Coronary artery embolization in closed-chest canines using flexible radiopaque plugs

A new technique induces localized myocardial infarction in closed-chest dogs by placing discrete plugs in coronary arteries without using cumbersome coaxial catheters or guide wires. Flexible plugs, essential to this method, are formed by extruding a dental impression polymer, rendered radiopaque with sodium iodide, into spaghetti-like strands. Segments of these strands can be injected through a catheter into a selected coronary artery. Contact with blood or saline causes plugs to swell. The mean increase in plug diameter due to swelling was 27 +/- 20%. Eight anesthetized dogs were embolized via carotid approach [6 left anterior descending (LAD), 1 left circumflex (LCX), and 1 LAD and LCX]. Plug positions were monitored fluoroscopically. One animal died at 2 days postembolization. The remaining seven dogs were killed after 14-37 days. Autopsies showed complete vessel occlusion and localized infarction. Infarcts resulting from coronary artery occlusion with one, two, or three plugs involved 2-26% of the left ventricular mass.     

Renal norepinephrine content decreases after chronic flow probe implantation.

Electromagnetic flow probes were chronically implanted around the left renal artery in 6 female beagle dogs. 3-10 months after implantation, renal cortex norepinephrine contents of right and left kidneys were compared (liquid chromatography). In 4 out of the 6 dogs investigated 3-6 months after implantation, the norepinephrine content of the left kidney was reduced by 27-72%. In 2 dogs investigated 7 and 10 months after implantation, the difference between left and right kidney was not significant. These findings should be taken into consideration when interpreting results from chronically instrumented kidneys.     

Pulmonary hypertension induced with an intra-arterial balloon: an alternate mechanism

The Laks catheter is a triple-lumen balloon catheter used to distend the canine main pulmonary artery while recording right ventricular pressure and the arterial pressure distal to the balloon. A rise in arterial pressure reported to occur during distension has been attributed to vasoconstriction rather than passive obstruction by the balloon. We tested this in six anesthetized dogs by inflating the Laks catheter-balloon while recording pressure distal to the balloon from the Laks catheter as well as from additional catheters in right and left pulmonary arteries placed retrogradely through lobar branches following thoracotomy. We found that balloon inflation increased pressures in the arterial port of the Laks catheter and in the left pulmonary artery catheter but reduced it in the right pulmonary artery. Tightening a snare around the right pulmonary artery had the same effects on pressures. Similar results were obtained while cardiac output was controlled by left ventricular bypass perfusion in four dogs. We conclude that the Laks catheter-balloon obstructs flow to the right lung and that the arterial pressure rise recorded in it during balloon inflation cannot be distinguished from that caused by occlusion of the right pulmonary artery.     

Effects of PEEP on pulmonary hemodynamics in intact dogs with oleic acid pulmonary edema

The effects of positive end-expiratory pressure (PEEP) on the pulmonary circulation were studied in 14 intact anesthetized dogs with oleic acid (OA) lung injury. Transmural (tm) mean pulmonary arterial pressure (Ppa)/cardiac index (Q) plots with transmural left atrial pressure (Pla) kept constant were constructed in seven dogs, and Ppa(tm)/PEEP plots with Q and Pla(tm) kept constant were constructed in seven other dogs. Q was manipulated by using a femoral arteriovenous bypass and a balloon catheter inserted in the inferior vena cava. Pla was manipulated using a balloon catheter placed by thoracotomy in the left atrium. Ppa(tm)/Q plots were essentially linear. Before OA, the linearly extrapolated pressure intercept of the Ppa(tm)/Q relationship approximated Pla(tm). OA (0.09 ml/kg into the right atrium) produced a parallel shift of the Ppa(tm)/Q relationship to higher pressures; i.e., the extrapolated pressure intercept increased while the slope was not modified. After OA, 4 Torr PEEP (5.4 cmH2O) had no effect on the Ppa(tm)/Q relationship and 10 Torr PEEP (13.6 cmH2O) produced a slight, not significant, upward shift of this relationship. Changing PEEP from 0 to 12 Torr (16.3 cmH2O) at constant Q before OA led to an almost linear increase of Ppa(tm) from 14 +/- 1 to 19 +/- 1 mmHg. After OA, Ppa(tm) increased at 0 Torr PEEP but changing PEEP from 0 to 12 Torr did not significantly affect Ppa(tm), which increased from 19 +/- 1 to 20 +/- 1 mmHg. These data suggest that moderate levels of PEEP minimally aggravate the pulmonary hypertension secondary to OA lung injury.     

Endocardial activation during ventricular fibrillation in normal and failing canine hearts

Because congestive heart failure (CHF) promotes ventricular fibrillation (VF), we compared VF in seven dogs with CHF induced by combined myocardial infarction and rapid ventricular pacing to VF in six normal dogs. A noncontact, multielectrode array balloon catheter provided full-surface real-time left ventricular (LV) endocardial electrograms and a dynamic color-coded display of endocardial activation projected onto a three-dimensional model of the LV. Fast Fourier transform (FFT) analysis of virtual electrograms showed no difference in peak or centroid frequency in CHF dogs compared with normals. The average number of simultaneous noncontiguous wavefronts present during VF was higher in normals (2.4 +/- 1.0 at 10 s of VF) than in CHF dogs (1.3 +/- 1.0, P < 0.005) and decreased in both over time. The wavefront "turnover" rate, estimated using FFT of the noncontiguous wavefront data, did not differ between normals and CHF and did not change over 5 min of VF. Thus the fundamental frequency characteristics of VF are unaltered by CHF, but dilated abnormal ventricles sustain fewer active wavefronts than do normal ventricles.     

PEEP inhibits hypoxic pulmonary vasoconstriction in dogs

The effects of an increase in alveolar pressure on hypoxic pulmonary vasoconstriction (HPV) have been reported variably. We therefore studied the effects of positive end-expiratory pressure (PEEP) on pulmonary hemodynamics in 13 pentobarbital-anesthetized dogs ventilated alternately in hyperoxia [inspired O2 fraction (FIO2) 0.4] and in hypoxia (FIO2 0.1). In this intact animal model, HPV was defined as the gradient between hypoxic and hyperoxic transmural (tm) mean pulmonary arterial pressure [Ppa(tm)] at any level of cardiac index (Q). Ppa(tm)/Q plots were constructed with mean transmural left atrial pressure [Pla(tm)] kept constant at approximately 6 mmHg (n = 5 dogs), and Ppa(tm)/PEEP plots were constructed with Q kept constant approximately 2.8 l.min-1.m-2 and Pla(tm) kept constant approximately 8 mmHg (n = 8 dogs). Q was manipulated using a femoral arteriovenous bypass and a balloon catheter in the inferior vena cava. Pla(tm) was held constant by a balloon catheter placed by left thoracotomy in the left atrium. Increasing PEEP, from 0 to 12 Torr by 2-Torr increments, at constant Q and Pla(tm), increased Ppa(tm) from 14 +/- 1 (SE) to 19 +/- 1 mmHg in hyperoxia but did not affect Ppa(tm) (from 22 +/- 2 to 23 +/- 1 mmHg) in hypoxia. Both hypoxia and PEEP, at constant Pla(tm), increased Ppa(tm) over the whole range of Q studied, from 1 to 5 l/min, but more at the highest than at the lowest Q and without change in extrapolated pressure intercepts. Adding PEEP to hypoxia did not affect Ppa(tm) at all levels of Q.(ABSTRACT TRUNCATED AT 250 WORDS)     

In vivo sampling of cardiac triglyceride from dogs during ethanol infusion

The feasibility of procuring and analyzing cardiac tissue for triglyceride in vivo was tested in anesthetized dogs. Measurements of triglycerides in samples obtained in vitro confirmed: reproducibility of triplicate analyses of the glycerideglycerol moiety of tissue triglyceride (SEM +/- 2.1%), homogeneity in and between ventricles (SEM +/- 1.8%), and agreement between right endocardial triglyceride and left myocardial triglyceride (difference not significant). Seven dogs received ethanol, 15-30 mg/kg/min, and five dogs received glucose or 0.85% NaCl for 2 hr. Cardiac output and filling pressure were measured from the left ventricle and tissue was taken from the right ventricle with a biopsy catheter before and during infusions. Three to four samples were obtained from each dog; the average weight was 14.4 mg and two to three biopsies were required for each sample. In the ethanol group, triglyceride increased after 15 min and continued to rise; the final triglyceride concentration correlated with the infusion rate. In the glucose-saline group, in vivo triglyceride concentration did not change and did not differ from postmortem triglyceride. Cardiac function declined in the ethanol group and was unaffected in the controls. Thus, multiple in vivo measurements of cardiac lipid are practical and safe and show that ethanol infusions cause early and progressive accumulation of triglyceride in heart muscle.     

Prevention of catheter-related infections by a new, catheter-restricted antibiotic filling technique

Catheter-related infections pose a hazard to both humans and laboratory animals. The aim of this study was to develop a technique preventing bacterial colonization of intravascular catheters. In 27 dogs a total of 70 catheters were implanted. On an average catheters were used for 207 days. Three protocols were compared: (1) flushing the catheters with a heparinized solution; (2) filling only the catheter lumen with alpha-chymotrypsin solution (225 units/ml); (3) filling only the catheter lumen with a solution containing a mixture of the aminoglycoside antibiotic gentamicin (20 mg/ml) and chymotrypsin (225 units/ml). Catheter fillings were always withdrawn before catheter use. Catheter exit sites were all treated with povidone iodine ointment once a day. Body temperatures and weights were recorded, bacteriological and electron microscopical examinations of catheters performed. Without gentamicin filling all catheters were colonized after a few weeks. The dogs showed clinical signs of chronic bacteraemia. Gentamicin filling eradicated colonization. No further bacteraemia was observed. We conclude that filling only the catheter lumen with a concentrated solution of chymotrypsin and gentamicin, combined with measures to prevent infections via the subcutaneous catheter tunnel, is an effective and safe technique to prevent catheter-related infections.     

Intermittent hypoxic training protects canine myocardium from infarction

This investigation examined cardiac protective effects of normobaric intermittent hypoxia training. Six dogs underwent intermittent hypoxic training for 20 consecutive days in a normobaric chamber ventilated intermittently with N2 to reduce fraction of inspired oxygen (FiO2) to 9.5%-10%. Hypoxic periods, initially 5 mins and increasing to 10 mins, were followed by 4-min normoxic periods. This hypoxia-normoxia protocol was repeated, initially 5 times and increasing to 8 times. The dogs showed no discomfort during intermittent hypoxic training. After 20 days of hypoxic training, the resistance of ventricular myocardium to infarction was assessed in an acute experiment. The left anterior descending (LAD) coronary artery was occluded for 60 mins and then reperfused for 5 hrs. At 30 mins of LAD occlusion, radioactive microspheres were injected through a left atrial catheter to assess coronary collateral blood flow into the ischemic region. After 5 hrs reperfusion, the heart was dyed to delineate the area at risk (AAR) of infarction and stained with triphenyl tetrazolium chloride to identify infarcted myocardium. During LAD occlusion and reperfusion, systemic hemodynamics and global left ventricular function were stable. Infarction was not detected in 4 hearts and was 1.6% of AAR in the other 2 hearts. In contrast, 6 dogs sham-trained in a chamber ventilated with compressed air and 5 untrained dogs subjected to the same LAD occlusion/reperfusion protocol had infarcts of 36.8% +/- 5.8% and 35.2% +/- 9.5% of the AAR, respectively. The reduction in infarct size of four of the six hypoxia-trained dogs could not be explained by enhanced collateral blood flow to the AAR. Hypoxia-trained dogs had no ventricular tachycardia or ventricular fibrillation. Three sham-trained dogs had ventricular tachycardia and two had ventricular fibrillation. Three untrained dogs had ventricular fibrillation. In conclusion, intermittent hypoxic training protects canine myocardium from infarction and life-threatening arrhythmias during coronary artery occlusion and reperfusion. The mechanism responsible for this potent cardioprotection merits further study.     

Lower-body work capacity and one-repetition maximum squat prediction in college football players

The purpose of this study was to assess lower-body muscular strength and work capacity after off-season resistance training and the efficacy of predicting maximal squat strength (1 repetition maximum [1RM]) from repetitions to fatigue. National Collegiate Athletic Association Division-II football players (n = 58) were divided into low-strength (LS, 1RM < 365 lb, n = 32) and high-strength (HS, 1RM ≥ 365 lb, n = 26) groups before training based on median 1RM squat performance. Maximal repetitions to failure (RTFs) were performed with a relative load of 70% of 1RM before training and 60, 70, 80, and 90% of 1RM after 12 weeks of a linear periodization resistance training program. As a team, 1RM squat (32 ± 27 lb), 70% RTF (4.5 ± 4.5 reps), and work capacity at 70% 1RM load (1,482 ± 1,181 lb reps) increased significantly after training. Likewise, training resulted in significant increases in 1RM, RTF at 70% 1RM, and work capacity (load × reps) in both LS (8 ± 33 lb, 3.9 ± 4.7 reps, 1,736 ± 1,521 lb reps, respectively) and HS (27 ± 21 lb, 4.9 ± 4.4 reps, 2,387 ± 1,767 lb reps, respectively), with no significant difference between groups. There was no relationship between the change in work capacity and the change in muscular strength for either the LS (r = 0.02) or HS (r = 0.06) group. Predicted 1RMs were best when RTFs were performed using 80% 1RM (5-17 RTFs), with an error of ±5% in 95% of the subjects. In conclusion, the changes in muscular strength associated with an off-season training program appear to have a positive influence on squat work capacity at 70% of 1RM and allow favorable prediction of 1RM using submaximal loads.     

Effects of digoxin on diaphragmatic strength generation

Contrary to hindlimb muscle, extracellular calcium plays an important role in diaphragmatic strength generation (J. Appl. Physiol. 58: 2054-61, 1985). Since the inotropic effect of digitalis appears to be related to cell membrane transport of calcium, we studied the effect of digoxin on diaphragmatic contractility in 20 anesthetized dogs. The diaphragm was electrically stimulated with intramuscular electrodes. The transdiaphragmatic pressure (Pdi) during supramaximal (50 V) 2-s stimulations applied over a frequency range of 10-100 Hz was measured with balloon catheters at functional residual capacity. Cardiac output was measured with a Swan-Ganz catheter and diaphragmatic blood flow (Qdi) by timed volume collections of left inferior venous effluent. The force generated by the sartorius muscle during electrical stimulations was studied concomitantly to Pdi. In 10 dogs (group A) 0.04 mg/kg of digoxin was infused in 10 min. In 10 other dogs (group B) 0.2 mg/kg was administered. All measurements were performed during control and 30, 60, 90, and 120 min after digoxin administration. In group A, digoxin plasmatic level at 60 min reached a therapeutic range in all dogs (1.8 +/- 0.3 ng/ml), whereas in group B, digoxin plasmatic level was higher (8 +/- 1.3 ng/ml). No significant change in cardiac output and Qdi was noted after administration of digoxin, either in the dogs of group A or those of group B.(ABSTRACT TRUNCATED AT 250 WORDS)     

A technique for measurement of retrograde coronary blood flow in intact anesthetized dogs.

A technique for measurement of retrograde coronary blood flow in intact anesthetized dogs is described. Occlusion of the coronary artery is produced by the inflation of a small rubber balloon at the tip of a no. 9 cardiac catheter placed under fluorescopy in a branch of the left coronary artery. Blood which bleeds back from the occluded coronary artery through the no. 9 catheter is diverted into a small reservoir of 1-ml capacity. The time to fill this reservoir is recorded electrically. Retrograde coronary blood flow is calculated from the time required to fill this reservoir. Results indicate good repeatability of meadurements. The technique seems to be a simple, adequate, and convenient means for assessing agents for possible vasodilator action on the collateral circulation in intact animals.     

Propranolol uptake by dog lung: effect of pulmonary artery occlusion and shock lung

First-pass lung uptake of propranolol in catheterized ambulant dogs was measured by comparison the difference between the ratios of [14C]propranolol to indocyanine green (ICG) before and after a single passage through the pulmonary circulation. Uptake was 56 +/- 5, 53 +/- 8, and 61 +/- 10% (mean +/- SD) when 0.02, 0.2, and 2.0 mg of propranolol were injected, respectively. There was a negative though unimportant correlation between percent uptake and cardiac output. Analysis of paired propranolol and ICG outflow curves confirmed the lack of saturation with increasing dose and suggested the involvement of simple diffusion. In four dogs uptake fell from 4.76 +/- 6.8 to 40.4 +/- 8.2% (0.05 greater than P greater than 0.025) during partial occlusion of the pulmonary circulation by a Swan-Ganz catheter balloon and rose again to 51.2 +/- 6.4% after relief of the occlusion. Propranolol uptake, measured serially in five dogs with indwelling catheters, was initially 55% but fell linearly to 30% over 2 wk (r = -0.59, P less than 0.001), and necropsy showed pathological features of shock lung. A similar study performed on three dogs within 4 days of catheterization showed no change in uptake of the drug. Propranolol uptake appears to reflect both quantitative and qualitative changes in the pulmonary endothelium. This method may therefore be valuable in studying the pulmonary endothelium in health and disease.     

Physical Activity and Welfare of Guide Dogs and Walking Activity of Their Partners

ABSTRACT

Appropriate physical activity is beneficial for physical and psychosocial wellbeing, and it is recommended for people to have 30 minutes of activity on most days of the week, to yield 150 minutes per week. Getting sufficient physical activity particularly challenges people with visual disabilities, and few health-promotion interventions have focused on adults with this disability. Recently, dog walking has been promoted in communities as a way to increase people's physical activity. We surveyed guide-dog partners to assess whether their guide dogs facilitated walking. We also assessed the welfare of these dogs, including their physical activity and social interactions with other dogs and people, especially as there is some concern that these dogs have too little freedom. For comparison, we assessed large and small companion dogs and their handlers, as well. A web-based survey was conducted among people living with guide dogs or companion (pet) dogs: large companion dogs (51 lb or more) and small companion dogs (50 lb or less). Guide-dog partners walked significantly more than handlers of either small or large companion dogs (Guide-dog partners met the healthy standard of 150 min per week of walking, at a level 10 times more than owners of large companion dogs). Guide dogs walked with their partners more frequently and for longer durations per day than owners of companion dogs. Guide dogs with their handlers met more people outside of their homes than did owners of companion dogs, but the groups did not differ in the number of dogs they greeted outside of the house. The frequencies of going to off-leash areas did not differ among the three groups. The findings indicate that having a dog as a guide can lead to a higher amount of walking among guide-dog partners, and that guide dogs have a higher quality of life, in terms of quantity of physical activity and social interactions, compared with large or small companion dogs.     

Effects of hypocalcemia on diaphragmatic strength generation

We studied the effects of hypocalcemia on diaphragmatic force and diaphragm blood flow (Qdi) in 12 anesthetized dogs. The diaphragm was electrically stimulated with intramuscular electrodes surgically implanted in the ventral surface of each hemidiaphragm. The transdiaphragmatic pressure (Pdi) during supramaximal (50 V) 2-s stimulations applied over a frequency range of 10-100 Hz was measured with balloon catheters during tracheal occlusion at functional residual capacity. A catheter was placed via the femoral vein into the left inferior phrenic vein, and Qdi was measured by timed volume collections of left inferior venous effluent. A catheter was introduced in a femoral artery to monitor blood pressure (BP). In five additional dogs, the force generated by the sartorius muscle during electrical stimulation was also studied concomitantly to diaphragmatic force. The animals were mechanically ventilated throughout the experiment, and the arterial blood gases and pH were maintained constant. Hypocalcemia was induced by a continuous infusion of EGTA (70 mg X kg-1 X h-1), which led to a progressive decrease (P less than 0.0001) of ionized calcium plasmatic level from 2.21 +/- 0.4 meq/1 during control to 1.69 +/- 0.06, 1.25 +/- 0.5, and 1.07 +/- 0.5 meq/1 after 30, 60, and 120 min, respectively. Hypocalcemia decreased progressively Pdi, which amounted to 84 +/- 3 (P less than 0.001) and 98 +/- 2% of control values for the low frequencies (10 and 20 Hz) and the high frequencies (50 and 100 Hz), respectively, after 30 min of EGTA infusion and to 74 +/- 5 and 79 +/- 6% for the low and high frequencies, respectively, after 120 min.(ABSTRACT TRUNCATED AT 250 WORDS)     

Wedge pressure in large vs. small pulmonary arteries to detect pulmonary venoconstriction

Pulmonary arterial wedge pressure measures the pressure where blood flow resumes on the venous side. By occlusion of a large artery, the point where blood flow resumes will be in or near the left atrium. However, by occlusion of a small artery, it is possible to shift the point where flow resumes to a more proximal site in the veins and thus measure a pressure within the small veins. Increased pulmonary venous pressure, as a result of partial obstruction in the large veins, may not be detected by wedging a Swan-Ganz catheter in a large artery but may be detected by wedging in a small artery. We demonstrated this phenomenon in open-chest dogs by mechanically obstructing the left lower lobar vein or by infusing histamine to cause a generalized pulmonary venoconstriction. The wedge pressure measured by a 7-F Swan-Ganz catheter, with its balloon inflated in the main left lower lobar artery, nearly equaled left atrial pressure. On the other hand, the wedge pressure measured with a 7-F, 5-F, or a PE-50 catheter advanced into a small artery (without a balloon) was considerably higher than left atrial pressure. These results suggest that high resistance in the pulmonary veins can be demonstrated with the Swan-Ganz catheter by comparing the pressures obtained with the catheter wedged in a small and large artery.     

Construction and evaluation of a coronary catheter for chronic implantation in dogs

Construction of a Silastic catheter and the procedure for chronic implantation in a coronary artery in dogs is described. In addition, studies designed to evaluate whether chronic coronary artery catheterization altered coronary vascular reactivity and myocardial function are presented. The results of these studies indicate that chronic implantation of the catheter in a coronary artery of conscious dogs does not significantly interfere with the normal reactivity of the coronary vascular bed, does not compromise regional or global left ventricular function, and does not induce collateral vessel development. This technique will be useful in studies involving the neural and metabolic regulation of the coronary circulation in animals subjected to exercise and/or exercise training.     

Continuous monitoring of right ventricular volume changes using a conductance catheter in the rabbit.

To assess the reliability of conductance (G) catheter for evaluating right ventricular (RV) volume changes, a miniature (3.5F) six-electrode catheter was developed and tested in 11 New Zealand rabbit hearts. In five animals the heart was excised; in six it was left in the thorax. RV conductance was recorded while the RV was filled with blood in 0.25-ml steps at different left ventricular (LV) volumes. Linear correlation of measured conductance vs. reference volumes was computed. RV conductance was highly correlated with reference volume [correlation coefficient (r) ranging from 0.991 to 0.999]. Slope of regression lines was not significantly affected by LV volume variations in 1-ml steps or by acute conductance changes of structures surrounding the heart, whereas the intercept was affected only by the 0- to 1-ml LV volume change. In four rabbits, RV conductance changes during a cardiac cycle [stroke volume- (SV) G] were compared in vivo with electromagnetic flow probe-derived estimates of SV (SVem) as stroke volume was varied by graded inferior vena caval occlusion. SV-G correlated well with SVem (r ranging from 0.92 to 0.96). This correlation persisted after the thorax was filled with saline; however, significant differences were found in individual slopes (P < 0.001). These results show that the conductance catheter has a potential to reliably monitor in vivo relative RV volume changes in small-animal hearts.     

Lack of alveolar O2 during lung reperfusion does not decrease edema formation

We previously reported that pulmonary arterial occlusion for 48 h followed by 4 h of reperfusion in awake dogs results in marked edema and inflammatory infiltrates in both reperfused and contralateral lungs (Am. Rev. Respir. Dis. 134: 752-756, 1986; J. Appl. Physiol. 63: 942-950, 1987). In this experiment we study the effects of alveolar hypoxia on this injury. Anesthetized dogs underwent thoracotomy and occlusion of the left pulmonary artery. Twenty-four hours later the dogs were reanesthetized, and a double-lumen endotracheal tube was placed. The right lung was continuously ventilated with an inspiratory O2 fraction (FIO2) of 0.35. In seven study animals the left lung was ventilated with an FIO2 of 0 for 3 h after the left pulmonary artery occluder was removed. In six control animals the left lung was ventilated with an FIO2 of 0.35 during the same reperfusion period. Postmortem bloodless wet-to-dry weight ratios were 5.87 +/- 0.20 for the left lower lobe and 5.32 +/- 0.12 for the right lower lobe in the dogs with hypoxic ventilation (P less than 0.05 for right vs. left lobes). These values were not significantly different from the control dog lung values of 5.94 +/- 0.22 for the left lower lobe and 5.11 +/- 0.07 for the right lower lobe (P less than 0.05 for right vs. left lobes). All values were significantly higher than our laboratory normal of 4.71 +/- 0.06. We conclude that reperfusion injury is unaffected by alveolar hypoxia during the reperfusion phase.     

Constant-flow ventilation in pigs

Constant-flow ventilation (CFV) is a ventilatory technique in which physiological blood gases can be maintained in dogs by a constant flow of fresh gas introduced via two catheters placed in the main-stem bronchi (J. Appl. Physiol. 53: 483-489, 1982). High-velocity gas exiting from the catheters can create uneven pressure differences in adjacent lung segments, and these pressure differences could lead to gas flow through collateral channels. To examine this hypothesis, we studied CFV in pigs, animals known to have a high resistance to collateral ventilation. In three pigs we examined steady-state gas exchange, and in six others we studied unsteady gas exchange at three flow rates (20, 35, and 50 l/min) and three catheter positions (0.5, 1.5, and 2.5 cm distal to the tracheal carina). During steady-state runs we were unable to attain normocapnia; the arterial CO2 partial pressure (PaCO2) was approximately 300 Torr at all flow rates and all catheter positions, compared with 20-50 Torr at similar flows and positions in dogs studied previously. The initial unsteady gas-exchange experiments indicated no consistent effect of catheter position or flow rate on the rate of rise of PaCO2. In three other pigs, the rates of rise of PaCO2 were compared with the rates observed with apneic oxygenation (AO). At the maximum flow and deepest position, the rate of rise of PaCO2 was lower during CFV than during AO. These data suggest that flow through collateral channels might be important in producing adequate gas transport during CFV; however, other factors such as airway morphometry and the effects of cardiogenic oscillations may explain the differences between the results in pigs and dogs.