Heart rate variability and hemodynamic alterations in canines with normal cardiac function during exposure to pressure support, continuous positive airway pressure, and a combination of pressure support and continuous positive airway pressure.

Variations in intrathoracic pressure generated by different ventilator weaning modes may significantly affect intrathoracic hemodynamics and cardiovascular stability. Although several investigators have attributed cardiovascular alterations during ventilator weaning to augmented sympathetic tone, there is limited investigation of changes in autonomic tone during ventilator weaning. Heart rate variability (HRV), the analysis of beat-to-beat changes in heart rate, is a noninvasive indicator of autonomic tone that might be useful in the identification of patients who are at risk for weaning difficulty due to underlying cardiac dysfunction. The authors describe HRV and hemodynamics in response to 3 ventilatory conditions: pressure support (PS) 10 cmH2O, continuous positive airway pressure (CPAP) 10 cmH2O, and a combination of PS 10 cmH2O and CPAP 10 cmH2O (PS + CPAP) in a group of canines with normal ventricular function. Six canines were studied in the laboratory. Continuous 3-lead electrocardiographic data were collected during baseline (controlled mechanical ventilation) and following transition to each of the ventilatory conditions (PS, CPAP, PS + CPAP) for analysis of HRV. HRV was evaluated using power spectral analysis to define the power under the curve in a very low frequency range (0.0033 to < 0.04 Hz, sympathetic tone), a low frequency range (0.04 to < 0.15 Hz, primarily sympathetic tone), and a high frequency range (0.15 to < 0.40 Hz, parasympathetic tone). A thermodilution pulmonary artery catheter measured cardiac output and right ventricular end-diastolic volume to describe global hemodynamics. There were significant increases in very low frequency power (sympathetic tone) with a concomitant significant reduction in high-frequency power (parasympathetic tone) with exposure to PS + CPAP. These alterations in HRV were associated with significantly increased heart rate and reduced right ventricular end-diastolic volume. Although there was a small but significant increase in cardiac output with exposure to PS, HRV was unchanged. These data indicate that there was a relative shift in autonomic balance to increased sympathetic and decreased parasympathetic tone with exposure to PS + CPAP. The increase in intrathoracic pressure reduced right ventricular end-diastolic volume (preload). This hemodynamic alteration generated a change in autonomic tone, so that cardiac output could be maintained. Individuals with autonomic and/or cardiovascular dysfunction may not be capable of this type of response and may fail to successfully wean from mechanical ventilation.

Cardiac power output during transition from mechanical to spontaneous ventilation in canines.

Hemodynamic instability during weaning from mechanical ventilation is one proposed cause of weaning failure. This study evaluated cardiac power output (CPO) as an indicator of significant hemodynamic alteration and cardiac reserve during the transition from controlled mechanical ventilation to spontaneous ventilation using three clinical weaning modes. It also compared CPO with commonly used indicators of hemodynamic instability during weaning. The data suggest that CPO is a sensitive indicator of significant hemodynamic alteration and could be used to optimize cardiovascular function during weaning from mechanical ventilation to increase the likelihood of weaning success. Key words: cardiac output, cardiac power output, hemodynamic monitoring, mechanical ventilation

Methodologic issues associated with secretion weight as a dependent variable in research using closed-system suction catheters.

BACKGROUND: Closed-system suction catheters (CSSC) were designed to eliminate the need to disconnect the patient from the ventilator during endotracheal suctioning (ETS). During data collection on an NIH-funded study, it was noted that moisture accumulated on the inside of the CSSC and sleeve when attached to the patient for 30 minutes. Because CSSC are not disconnected, they present unique methodologic problems related to measurement of secretions as a dependent variable in clinical research. OBJECTIVES: To describe a valid, reliable, and practical method for weighing secretions obtained during ETS using a CSSC; and to determine the change in weight of a CSSC after its attachment for 30 minutes to a mechanically ventilated patient. METHODS: After being weighed, a CSSC and sputum trap were attached to the endotracheal tube of a mechanically ventilated adult and remained attached for 43 minutes (30 minutes to allow positive end expiratory pressure and oxygenation levels to return to normal and 13 minutes to mimic the time frame for the ETS procedure used in another study). No ETS occurred. The CSSC and sputum trap were then removed and reweighed. RESULTS: A convenience sample consisted of 50 adults who were critically ill and mechanically ventilated. Independent variables included tidal volume, pressure support, body temperature, and respiratory rate. The dependent variable was wet weight of the CSSC, determined by subtracting the preprotocol catheter weight from the postprotocol catheter weight. The mean wet weight for all catheters was 0.5142 +/- 0.1215 grams. In a subset of 37 patients, two wet weights (74 paired observations) were determined. The mean wet weight for these catheters was 0.54014 +/- 0.1404 grams. The paired wet weights were statistically different (t = 2.433; df = 36; p = 0.02). Pearson correlation coefficients and beta coefficients were computed. While tidal volume and pressure support were highly correlated (r = 0.678; p = 0.011), there were no other statistically significant associations. CONCLUSIONS: The amount of secretions is a common dependent variable in ETS research. During the time that CSSC are attached to the patient and ventilator, moisture from either the ventilator's humidification system or the patient accumulates in the CSSC. This wet weight is not actually part of the secretions retrieved during ETS and should not be considered in the actual weight of secretions. Further study on the determinants of wet weight is warranted.

A comparison of hemodynamic changes during the transition from mechanical ventilation to T-piece, pressure support, and continuous positive airway pressure in canines.

The immediate transition from positive pressure mechanical ventilation to spontaneous ventilation may generate significant cardiopulmonary hemodynamic alterations based on the mode of weaning selected, particularly in individuals with preexisting cardiac dysfunction. The purpose of this study was to compare hemodynamic responses associated with the initial transition to 3 modes of ventilator weaning (spontaneous ventilation/T-piece, pressure support [PS], and continuous positive airway pressure [CPAP]). Right ventricular hemodynamic responses were evaluated with a thermodilution pulmonary artery catheter; while left ventricular hemodynamic responses were measured by a transducer-tipped Millar catheter and conductance catheter. Two groups of canines were studied. Group 1: normal biventricular function (n = 10) and group 2: propranolol-induced biventricular failure (n = 10). Dependent variables were measured at baseline on controlled mechanical ventilation (MV) and following the initial transition to each of 3 randomized spontaneous ventilatory conditions: T-piece, PS 5 cmH2O, and CPAP 5 cmH2O. Both groups significantly increased cardiac output in response to T-piece. Right ventricular stroke work was also significantly increased with T-piece and CPAP in both groups of subjects. Left ventricular response depended on baseline ventricular function. Baseline ventricular function influenced hemodynamic response to the immediate transition from mechanical to spontaneous ventilation. There were also differential hemodynamic responses based on the ventilatory mode. Consideration of baseline cardiac function may be an important factor in the selection of an appropriate mode of spontaneous ventilation following controlled MV.

Effect of short-duration hyperventilation during endotracheal suctioning on intracranial pressure in severe head-injured adults.

A repeated measures randomized within-group design was used to determine the effectiveness of controlled short-duration hyperventilation (HV) in blunting the increase of intracranial pressure (ICP) during endotracheal suctioning (ETS). A multimodal continuous real-time computerized data acquisition procedure was used to compare the effects of two HV ETS protocols on ICP, arterial pressure, cerebral perfusion pressure (CPP), heart rate, and arterial oxygen saturation in severe head-injured adult patients. The results indicated that short-duration HV for 1 minute, which decreases the PaCO2, reduced ETS-induced elevations in ICP while maintaining CPP. However, it is not clear whether short-duration HV is neuroprotective, particularly in ischemic regions of the brain. Therefore, before a change in practice is implemented on the use of short-duration HV as a prophylactic treatment against ETS-induced elevations in ICP, additional questions on cerebral oxygen delivery and uptake need to be answered.

Head-injured adults: recommendations for endotracheal suctioning.

Endotracheal suctioning (ETS) is a conventional nursing intervention used to decrease pulmonary complications in the critically ill patient. ETS provides a particular dilemma for the head-injured patient because it increases intracranial pressure and may put the patient at increased risk for intracranial hypertension and cerebral ischemia. Research in endotracheal suctioning targets understanding the ETS response in the severely head-injured patient. This article reviews the major research focused on suctioning the head-injured patient. The guidelines for practice based on this research include preoxygenating patients prior to suctioning, limiting suction duration to 10 seconds, limiting suction passes to 1-2 per procedure, using hyperventilation with caution, not rotating the head, keeping negative suction pressure under 120 mm Hg, and not using suction catheters with outer to inner diameter ratios greater than .50.

Endotracheal suctioning in adults with head injury.

The purpose of this study was to determine the method of endotracheal suctioning (ETS) that resulted in the least compromise to the cerebrovascular status of adult patients with severe head injuries. A two-group (two vs three ETS), two-protocol (100% tidal volume [VT] vs 135% VT) design was used. The dependent variables were mean intracranial pressure (MICP), mean arterial pressure (MAP), cerebral perfusion pressure (CPP), heart rate (HR), and oxygen saturation (SaO2). By random assignment, 14 subjects were in the two-ETS group and 16 subjects were in the three-ETS group. Intracranial pressure response to ETS in these patients with head injury can be characterized as falling into three patterns: (1) a rise in baseline beginning with ETS and continuing throughout the ETS sequences; (2) intracranial pressure spiking during the suctioning component of the protocol; (3) a combination of both a rising baseline and spiking. There was a significant (p less than or equal to 0.001) increase from baseline for both two- and three-ETS groups with both hyperoxygenation protocols (100% VT vs 135% VT) for MICP, MAP, HR, and CPP. No significant difference was found for SaO2 for either of the protocols regardless of number of suction passes. No significant differences were found between two- and three-ETS groups for any of the dependent variables. All groups, however, regardless of number of suction passes, demonstrated a cumulative increase in MICP, MAP, and CPP with each consecutive suction sequence.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of repeated endotracheal suctioning on arterial blood pressure.

The purpose of this study was to examine the effects of three lung hyperinflation/suction sequences on mean arterial pressure (MAP). The results indicate both lung hyperinflation (LHS) and suction sequences (SS) significantly increased MAP (p = .05) from baseline. There was a mean increase in MAP of 13.72 mm Hg over the three lung hyperinflation/suction sequences. The results showed a significant increase in MAP from baseline between SS1 to LHS2 (3.62 mm Hg), from LHS2 to SS2 (4.36 mm Hg), and from LHS3 to SS3 (2.84 mm Hg) at p = .05. The increase in MAP was cumulative with each successive lung hyperinflation/suction sequence. Consequently, the number of repeated lung hyperinflation/suction episodes should be limited to only those necessary to maintain airway patency.

Effect of lung hyperinflation and endotracheal suctioning on heart rate and rhythm in patients after coronary artery bypass graft surgery.

We examined the effect of lung hyperinflation and suction on PaO2, heart rate, and rhythm in patients after coronary artery bypass graft surgery (N = 26). Three lung hyperinflation breaths, at one of five randomly ordered volumes (tidal volume, 12 cc/kg, 14 cc/kg, 16 cc/kg, or 18 cc/kg of lean body weight) were delivered, by a ventilator (fraction of inspired oxygen 1.0), followed by 10 seconds of continuous suction. Lung hyperinflation and suctioning were repeated three times. Repeated-measures analysis of variance revealed a statistically significant increase (p = 0.000) in PaO2 immediately after the third suction pass, which was volume dependent (p = 0.009). A statistically significant (p less than 0.001) increase in heart rate from baseline occurred over the three lung hyperinflation-suctioning sequences that was not volume dependent. The mean increase in heart rate was 6.8 beats/min. The majority of rhythm changes for lung hyperinflation and suctioning were from normal sinus rhythm to sinus tachycardia. Suction was associated with a greater incidence of rhythm (53.9%) and arrhythmia (80.8%) changes. The most frequent arrhythmia was premature atrial contraction.

Endotracheal suctioning-induced heart rate alterations.

The purpose of the experimental study was to identify mechanical and/or neural factors contributing to the heart rate alterations associated with endotracheal suctioning (ETS). Effects of catheter insertion alone and catheter insertion with the application of negative pressure on the partial pressure of arterial oxygen (PaO2) and heart rate prior to and following vagal blockade with atropine were examined. Eleven newborn swine less than 24 h of age were randomly exposed to (a) suction catheter insertion alone and (b) suction catheter insertion and the application of suction. Preoperatively, the subjects were anesthetized with ketamine and xylazine (.05 mL/kg). The ECG in lead II was recorded on a Gould 2600S recorder. Arterial blood (aB) O2 levels were withdrawn 1 min before and at 20 s, 1 min, 2 min, and 3 min following ETS and analyzed on a calibrated Corning 158 semi-automated aB gas machine. There was a significant decline in PaO2 from baseline at 20 s post ETS in both the no suction and suction protocols, p less than .05. The application of suction caused a greater decline in mean PaO2 (41 torr) compared to no suction (13 torr) at 20 s post ETS which was similar in pattern following vagal blockade (suction: 37 torr; no suction: 2 torr). Following the first pass of the catheter, mean heart rate (HR) declined 14 beats per min with suction and 9 beats per min with no suction. The difference was obliterated following the second pass of the catheter. There was no difference in the HR following vagal blockade.

Differential effects of continuous versus intermittent suction on tracheal tissue.

The purpose of this study was to determine the differential effect of continuous versus intermittent application of negative pressure on tracheal tissue during endotracheal suctioning. The sample consisted of 12 mongrel dogs, randomly assigned to group 1 (N = 5), continuous suction, or group 2 (N = 5), intermittent suction. All animals were orally intubated (40F endotracheal tube). Two control animals were intubated and not suctioned. Animals in group 1 and 2 were suctioned every 15 minutes for 4 hours for a total of 16 suction passes. Endotracheal suctioning was performed by using a 14F suction catheter either continuously (10 seconds) or intermittently (2 seconds with, 1 second without for a total of 10 seconds) at a suction pressure of 200 mm Hg and a suction flow rate of 16 L/min. Tracheal tissue samples were examined for simplified and major simplified damage, ulceration, and ulceration with necrosis. Results indicated that all forms of damage were present with both suctioning techniques. No significant differences were found between group 1 and group 2 (Wilcoxon rank sum) for any of the alterations. Results indicate that both continuous and intermittent application of negative pressure with endotracheal suction produces significant damage to tracheal tissue.

The effect of lung hyperinflation and endotracheal suctioning on cardiopulmonary hemodynamics.

The purpose of this study was to examine the effect of sequential lung hyperinflation breaths followed by suction on mean arterial pressure (MAP), cardiac output (CO), pulmonary artery pressure (PAP), and pulmonary airway pressure (Paw) to elucidate the mechanism for the increase in MAP seen with lung hyperinflation and suction. Thirty-four postoperative coronary artery bypass graft patients were randomly exposed to three lung hyperinflation breaths at one of five volumes (tidal volume, 12cc/kg, 14cc/kg, 16cc/kg, and 18cc/kg of lean body weight) using a ventilator followed by 10 s of suctioning repeated for a total of three times. There was a mean increase in MAP (13.71 torr), CO (12.2%), PAP (4 torr), and Paw (23.5 torr) above baseline over the three sequences. The mechanism for the increase in MAP with lung hyperinflation may be attributed to transient increases in intrathoracic pressure, resulting in increased left ventricular preload and CO.

Ventilator versus manual resuscitation bag as the method for delivering hyperoxygenation before endotracheal suctioning.

A critical review and analysis of the current research on the efficacy of the ventilator versus the manual resuscitation bag (MRB) as the method of delivering hyperoxygenation/hyperinflation breaths before, during, and/or after endotracheal suctioning (ETS) is presented. Current research findings indicate that hyperoxygenation/hyperinflation breaths at 100% oxygen (O2) delivered via the ventilator have resulted in elevated blood-O2 levels which are either superior or equivalent to the MRB in preventing suction-induced hypoxemia. Delivery of hyperoxygenation/hyperinflation breaths using the MRB results in increased airway pressure, and increased hemodynamic consequences. Guidelines of clinical practice, based on current research findings, are presented. Areas for further research are identified.

Congenital aneurysm of the left atrial wall in infancy.

A 5-month-old male infant with congenital left atrial aneurysm is reported. The youngest patient previously reported was 2 1/2 years old. This case is unusual because the patient was an infant and because of the degree of mitral insufficiency and acute cardiac decompensation observed in the patient.

Quantifying the minimum discard sample required for accurate arterial blood gases.

The purpose of this study was to determine the minimum amount of discard sample required to remove all heparinized normal saline from an indwelling arterial catheter to obtain an accurate arterial blood gas (ABG) sample. The pH and partial pressure of oxygen (PO2) and carbon dioxide (PCO2) were adjusted to known values prior to the onset of the study. The discard sample size ranged from zero to 5 cubic centimeters (cc), increasing at 0.5-cc increments. Twenty-nine sets of 11 consecutive discard/ABG samples, totaling 319 ABG specimens, were drawn over 20 minutes. Based on the data, a 2-cc discard is sufficient to guarantee accurate blood gases when withdrawing blood from an arterial catheter with a 1-cc dead space volume.

Effects of lung hyperinflation on mean arterial pressure and postsuctioning hypoxemia.

Our purpose was to determine the effect of five different lung hyperinflation volumes (tidal volume, 12 cc/kg, 14 cc/kg, 16 cc/kg, and 18 cc/kg lean body weight) on mean arterial pressure and postsuctioning hypoxemia (arterial blood gases). Subjects received three consecutive lung hyperinflations at one of the five randomly ordered volumes in 15 seconds via a ventilator "sigh" control at a fraction of inspired oxygen of 1.0. The three lung hyperinflations were followed by 10 seconds of continuous suction (flow rate 16 L/min). The procedure was repeated three times. The sample consisted of eight men and women 4 hours after coronary artery bypass graft surgery. Data indicated a statistically significant (by analysis of variance, p = 0.000) mean increase of 15 mm Hg in mean arterial pressure over the three lung hyperinflation sequences that was not volume dependent. There was a significant increase (p = 0.0001) in arterial oxygen pressure at 0 seconds after suctioning that increased with each increasing lung hyperinflation volume.

Management of asymptomatic intracardiac missiles using echocardiography.

A child sustained a low-velocity airgun pellet injury to the left ventricle. No cardiovascular compromise was produced. The foreign body was localized by two-dimensional echocardiography to the left ventricular chamber near the mitral valve, and subsequently removed through a left atriotomy incision. In asymptomatic patients, missiles clearly embedded within a chamber wall may be observed; all others should be removed. Two-dimensional echocardiography is recommended for localization.