The effects of CO2 on respiratory mechanics in anesthetized paralyzed humans

BACKGROUND

There is little information concerning the carbon dioxide-related effects on respiratory mechanics in anesthetized, paralyzed subjects; however, hypocapnia or hypercapnia is often permitted in patients with severe brain injury or acute respiratory distress syndrome. Therefore, the carbon dioxide dependence of respiratory mechanics in healthy anesthetized, paralyzed subjects was investigated.

METHODS

Interrupter resistance (Rint), additional tissue viscoelastic resistance (deltaR), and quasi-static elastance (Est) of lung (L) and chest wall were assessed by means of the rapid end-inspiratory occlusion method in two groups of seven healthy paralyzed subjects anesthetized with diazepam or isoflurane. They underwent ventilation with a fixed pattern and hyperoxic gas mixtures with different fractions of inspired carbon dioxide (FICO2) to produce a partial pressures of arterial carbon dioxide (PaCO2) of 24.4 +/- 3.4, 39.6 +/- 3.2, and 62 +/- 4.1 (SD) mmHg.

RESULTS

Chest wall mechanics and Est,L were unaffected by PaCO2 changes. With diazepam anesthesia, Rint,L decreased linearly, with increasing PaCO2, from 2.3 to 1.4 cm H2O.s.l(-1), whereas deltaR,L decreased from 2 to 1.7 cm H2O.s.l(-1), though not significantly. With isoflurane anesthesia, the decrease of Rint,L (0.2 +/- 0.5 cm H2O.s.l(-1)) was not significant, and deltaRL remained unchanged. With diazepam, Rint,L was 45 (hypercapnia) to 110% (hypocapnia) greater than with isoflurane.

CONCLUSIONS

Changes of PaCO2 from 20-65 mmHg cause increasing bronchodilation in anesthetized, paralyzed subjects, this effect being attenuated or abolished by drugs (e.g., halogenated anesthetics) that depress smooth muscle tone substantially. The carbon dioxide bronchodilating effects are probably direct for peripheral structures and are paralleled by a tendency of lung tissue resistance to decrease. Because local PaCO2-related changes in bronchomotor tone promote VA/Q matching, this mechanism should be impaired by anesthetics that cause bronchodilation.

Volume and time dependence of respiratory system mechanics in normal anaesthetized paralysed humans

The purpose of the present investigation was to assess the effect of large tidal volumes and mean lung volumes on the viscoelastic properties of the respiratory system in normal humans; and to verify if in this case the results could be satisfactorily described by a simple linear viscoelastic model of the respiratory system. Twenty-eight subjects (7 females), aged 14-28 yrs, were studied before orthopaedic surgery on the lower limbs. None were obese, or had clinical evidence of cardiopulmonary disease. The interrupter conductance and the viscoelastic constants of the respiratory system were assessed using the rapid end-inspiratory airway occlusion method during mechanical ventilation with tidal volumes up to 3 L and applied end-expiratory pressures up to 23 cmH2O. It was found that the interrupter conductance increased linearly with lung volume over a larger range than used previously; and the viscoelastic resistance and time constant did not change over the entire range of tidal volumes and end-expiratory pressures studied. In conclusion, in normal anaesthetized, paralysed subjects a simple linear viscoelastic model satisfactorily described the viscoelastic behaviour of the respiratory system over the whole range of volume studied.

Assessment of respiratory system viscoelasticity in spontaneously breathing rabbits

Airflow, volume (V), inspiratory time (TI), tracheal pressure (Ptr), abdomen (Dab) and rib cage diameters (Drc), peak diaphragm (Adi) and parasternal muscle activity (Aic) were measured in thirteen anaesthetized and vagotomized rabbits and in six vagotomized rabbits with cordotomy at T1 during unimpeded inspirations followed by rapid end-inspiratory airway occlusion, relaxation against closed airways, and inspiratory effort. To modify the inspiratory flow pattern, such sequences were performed at different volume, levels of chemical drive, and body temperatures (BT). Under all conditions, Adi, Aic, TI, Drc and Dab at iso-volume were the same for unimpeded and occluded inspirations; end-inspiratory Ptr was lower for occluded than for unimpeded inspirations, the difference (Pdiff) being larger the lower the volume at which occlusions were performed and the higher the chemical drive and BT. After paralysis, the viscoelastic constants of the respiratory system, modelled as a Kelvin body, were assessed according to the rapid airway occlusion method and used together with the inspiratory flow waveform to predict the end-inspiratory viscoelastic pressure (Pvisc) of unimpeded inspirations. Since the slope of the Pdiff vs. Pvisc relationship never differed from unity, Pdiff under the specified conditions should represent the effective Pvisc of unimpeded inspirations.

[Subarachnoid hemorrhage: protective hypotension in delayed surgery]

Systemic hypertension is frequently observed in patients with subarachnoid haemorrhage (SAH). Continuing systemic hypertension might augment the risk of rebleeding and also increase the blood flow and blood volume, resulting in more marked cerebral edema and intracranial hypertension. However, reduction of blood pressure might also decrease cerebral perfusion pressure in patient with an impaired autoregulation and in this way enhance the risk of cerebral ischemia. Anti-hypertensive therapy is not recommended to prevent rebleeding after SAH. The agents of choice for reduction of arterial blood pressure might be mixed alfa and beta adrenergic antagonists and barbiturates.

Chest wall interrupter resistance in anesthetized paralyzed humans

Tracheal (Ptr) and esophageal (Pes) pressure and flow were measured in 12 supine anesthetized paralyzed normal subjects aged 16-22 yr. The subjects were ventilated with a fixed inflation volume (range 0.57-0.62 liter) and with different constant flows ranging between 0.24 and 1.12 l/s. A rapid airway shutter (closing time 10-15 ms) was used to briefly occlude (0.4-0.9) the airways at end inspiration for 33-44 consecutive breaths. At each flow level, Ptr and Pes records obtained during end-inspiratory occlusions were ensemble averaged to allow for the cardiac artifacts. The interrupter resistances of the chest wall and respiratory system were assessed as the rapid fall in Pes and Ptr with occlusion divided by the flow preceding the occlusion. Interrupter resistances of both the chest wall and lung were independent of flow and averaged 0.4 +/- 0.1 and 1.5 +/- 0.4 (SD) cmH2O.s.l-1, respectively. The contribution of the chest wall to the total interrupter resistance was approximately 27% at flows < or = 1 l/s.

[Pathology requiring admission to the neurosurgical intensive care]

A patient's admission to an ICU should benefit the patient (either short or long term). Consequently patients admitted to the ICU should be only those neurological-neurosurgical ones with one or more organ failure who need immediate or prolonged therapy with adequate monitoring and technical support not available in other wards. Normally admission depends on may other factors, the most important ones seem to be medico-legal responsibilities of the possible refusal of admission even if due to clinical reasons, organizational problems such as equipment and bed availability. There is a need for pre- and post-surgical monitoring for high risk patients. We routinely admit patients to our ICU according to the following directions: respiratory and or cardio-circulatory failure connected to the neurological pathology, impaired consciousness, neurological signs and symptoms indicating deterioration, loss of airway protective reflexes, seizured in neurological-neurosurgical patients, severe electrolyte, metabolic or nutritional disturbances which may affect the CNS; neurosurgical postoperative control, performance of complex techniques (central venous and arterial catheterization, ICP monitoring), management of multiorgan donors suitable for therapeutic transplant. The groups of pathologies observed in our ICU during 1991 where the follow-up: 442 neurological-neurosurgical patients--head trauma 141 (32%)--cerebral tumours 88 (19.9%)--vasculopathies 135 (30.6%)--others 77 (17.5%). 228 (51.5%) patients stayed for a period less than three days. According to our experience we can affirm that causes for patient's admission to neuro-ICUs are various, the legal organizational reasons have a great importance compared to the clinical ones.

Effect of PEEP on respiratory mechanics in anesthetized paralyzed humans

With the use of the technique of rapid airway occlusion during constant flow inflation, respiratory mechanics were studied in eight anesthetized paralyzed supine normal humans during zero (ZEEP) and positive end-expiratory pressure (PEEP) ventilation. PEEP increased the end-expiratory lung volume by 0.49 liter. The changes in transpulmonary and esophageal pressure after flow interruption were analyzed in terms of a seven-parameter "viscoelastic" model. This allowed assessment of static lung and chest wall elastance (Est,L and Est,W), partitioning of overall resistance into airway interrupter (Rint,L) and tissue resistances (delta RL and delta RW), and computation of lung and chest wall "viscoelastic constants." With increasing flow, Rint,L increased, whereas delta RL and delta RW decreased, as predicted by the model. Est,L, Est,W, and Rint,L decreased significantly with PEEP because of increased lung volume, whereas delta R and viscoelastic constants of lung and chest wall were independent of PEEP. The results indicate that PEEP caused a significant decrease in Rint,L, Est,L, and Est,W, whereas the dynamic tissue behavior, as reflected by delta RL and delta RW, did not change.

Pulmonary and chest wall mechanics in anesthetized paralyzed humans

Pulmonary and chest wall mechanics were studied in 18 anesthetized paralyzed supine humans by use of the technique of rapid airway occlusion during constant-flow inflation. Analysis of the changes in transpulmonary pressure after flow interruption allowed partitioning of the overall resistance of the lung (RL) into two compartments, one (Rint,L) reflecting airway resistance and the other (delta RL) representing the viscoelastic properties of the pulmonary tissues. Similar analysis of the changes in esophageal pressure indicates that chest wall resistance (RW) was due entirely to the viscoelastic properties of the chest wall tissues (delta RW = RW). In line with previous measurements of airway resistance, Rint,L increased with increasing flow and decreased with increasing volume. The opposite was true for both delta RL and delta RW. This behavior was interpreted in terms of a viscoelastic model that allowed computation of the viscoelastic constants of the lung and chest wall. This model also accounts for frequency, volume, and flow dependence of elastance of the lung and chest wall. Static and dynamic elastances, as well as delta R, were higher for the lung than for the chest wall.