fick method, cardiac output was calculated from Vo2 together with arterial and venous oxygen saturations. In the modified CO2 Fick methods, cardiac output values were calculated from arterial and venous oxygen saturations with Vco2, divided by either: a) an assumed value of the respiratory exchange ratio equal to 0.8 for all patients (method 1); b) the patients measured value of the respiratory exchange ratio (method 2); or c) a constant, determined from an initial, simultaneous measurement of thermodilution cardiac output, Vco2, and oximetry saturations. Data were examined by linear regression analysis and bias and precision calculations. Main Results: Thermodilution cardiac output was more related to cardiac outputs calculated with the 3 modified CO2 Fick methods than to the oxygen Fick cardiac output. Thermodilution cardiac output was closely related to the modified CO2 Fick cardiac output calculated via method 3. For this method, with pulse and venous reflectance oximetry saturations, linear regression yielded an r2 = .85, a standard error of the estimate of 0.88 L/min (n = 111) and a bias and precision of 0.11 and 0.97 L/min, respectively. Thermodilution cardiac output was less closely related to oxygen Fick cardiac output, which, when calculated with pulse and venous reflectance oximetry saturations, yielded an r2 = .50, a standard error of the estimate of 1.47 L/min (n = 128), and a bias and precision of 0.01 and 1.85 L/min, respectively. Conclusions: We conclude from this study that thermodilution cardiac output is more closely related to cardiac output calculated from modified CO2 Fick methods than to oxygen Fick cardiac output. Since cardiac output calculated with the modified CO2 Fick method 3 obviates the difficulties associated with measuring Vo2 accurately and requires neither an assumption of nor measurement of the respiratory exchange ratio, method 3 may prove to be clinically useful for continuous cardiac output monitoring via oximetry in ICU patients. (Crit Care Med 1994; 22:86‐95)

The characteristics of five patients with obstructive Sleep apnea whose apnea-hypopnea index deteriorated after uvulopalatopharyngoplasty

The objective of this retrospective, consecutive, case series design study was to determine the number of unselected patients with obstructive sleep apnea (OSA) who deteriorated after uvulopalatopharyngoplasty (UPPP). Sixteen of 27 patients at the Sleep Clinic at Veterans Affairs Medical Center who underwent UPPP for OSA and who completed both a pre- and postpolysomnogram were studied. After comparing the apnea-hypopnea index (AHI) before and after UPPP, three groups of patients were identified: deteriorators, unchanged, and improvers. In five patients (31%), the AHI increased by more than 10% after UPPP (deteriorators); in four (25%), the AHI showed a change in either direction of less than 10% (unchanged); and in the remaining seven (44%), the AHI decreased by more than 10% after UPPP (improvers). The AHI deteriorated in five of 16 (31%) unselected patients with OSA in our clinic population who underwent UPPP. The mean pre-UPPP AHI was lower in the patients who deteriorated relative to all other patients (P = 0.02). We suggest that patients who undergo UPPP should have a post-UPPP polysomnogram to determine whether they have improved or deteriorated after the procedure and that alternative forms of treatment may be needed in some patients.

Optimal arrangement of magnetic coils for functional magnetic stimulation of the inspiratory muscles in dogs

In an attempt to maximize inspiratory pressure and volume, the optimal position of a single or of dual magnetic coils during functional magnetic stimulation (FMS) of the inspiratory muscles was evaluated in twenty-three dogs. Unilateral phrenic magnetic stimulation (UPMS) or bilateral phrenic magnetic stimulation (BPMS), posterior cervical magnetic stimulation (PCMS), anterior cervical magnetic stimulation (ACMS) as well as a combination of PCMS and ACMS were performed. Trans-diaphragmatic pressure (Pdi), flow, and lung volume changes with an open airway were measured. Transdiaphragmatic pressure was also measured with an occluded airway. Changes in inspiratory parameters during FMS were compared with 1) electrical stimulation of surgically exposed bilateral phrenic nerves (BPES) and 2) ventral root electrical stimulation at C5-C7 (VRES C5-C7). Relative to the Pdi generated by BPES of 36.3/spl plusmn/4.5 cm H/sub 2/O (Mean /spl plusmn/ SEM), occluded Pdi(s) produced by UPMS, BPMS, PCMS, ACMS, and a combined PCMS + ACMS were 51.7%, 61.5%, 22.4%, 100.3%, and 104.5% of the maximal Pdi, respectively. Pdi(s) produced by UPMS, BPMS, PCMS, ACMS, and combined ACMS + PCMS were 38.0%, 45.2%, 16.5%, 73.8%, and 76.8%, respectively, of the Pdi induced by VRES (C5-C7) (48.0/spl plusmn/3.9 cm H/sub 2/O). The maximal Pdi(s) generated during ACMS and combined PCMS + ACMS were higher than the maximal Pdi(s) generated during UPMS, BPMS, or PCMS (p<0.05). ACMS alone induced 129.8% of the inspiratory flow (73.0/spl plusmn/9.4 L/min) and 77.5% of the volume (626/spl plusmn/556 ml) induced by BPES. ACMS and combined PCMS + ACMS produce a greater inspiratory pressure than UPMS, BPMS or PCMS. ACMS can be used to generate sufficient inspiratory pressure, flow, and volume for activation of the inspiratory muscles.