Weaning from mechanical ventilation

Patients that require positive pressure ventilation to maintain sufficient alveolar ventilation or pulmonary gas exchange may eventually reach a point in the course of their care wherein mechanical ventilation is no longer necessary. This process of transferring the work of breathing from the ventilator back to the patient is referred to as ventilator weaning. The term "ventilator weaning" may be used to refer to all methods by which this transfer of workload may be accomplished. In many patients, particularly those with short-lasting or readily correctable causes of respiratory insufficiency (e.g., general anesthesia), the discontinuation of positive pressure ventilation may be easily achieved. Indeed, in patients awakening from general anesthesia, the axiom "awake enough to blink, awake enough to breath" may prove to be a sufficient guideline. However, in those patients requiring long-term mechanical ventilatory support, the process can prove to be both frustrating and exceptionally challenging. It is of crucial importance to identify those patients that may be successfully weaned because of both the financial impact of prolonged intensive care unit hospitalization and the risks imposed on the patient by the process of positive pressure ventilation. To be able to predict which patients may be ready to be weaned from the ventilator requires an understanding of the balance between the work of breathing (ventilatory load) and the ability of the patient's respiratory pump to meet those needs (ventilatory capacity). The management of patients experiencing difficulty during the weaning process requires that the clinician recognize imbalances between ventilatory load and capacity and to correct these imbalances once identified.     

Principles of mechanical ventilation.

Mechanical ventilation is an enormous undertaking for a veterinary hospital in general and for any patient in particular. It is a team effort requiring large amounts of space, supplies, labor, and time. It requires committed owners and clinicians who communicate clearly with each other. It also requires a significant financial commitment initially from the hospital to obtain the equipment and expertise and then from the owner to maintain the patient. All members of the patient care team should have a basic understanding of respiratory physiology and ventilator mechanics. Clear goals for therapy and end points should be established. If they cannot be met, the goals should be reassessed in light of changes in patient condition. Weaning may be difficult and long, but once successful, it is most rewarding for the patient, family, clinician, and team.     

An evaluation of apnea or spontaneous ventilation in early recovery following mechanical ventilation in the anesthetized horse

Objective To compare arterial oxygen and carbon dioxide tensions in apneic and spontaneously ventilating horses recovering from anesthesia. Study design Randomized clinical trial. Animal population Forty‐two healthy horses averaging 466 ± 106 kg and 6 ± 5 years of age. Methods Anesthetized horses undergoing a variety of surgical procedures and receiving positive pressure ventilation (IPPV) were divided into two equal groups. One group was allowed to return to spontaneous ventilation prior to disconnection from the anesthetic circuit (weaned). The other group remained apneic during transport to a recovery stall. Arterial blood gas data were collected at five time points: 20 minutes before moving to a recovery stall (t = ? 20); at the time the anesthetic circuit was disconncted (t = 0); at 3 and 5 minutes post‐disconnection (t = 3 and t = 5) and at the time of the first spontaneous breath (t = sv). The data were analyzed using an anova method for repeated measures and paired, two‐tailed t‐tests. Significance was assumed when p < 0.05. Results The apneic group took a mean of 5 minutes 18 seconds (± 135 seconds) before starting spontaneous ventilation. This group maintained significantly higher PaO₂ levels at intermediate time points (t = 0 and t = 3) but no difference was noted after 5 minutes. PaCO₂ levels were higher in the weaned group at time 0 minutes, returning to a comparable level to the apneic group at t = 3 minutes. Conclusions and clinical relevance Horses can survive a short period of apnea during transport from the surgery suite to recovery stall and may benefit from a reduced incidence of transient hypoxemia compared with spontaneously ventilating horses. This information has practical implications for the anesthetist evaluating the options for discontinuing IPPV when horses are moved to a recovery stall.     

Tracheal gas isufflation-aided mechanical ventilation during carbon dioxide-induced pneumoperitoneum in rabbits

Despite numerous benefits of laparoscopic procedures, the serious hypercapnia and respiratory acidosis in hypercapnic patients with decreased pulmonary compliance during carbon dioxide-induced pneumoperitoneum (CDP) may be developed. Tracheal gas insufflation (TGI) has been shown to be a useful adjunct to controlled mechanical hypoventilation. This study was undertaken to identify whether TGI superimposed on controlled mechanical ventilation (CMV) improve ventilatory efficiency during CDP in rabbits. Sixteen paralyzed and anesthetized rabbits were used. The animals were assigned to two groups-CMV group: CMV alone; TGI group: CMV superimposed by TGI with flow rate of 2L/min. The animals were insufflated to intra-abdominal pressure of 8 mmHg with CO2 gas. Then, tidal volume (V(T)) was changed to maintain the set peak inspiratory pressure (PIP) value, while other ventilatory settings were kept constant. The set PIP value corresponding to 30, 60, and 90 min after the start of peritoneal insufflation of CO2 were 15, 22, and 25 cm H2O, respectively. During CDP with TGI, PaCO2 decreased significantly (p<0.01) from CMV without TGI of 82.1 +/- 14.1 to 47.5 +/- 5.5, 58.1 +/- 9.9 to 40.0 +/- 4.6, 47.1 +/- 9.4 to 32.7 +/- 5.1 mmHg at PIP of 15, 22, and 25 cm H2O, respectively. The inspired V(T) decreased significantly (p<0.05) from CMV without TGI of 18.4 +/- 3.9 to 12.8 +/- 2.8 ml at PIP of 15 cm H2O. TGI superimposed on CMV is more effective than CMV alone in enhancing ventilatory efficiency during CDP in rabbits.     

Arterial blood gas tensions during recovery in horses anesthetized with apneustic anesthesia ventilation compared with conventional mechanical ventilation

ObjectiveTo compare PaO₂ and PaCO₂ in horses recovering from general anesthesia maintained with either apneustic anesthesia ventilation (AAV) or conventional mechanical ventilation (CMV).Study designRandomized, crossover design.AnimalsA total of 10 healthy adult horses from a university-owned herd.MethodsDorsally recumbent horses were anesthetized with isoflurane in oxygen [inspired oxygen fraction = 0.3 initially, with subsequent titration to maintain PaO₂ ≥ 85 mmHg (11.3 kPa)] and ventilated with AAV or CMV according to predefined criteria [10 mL kg^–1 tidal volume, PaCO₂ 40–45 mmHg (5.3–6.0 kPa) during CMV and < 60 mmHg (8.0 kPa) during AAV]. Horses were weaned from ventilation using a predefined protocol and transferred to a stall for unassisted recovery. Arterial blood samples were collected and analyzed at predefined time points. Tracheal oxygen insufflation at 15 L minute^–1 was provided if PaO₂ < 60 mmHg (8.0 kPa) on any analysis. Time to oxygen insufflation, first movement, sternal recumbency and standing were recorded. Data were analyzed using repeated measures anova, paired t tests and Fisher’s exact test with significance defined as p < 0.05.ResultsData from 10 horses were analyzed. Between modes, PaO₂ was significantly higher immediately after weaning from ventilation and lower at sternal recumbency for AAV than for CMV. No PaCO₂ differences were noted between ventilation modes. All horses ventilated with CMV required supplemental oxygen, whereas three horses ventilated with AAV did not. Time to first movement was shorter with AAV. Time to oxygen insufflation was not different between ventilation modes.ConclusionsAlthough horses ventilated with AAV entered the recovery period with higher PaO₂, this advantage was not sustained during recovery. Whereas fewer horses required supplemental oxygen after AAV, the use of AAV does not preclude the need for routine supplemental oxygen administration in horses recovering from general anesthesia.     

Indications and outcomes for puppies undergoing mechanical ventilation: 59 cases (2006 to 2020)

The medical records of 59 puppies from 6 hospitals undergoing mechanical ventilation (MV) between 2006 and 2020 were reviewed to describe the signalment, underlying disease, duration of ventilation, and outcome. The most common underlying diseases were pneumonia (n = 18), non-cardiogenic pulmonary edema (n = 16), and trauma (n = 8). Twenty-six (44%) puppies were weaned from the ventilator. The overall survival rate was 39% (23/59) including 19 non-brachycephalic dogs and 4 brachycephalics. Median duration of mechanical ventilation was 27 hours (range: 4 to 144 hours). Brachycephalic dogs were less likely to survive than nonbrachycephalic dogs (P = 0.032). English bulldogs were over-represented with pneumonia. No association between age and survival to discharge (P = 0.716) or outcome (P = 0.579) was detected. The survival rate, and underlying disease process and severity for mechanically ventilated puppies was similar to previous studies in adult dogs.     

Indication,management, and outcome of brachycephalic dogs requiring mechanical ventilation

Objectives – To evaluate the frequency, and need for mechanical ventilation (MV) in a population of brachycephalic dogs (BD) compared with non‐BD. Also, to describe the pre‐MV abnormalities, ventilator settings used, the cardiovascular and pulmonary monitoring results and complications encountered in the same BD population. In addition, we sought to identify factors associated with successful weaning and describe outcomes of BD requiring MV. Design – Retrospective observational study (1990–2008). Setting – University Small Animal Teaching Hospital. Animals – Fifteen BD managed with MV. Interventions – None. Measurements and Main Results – Signalment, indication for MV, ventilator settings, arterial blood gas values, duration of MV, complications, and outcome were recorded for each patient enrolled in study. BD were more likely to receive MV than non‐BD (P=0.036). Out of the 15 dogs that fulfilled the inclusion criteria 7 (47%) underwent MV for impending respiratory fatigue, 6 (40%) for hypoxemia and 2 for hypercapnea. The most common underlying disease was aspiration pneumonia. Duration of MV ranged from 2 to 240 hours (median 15 hours). Seven patients were weaned (47%). Seven dogs had a temporary tracheostomy tube and 5 of them (71%) were weaned. Dogs that were weaned had a significantly greater preweaning trial PaO₂/FiO₂ ratio than those that were not (359 ± 92 versus 210 ± 57 mm Hg, P=0.025). No significant difference for weaning success between dogs with and those without a tracheostomy was detected (P=0.132). The discharge rate was 27% (all from the respiratory fatigue group). Conclusion – Among all dogs admitted to ICU, BD were more likely to receive MV than non‐BD. Aspiration pneumonia was frequently identified as the underlying cause of respiratory compromise. The survival rate for BD undergoing MV was not markedly different from previous studies. Weaning of BD from MV may be facilitated by employing preemptive strategies such as performing tracheostomy tube placements.     

Successful management of saltwater submersion injury in a dog using mechanical ventilation

Objective– To describe successful management of saltwater near-drowning in a dog using mechanical ventilation. Case Summary– A 7-year-old spayed female Golden Retriever weighing 37 kg was referred for mechanical ventilation after saltwater submersion injury. The dog had a history of rare seizures characterized by pre-ictal agitation. On the morning of the event, the dog became agitated and ran toward Puget Sound. The owners discovered the dog unconscious in approximately 25.4 cm (10 in) of water, with her head submerged. The owners estimated that the dog was submerged for approximately 30 seconds. The dog was presented immediately to the nearest emergency facility where initial diagnostic testing and treatment included venous blood gases, nasal oxygen, and IV fluids. The dog was dyspneic despite nasal oxygen administration and was referred for mechanical ventilation. Upon arrival the patient was cyanotic with an arterial partial pressure of oxygen of 38 mm Hg (reference interval 85–100 mm Hg) and oxygenation saturation of 62% (reference interval >95%). Thoracic radiographs were taken and revealed severe, bilateral pulmonary infiltrates. The patient was ventilated for 70 hours and was discharged 4 days later. Complications included pneumonia and phlebitis at the site of a cephalic IV catheter. Follow up thoracic radiographs 10 days after discharge were within normal limits and the owners report a full recovery at 1 year. New or Unique Information Provided– Submersion injury can result in acute respiratory distress syndrome in dogs. Mechanical ventilation provided critical support during pulmonary recovery in this dog.     

Treatment of a dog with severe baclofen intoxication using hemodialysis and mechanical ventilation

Objective: This case report describes the successful management of a dog with coma and respiratory depression due to severe baclofen intoxication. Case summary: A Doberman Pinscher mixed breed dog ingested 500 mg (20 mg/kg) of baclofen. Signs of severe intoxication included coma and profound respiratory muscle weakness. The dog was supported with positive pressure ventilation and treated with one session of hemodialysis. Weaning from the ventilator was achieved within 4 hours of hemodialysis, and recovery from coma occurred over the following 12–36 hours. The dog regained full neurologic function and was normal at discharge following 3 days of hospitalization. New or unique information provide: Severe central nervous system depression and respiratory depression due to baclofen intoxication can be life threatening. In addition to other supportive care, hemodialysis may hasten recovery and ventilatory support may be essential to achieve a positive outcome. With successful treatment, toxicity can be decreased and the associated life‐threatening central nervous system and ventilatory depression can resolve. Prognosis for return of normal function is excellent.     

Some cardiopulmonary effects of capnoperitoneum in anesthetized guinea pigs (Cavia porcellus): spontaneous ventilation versus intubation and mechanical ventilation

ObjectiveTo compare cardiopulmonary variables and blood gas analytes in guinea pigs (Cavia porcellus) during anesthesia with and without abdominal carbon dioxide (CO₂) insufflation at intra-abdominal pressures (IAPs) 4 and 6 mmHg, with and without endotracheal intubation.Study designProspective experimental trial.AnimalsA total of six intact female Hartley guinea pigs.MethodsA crossover study with sequence randomization for IAP and intubation status was used. The animals were sedated with intramuscular midazolam (1.5 mg kg^–1) and buprenorphine (0.2 mg kg^–1) and anesthetized with isoflurane, and an abdominal catheter was inserted for CO₂ insufflation. Animals with endotracheal intubation were mechanically ventilated and animals maintained using a facemask breathed spontaneously. After 15 minutes of insufflation, the following variables were obtained at each IAP: pulse rate, respiratory rate, rectal temperature, oxygen saturation, end-tidal CO₂ (intubated only), peak inspiratory pressure (intubated only), noninvasive blood pressure and blood gas and electrolyte values, with a rest period of 5 minutes between consecutive IAPs. After 4 weeks, the procedure was repeated with the guinea pigs assigned the opposite intubation status.ResultsIntubated guinea pigs had significantly higher pH and lower partial pressure of CO₂ in cranial vena cava blood (PvCO₂) than nonintubated guinea pigs. An IAP of 6 mmHg resulted in a significantly higher PvCO₂ (65.9 ± 19.0 mmHg; 8.8 ± 2.5 kPa) than at 0 (53.2 ± 17.2 mmHg; 7.1 ± 2.3 kPa) and 4 mmHg (52.6 ± 10.8 mmHg; 7.01 ± 1.4 kPa), mean ± standard deviation, with intubated and nonintubated animals combined.Conclusions and clinical relevanceAlthough the oral anatomy of guinea pigs makes endotracheal intubation difficult, capnoperitoneum during anesthesia induces marked hypercapnia in the absence of mechanical ventilation. An IAP of 4 mmHg should be further evaluated for laparoscopic procedures in guinea pigs because hypercapnia may be less severe than with 6 mmHg.     

Assessing the influence of mechanical ventilation on blood gases and blood pressure in rattlesnakes

ObjectiveTo characterize the impact of mechanical positive pressure ventilation on heart rate (HR), arterial blood pressure, blood gases, lactate, glucose, sodium, potassium and calcium concentrations in rattlesnakes during anesthesia and the subsequent recovery period.Study designProspective, randomized trial.AnimalsTwenty one fasted adult South American rattlesnakes (Crotalus durissus terrificus).MethodsSnakes were anesthetized with propofol (15 mg kg⁻¹) intravenously, endotracheally intubated and assigned to one of four ventilation regimens: Spontaneous ventilation, or mechanical ventilation at a tidal volume of 30 mL kg⁻¹ at 1 breath every 90 seconds, 5 breaths minute⁻¹, or 15 breaths minute⁻¹. Arterial blood was collected from indwelling catheters at 30, 40, and 60 minutes and 2, 6, and 24 hours following induction of anesthesia and analyzed for pH, PaO₂, PaCO₂, and selected variables. Mean arterial blood pressure (MAP) and HR were recorded at 30, 40, 60 minutes and 24 hours.ResultsSpontaneous ventilation and 1 breath every 90 seconds resulted in a mild hypercapnia (PaCO₂ 22.4 ± 4.3 mmHg [3.0 ± 0.6 kPa] and 24.5 ± 1.6 mmHg [3.3 ± 0.2 kPa], respectively), 5 breaths minute⁻¹ resulted in normocapnia (14.2 ± 2.7 mmHg [1.9 ± 0.4 kPa]), while 15 breaths minute⁻¹ caused marked hypocapnia (8.2 ± 2.5 mmHg [1.1 ± 0.3 kPa]). Following recovery, blood gases of the four groups were similar from 2 hours. Anesthesia, independent of ventilation was associated with significantly elevated glucose, lactate and potassium concentrations compared to values at 24 hours (p < 0.0001). MAP increased significantly with increasing ventilation frequency (p < 0.001). HR did not vary among regimens.Conclusions and clinical relevanceMechanical ventilation had a profound impact on blood gases and blood pressure. The results support the use of mechanical ventilation with a frequency of 1–2 breaths minute⁻¹ at a tidal volume of 30 mL kg⁻¹ during anesthesia in fasted snakes.     

Regional distribution of ventilation in horses in dorsal recumbency during spontaneous and mechanical ventilation assessed by electrical impedance tomography: a case series

ObjectiveTo evaluate the regional distribution of ventilation in horses during spontaneous breathing and controlled mechanical ventilation (CMV) using electrical impedance tomography (EIT).Study designProspective, experimental case series.AnimalsFour anaesthetized experimental horses.MethodsHorses were anaesthetized with isoflurane in an oxygen-air mixture and medetomidine continuous rate infusion, placed in dorsal recumbency with an EIT belt around the thorax, and allowed to breathe spontaneously until PaCO₂ reached 13.3 kPa (100 mmHg), when volume CMV was started. For each horse, the EIT signal was recorded for at least 2 minutes immediately before (T1), and at 30 (n = 3) or 60 (n = 1) minutes after the start of CMV (T2). The centre of ventilation (CoV), dependent silent spaces (DSS) (likely to represent atelectatic lung areas), non-dependent silent spaces (NSS) (likely to represent lung areas with low ventilation) and total ventilated area (TVA) were evaluated. Cardiac output (CO) was measured and venous admixture and oxygen delivery (DO₂) were calculated at T1 and T2. Data are presented as median and range.ResultsAfter the initiation of CMV, the CoV moved ventrally towards the non-dependent lung by 10% [from 57.4% (49.6–60.2%) to 48.3% (41.9–54.4%)]. DSS increased [from 4.1% (0.2–13.9%) to 18.7% (7.5–27.5%)], while NSS [21.7% (9.4–29.2%) to 9.9% (1.0–20.7%)] and TVA [920 (699–1051) to 837 (662–961) pixels] decreased. CO, venous admixture and DO₂ also decreased.Conclusions and clinical relevanceIn spontaneously breathing anaesthetized horses in dorsal recumbency, ventilation was essentially centred within the dependent dorsal lung regions and moved towards non-dependent ventral regions as soon as CMV was started. This shows a major lack of ventilation in the dependent lung, which may be indicative of atelectasis.     

Successful cardiopulmonary resuscitation and use of short-term mechanical ventilation following inadvertent ketamine overdose in a cat

Objective: To describe the clinical manifestations and successful outcome following an inadvertent overdose of ketamine to a cat. Case summary: A 4‐year‐old neutered male domestic shorthair cat was evaluated for a urethral obstruction. Because of an inadvertent miscalculation of ketamine, 20 times the intended dose was administered intravenously, which resulted in cardiopulmonary arrest. Cardiopulmonary‐cerebral resuscitation was successful, and short‐term mechanical ventilation, fluids and intensive monitoring were utilized to achieve full recovery and subsequent discharge of the animal. New or unique information provided: Ketamine is a common anesthetic agent used in cats that is considered to have a wide therapeutic index and minimal cardiopulmonary depressant effects at recommended doses. Successful management of inadvertent ketamine overdose has been reported in children, but not in cats. Prompt CPCR and short‐term mechanical ventilation may be necessary to treat a significant ketamine overdose. In cats, yohimbine may act as a partial antagonist of ketamine.     

Controlled mechanical ventilation in equine anaesthesia: Physiological background and basic considerations (Part 1)

Controlled mechanical ventilation (CMV) is routinely used in equine anaesthesia, with many different options available to mechanically deliver breaths. The complexity of respiratory pathophysiology in anaesthetised horses and the wide range of devices available is described in this scoping review. The first part of the review outlines basic equine respiratory physiology and pathophysiology during anaesthesia to illustrate what makes horses prone to inefficient gas exchange and ventilation when they are recumbent. The difference between spontaneous ventilation and CMV is reviewed and basic considerations of CMV are explored in more detail.     

盐酸吸入联合机械通气建立暴喘症大鼠模型

为了建立盐酸吸入联合机械通气暴喘症模型大鼠,通过盐酸吸入联合机械通气建立暴喘症大鼠模型。SD大鼠随机分为正常对照组、小通气量模型组、大通气量模型组。正常对照组不给予盐酸及机械通气;小通气量模型组和大通气量模型组分别给予盐酸及不同机械通气,6 h后观察并取肺脏进行湿干重比、肺BALF液嗜中性粒细胞占白细胞比例和组织病理学检查。结果模型组大鼠呼吸频率加快,出现呼吸窘迫症状,甚至口唇及指端发绀;与正常对照组比较,小通气量模型组、大通气量模型组大鼠左肺组织湿干重比显著高于对照组(P0.05、P0.01),大鼠肺BALF液嗜中性粒细胞占白细胞比例显著高于对照组(P0.01),肺脏组织病理出现肺泡壁毛细血管扩张、充血及肺水肿等症状,表明利用盐酸吸入联合机械通气成功建立了暴喘症大鼠模型。